Inorganic chemistry questions

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• AQA 7405 · June 2025 · Paper 1 · Q1 · 11 marks

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  Over half the students scored both marks in part 1; some students were unfamiliar with the correct formula of titanium (IV) chloride and were unable to write a balanced equation. Some students did not know the role of magnesium. Answers to part 2 were good; common errors included incorrect formulas. In part 3 many students knew that magnesium hydroxide was less soluble than sodium hydroxide; common errors included reference to the base strength without linking their ideas to solubility or comparing the reactivity of NaOH and Mg(OH)2 Fewer than half the students scored both marks in part 4; some students did not attempt an ionic equation and used NaOH as their reagent. The bromide was well known in part 5, but many students did not write an ionic equation or omitted the state symbols in their equation. The most common answer for the explanation of acidification was to remove carbonate ions. Occasionally, students answered in terms of false positive results but failed to mention the ions involved. Part 6 was well answered.

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  This question is about some Group 2 elements and their compounds.

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• AQA 7405 · June 2025 · Paper 1 · Q2 · 8 marks

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  About 50% of the students gained both marks in part 1; some students lost the second mark for not giving a comparative response or by not referring to the attraction between the nucleus and the outermost electron. Part 2 was well answered by the majority of students. In part 3, about one third of students correctly identified magnesium; argon, aluminium or silicon were common incorrect answers. Phosphorus was regularly identified in part 4 and most students recognised the jump in ionisation energy from 5 → 6; many students did not explain that a jump in the ionisation energy meant that an electron was being removed from an inner energy level. About half the © 2025 AQA students scored the mark in part 5; common incorrect answers included log 3.5 = 0.54 and 3.16 × 10–4.

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  This question is about ionisation energies.

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• AQA 7405 · June 2025 · Paper 1 · Q3 · 23 marks

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  Just over 60% of students answered part 1 correctly. In part 2 many students lost this mark by not identifying the electron pair/lone pair was donated from oxygen to copper. Many incorrect observations were seen in part 3. The students who started with CO32– usually gave a correct equation; ions with incorrect formula CO3– were common. In part 4 most students gave the correct shapes; the most common error was square planar instead of tetrahedral. Some students did not refer to ligand size and instead gave ideas based on electron pair repulsion theory; some referred to chlorine atoms/molecules rather than chloride ions. Some students did not refer to d orbital splitting. Many students referred to light being absorbed but did not identify the light as visible or white. Many students knew that the colour seen was light that had not been absorbed. Few students were able to write about the energy gap and what was responsible for the change in the energy gap, although some did reference the equation ∆E = h𝜈𝜈. Fewer than 20% scored both marks in part 5. Many students gave an incorrect observation. Students found the equation challenging; chlorine was often given as a product, some students had 2 mol of ammonia and 4 mol of water instead of the other way round and the copper complex often lacked a charge. In part 6 many students could not give a balanced equation; H+ ions were often omitted from the left-hand side and water from the right-hand side. In part 7 most students gave the correct answer; a common error was to average three of the four titres instead of the two that were concordant. The calculation in part 7 proved challenging; most students were able to score the first three marks but then failed to divide by 2 and were not able to make progress. Some students used 25.0 cm3 for their titre. Students who had calculated an Mr greater than 275 usually divided this by 2 to get an Ar but often did not assign this to a metal with a +1 oxidation state. Students found part 9 challenging; common errors were to have a 4- coordinate copper or to bond the copper to a C2O42– ion via 1 coordinate bond. Many students did not specifically draw the coordinate bonds originating from the O atom of the water.

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  This question is about the chemistry of copper.

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• AQA 7405 · June 2025 · Paper 1 · Q5 · 10 marks

  10 marks

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  Many students failed to mention that van der Waals’ forces were between molecules in part 1; a few students referred to chloride and bromide ions. Some students confused the boiling point with ionisation energy and wrote about higher nuclear charge in bromine so stronger attraction between the outer electrons and the nucleus. A number of students thought that the covalent bonds break during a change of state and some students compared the reactivity of chlorine and bromine. Part 2 was generally well answered. Many students drew an acceptable diagram in part 3; a common wrong answer had three bonding pairs and one lone pair. The explanation proved more challenging; some students mentioned electron pairs repelling, but few qualified this with the electron pairs repel to be as far apart as possible. Part 4 differentiated well; observations were often incorrect eg white and cream precipitates, and HBr described as a brown gas. A correct equation for the acid -base equation was more successful than the redox equation. The redox equation was often unbalanced or contained incorrect formulas eg NaSO4 and BrSO4. In part 5 very few students gave an equation to form copper(I) iodide.

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  This question is about the halogens and some of their compounds.

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• AQA 7405 · June 2025 · Paper 1 · Q7 · 15 marks

  15 marks

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  Part 1 was well answered. In part 2 many students did not state that layers of atoms/ions can slide over each other; some students stated that metals were malleable or ductile or that vanadium was a metal. The equations in part 3 were generally well done; common errors included unbalanced equations with the correct species, or equations that had the wrong oxidation states for the vanadium oxides, many including positive ions e.g. VO+. In part 4, the majority of students did not recognize that the reduction of the vanadium species with tin proceeds in two steps and stops at V(III). A correct understanding of how EMF values determine the spontaneity of redox reactions was essential for full marks. Many students who did start and end with the correct vanadium species did not balance the equation. In part 5 many students clearly explained that the high melting point of ionic compounds is due to the strong electrostatic attraction between oppositely charged ions. Some students demonstrated confusion about the nature of the bonding; common errors included describing the bonding as molecular with van der Waals’ forces, macromolecular with covalent bonds, or metallic bonding. Few students were able to construct and balance the initial equation in part 6 to show the formation of the hexa-aqua complex; VO, VO2+ and H2SO4 were all common products. A common error was the misidentification of SO₄²⁻ as the acidic species, likely due to its association with sulfuric acid. However, in this context, sulfate is simply a spectator ion and does not contribute to acidity. In part 7 many students were confused by the instruction to use the symbol Q and instead attempted to write equations using the molecular formula. Many students used high energy electrons in their equation to knock an electron off the molecule and those who used the full formula did not add an H+ to the ion they formed or forgot to add the charge. Equations such as Q + H+ → QH and Q + H+ → Q+ were common. Few students gained M2, as many ignored the instruction to use a mass of 51 for vanadium. Instead, they used the relative atomic mass from the Periodic Table, which led to an incorrect calculated value of 601.9 rather than the expected result based on the given mass. More than 60% of students gained both marks in part 8. The students who found the question challenging could not give an expression linking the abundances of the two isotopes; 49x + 51y = 50.97 and 49x + 51x = 50.97 were common. Some students did not add the abundances of their ions to give 100 (or 1).

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  This question is about vanadium and some of its compounds.

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• AQA 7405 · June 2024 · Paper 1 · Q2 · 10 marks

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  Students found parts 1, 2 and 3 challenging; fewer than 40% were successful on each of these parts. In part 1 many students suggested that the strontium hydroxide had fully dissolved or was in excess, not realising that a saturated solution was needed. Common insufficient answers in part 2 included removing the undissolved strontium hydroxide so it didn’t affect the titre, but students did not state that the titre would be larger. In part 3, very few answers referred to the solution reacting with carbon dioxide in the air; common incorrect answers stated that the solution would react with oxygen/air. Evaporation of the strontium hydroxide (solution) was accepted in the mark scheme and seen quite regularly as an answer. Answers to part 4 were very good. 33% of students gained all the marks in part 5: the question discriminated extremely well. Most students calculated the average titre: some used 3 or 4 values rather than the two concordant values. Other common errors included multiplying by 2, using an incorrect Mr and failing to multiply by 4.

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  This question is about an experiment to determine the solubility of strontium hydroxide in water at 20 degrees C. Solid strontium hydroxide is added to approximately 1 dm3 of distilled water in a stoppered flask kept at 20 degrees C. The mixture is checked daily and more solid is added whenever none remains. Once no more solid is needed, the mixture is filtered into another stoppered flask. A 25.0 cm3 sample of filtrate is pipetted into a conical flask, indicator is added, and the sample is titrated with 0.100 mol dm^-3 hydrochloric acid. The titration is repeated with further samples.

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• AQA 7405 · June 2024 · Paper 1 · Q3 · 14 marks

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  In part 1 most students knew they needed an octahedral shape and bidentate ligands for the complex, but could not draw the ethanedioate ligands correctly: errors included missing C or O atoms or including C=C. Many students could not recall the chelate effect in part 2: some students stated that the complex formed is more stable because stronger bonds were formed. Part 3 discriminated well: 23% of students gave a level 3 answer and most gained 1 or 2 marks by drawing a graph with labelled axes. Incorrect curves on the graph included a straight line down, a horizontal line at the start then a curve down of reducing steepness. The repulsion between the negative ions was well understood by many students but the attraction between iron(II) ions and persulfate ions was only stated by the more able students. The idea that variable oxidation states are important for transition metal catalysis is usually well known but few students stressed this in their answers. The equations were often partially recalled with errors in balancing; SO4– was a common incorrect formula for the sulfate ion. Invalid references to autocatalysis and heterogeneous catalysis were sometimes seen. Many students gave the correct answer to part 4: common incorrect answers included Fe(NH3)2(H2O)4 and Fe(OH–)2(H2O)4. In part 5 the colour of the precipitate was known quite well: some students gave effervescence as an observation with or without the precipitate and therefore could not score. In the equation many students incorrectly attempted to give acid base reactions. 31% of students gave a correct equation in part 6: common errors included unbalanced equations or giving Na atoms as a product. CO3– was often seen for the carbonate ion in both parts 4 and 5.

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  This question is about aqueous ions of the metal iron.

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• AQA 7405 · June 2024 · Paper 1 · Q5 · 10 marks

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  67% of students gave a correct equation in part 1. The equation in part 2 proved more difficult: errors included HCl or HOCl as the product and a few students who had all the species correct failed to balance the equation. In part 3 most students gave the first equation. Errors in the second equation included not balancing the chlorine atoms, putting H+ on the wrong side of the equation and omitting the electrons. In the third equation 5Cl2 or 7Cl2 were common errors. A few students missed the charge on the chlorate(V) ion. In part 4, 39% of students scored both the marks: errors in the identification of the role of the chloride ion included acid, oxidising agent, reducing agent and catalyst. Students found part 5 challenging: errors included shapes with 0, 1 or 2 lone pairs or four chlorine atoms. In part 6, 24% of students gained both marks. Many students who gave the correct bond angle did not give a correct explanation: many responses referred to lone pairs repelling more than bond pairs.

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  This question is about chlorine.

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• AQA 7405 · June 2024 · Paper 1 · Q6 · 10 marks

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  In part 1, 15% of students scored both marks. Very few students answered in terms of the EꝊ of Zn2+. Many students referred to the EꝊ of Zn and could not gain any marks. Students who wrote about EꝊ of Zn2+ compared it to the EꝊ of VO2+ and not to the EꝊ of V3+. There was a lack of understanding of the concept that while Zn is the reacting species, the electrode potential refers to the reduction of Zn2+. Part 2 was well answered. Most students scored the mark for the cell EMF in part 3. In the cell representation, the order of VO2+ and V3+ were often the wrong way round or the Pt electrode was missing. The half- equation was often the wrong way round. The calculation in part 4 proved challenging but 34% of students managed to score all the marks. Many students used the reacting ratio correctly but then went on to use a second reacting ratio to calculate the amount, in moles, of vanadate(V) and lost the second mark. Some students used an incorrect Mr in the calculation.

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  This question is about vanadium ions. Use these standard electrode potentials: O2(g) + 4H+(aq) + 4e- -> 2H2O(l), E = +1.23 V; VO2^+(aq) + 2H+(aq) + e- -> VO^2+(aq) + H2O(l), E = +1.00 V; VO^2+(aq) + 2H+(aq) + e- -> V3+(aq) + H2O(l), E = +0.34 V; V3+(aq) + e- -> V2+(aq), E = -0.26 V; Fe2+(aq) + 2e- -> Fe(s), E = -0.44 V; Zn2+(aq) + 2e- -> Zn(s), E = -0.76 V; V2+(aq) + 2e- -> V(s), E = -1.20 V; Mg2+(aq) + 2e- -> Mg(s), E = -2.38 V. Here VO2^+ denotes one V atom, two O atoms and an overall + charge, whereas VO^2+ denotes one V atom, one O atom and an overall 2+ charge.

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• AQA 7405 · June 2024 · Paper 1 · Q9 · 17 marks

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  Errors in answers to part 1 included more shielding or more orbitals: these are correct statements but insufficient for the mark as they do not explain the increase in atomic size. The equation in part 2 was generally good: errors included missing state symbols and MgO(aq) or MgO(g) as a product. Some students gave magnesium hydroxide as a product. In part 3 most students knew that electrons were easier to lose from barium and/or that barium sulfate is insoluble. Very few students stated that a precipitate would form a layer on the barium and stop the reaction. 38% of students scored all the marks in the calculation in part 4 and most students gained a few marks. Errors included incorrect rearrangement of the equation and not using the correct mole ratio. An answer of 59 was common using a 1:1 ratio but this meant that 124 could not be subtracted and the metal could not be identified. The equation in part 5 was generally correct: errors included using H instead of H2. Very few students realised that there was ionic bonding in the compound and described metallic bonding or intermolecular forces. The correct equation in part 6 was given by 47% of students: common errors included using one mole of NaOH or not balancing the equation. 19% of students gave the correct half-equation in part 7. Li+ + e– → Li and Li+ + CoO2 → LiCoO2 were common incorrect answers. Many students identified that the reaction is reversible in part 8: some students confused the cell with a fuel cell and stated that reactants could be continuously supplied.

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  This question is about metals and their compounds.

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• AQA 7405 · June 2024 · Paper 3 · Q5 · 9 marks

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  [05.2] 60% of students scored two or more marks for this question with the most common error being the mis-conversion of nm into m. Some students read off the absorbance at the peak of the curve instead of the wavelength. Section B – Multiple Choice Some students need to pay more careful attention to the instructions given at the start of Section B about how to complete the answers by shading in the lozenge, crossing out an incorrect answer and then circling a crossed-out answer if it is then decided to be correct. There were again quite a few instances of crossed out and circled answers alongside shaded lozenges – which has to be treated as two answers and so marked incorrect. Multiple-choice items that proved to be very accessible (answered correctly by at least 75% of students) were B8, B16, B23, B24, B29, B33 and B35. The most challenging items (answered correctly by fewer than 45% of students) were B7, B11, B28 and B34.

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  This question concerns colour in transition-metal complexes and quantitative colorimetry.

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• AQA 7405 · June 2023 · Paper 1 · Q1 · 10 marks

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  Although many students gave the correct definition in 01.1, several answers did not refer to an atom or ion and many answers gave the definition of a transition metal instead of a transition metal complex. In 01.2, the name of the shape was often not known even if the diagram was correct; common errors included diagrams with no lone pairs. Only the most able students could give the correct explanation in 01.3; many students referred to the oxidation state of Cr(PF3)6 or the PF3 molecule. In 01.4, the oxidation state was given correctly by 49% of students; the most common incorrect answer was +2. Many students gave the correct answer in 01.5; common errors were hydrogen bonds and dative covalent bonds. Many students gained partial marks in 01.6. There were several errors seen in students’ answers, eg Cl2 as a ligand, omitting the charge on the ion, drawing two cis- or two trans-isomers, and attempts at drawing optical isomers. Students found 01.7 challenging; the correct equation was given by only 14% of students.

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  This question is about complexes of the transition metal chromium.

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• AQA 7405 · June 2023 · Paper 1 · Q3 · 6 marks

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  The equation in 03.1 was well known. The equation in 03.2 was less well known; some students gave H2SO4 as the product. Many students found 03.3 difficult and did not draw the displayed formula of the anion. In 03.4 several students did not give the correct equation; the most common errors were to form magnesium hydroxide and hydrogen or to give the observation as fizzing or a white precipitate. If students knew the formula of the product in 03.5 they invariably gave a correct equation; many students did not know the formula of magnesium phosphate and therefore could not give a balanced equation.

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  This question is about Period 3 elements and their oxides.

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• AQA 7405 · June 2023 · Paper 1 · Q7 · 17 marks

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  Answers to 07.1 were good. In 07.2, the correct equation was given by 35% of students; common errors included 6 waters on the LHS of the equation and missing the 2– charge on SO42–. Students generally found the explanation in 07.3 challenging; many students discussed the 3+ charge on the complex stating that the complex was polarising, with no reference to Al3+. The second mark was not often gained. Answers to 07.4 were quite good. In 07.5, the observation was often correct but the equation was less well known. A few students gave the correct answer in 07.6; Al2(SO4)3 was a common incorrect answer. In 07.7, the nitrogen atoms were generally correctly identified; common errors included circling both O species. Question 07.8 discriminated very well. Most students could calculate the amount, in moles, of Zn2+ and EDTA4– and many then subtracted correctly to get M3; common errors in answers included not multiplying by 10, not dividing by 2 and not giving the answer as an integer. Some students incorrectly calculated the Mr of Al2(SO4)3 even though this value was given in the question.

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  This question is about complexes containing the aluminium ion.

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• AQA 7405 · June 2023 · Paper 3 · Q4 · 6 marks

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  As last year, only about 10% of students were able to access level 3 and score 5 or 6 marks. In this case, the most common omissions from the indicative content were 2b and 2c which related to the explanation of the reason for effervescence with L but not with M when carbonate was added. Many students thought that it was enough to simply say that CO2 is produced with L but not M and did not go on to discuss the reasons for this in terms of the greater acidity of Fe3+(aq) compared with Fe2+(aq). Many students did, however, recognise the important distinction that the 3+ ion with carbonate produces iron(III) hydroxide but the 2+ ion with carbonate forms iron(II) carbonate. The quality of equations varied and there were sometimes contradictions between a statement that, for example, a reaction produces a precipitate of Fe(H2O)4(OH)2 but then an equation showing the formation of Fe(H2O)4(NH3)2. Hydroxide ions were also often seen as a reactant in the reaction equations for Fe2+ and Fe3+ with ammonia. It was unexpected to discover that a significant minority of students did not recognise the tests for ions that were included in the question – especially as some of these tests are first encountered at GCSE.

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  Solutions L and M contain iron salts. With aqueous ammonia, L gives a red-brown precipitate and M gives a green precipitate; both are insoluble in excess. With aqueous sodium carbonate, L gives a red-brown precipitate and effervescence, while M gives a green precipitate without effervescence. After acidification, barium chloride gives no change with L and a white precipitate with M; silver nitrate gives a white precipitate with L and no change with M.

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• AQA 7405 · June 2023 · Paper 3 · Q6 · 7 marks

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  In 06.1, the most common reason for a lost mark was a lack of clarity in explaining that 0.005 mol was the required amount of Ca(OH)2 needed to react with the 0.01 mol of HCl. M2 was not awarded just for seeing 0.01/2 unless it was clear why this was done. The application of a generous mark scheme in 06.2 meant that many students were able to access some marks despite quite significant misunderstanding as to what was required. Many failed to realise that the concentration of Ca(OH)2 in the saturated solution could be found directly from the solubility data given. Section B – Multiple Choice Questions Some students need to pay more careful attention to the instructions given at the start of Section B about how to complete the answers by shading in the lozenge, crossing out an incorrect answer and then circling a crossed out answer if it is then decided to be correct. There were again quite a few instances of crossed out and circled answers alongside shaded lozenges – which has to be treated as two answers and so marked incorrect. Overall, Section B proved to be rather accessible, with a mean score of 19.3 out of 30. Questions that were particularly accessible, with success rates of at least 80%, were 6, 8, 11, 14, 24, 28 and 34. The trickiest questions, where 40% or fewer students gave the correct response, were 13 and 31.

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  This question concerns calcium hydroxide, neutralisation and the pH of a saturated solution. Use Kw = 6.8 x 10^-15 mol^2 dm^-6 at the stated temperature where required.

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• AQA 7405 · June 2022 · Paper 1 · Q6 · 15 marks

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  Most students knew the answer to 06.1. In 06.2, most students knew the equation but did not mention electrons in their explanation; many referred to oxidation numbers. Of those who mentioned electrons in the explanation, some stated that chlorine gained electrons but then thought oxygen or hydrogen lost electrons. Many students stated that a brown or purple gas was given off in 06.3 and many students did not give an ionic equation; 33% of students scored both marks. In 06.4, most students knew the role of sulfuric acid. The equations were not well known, however, and some students gave half-equations. Several students gave a correct equation for sulfur rather than a gas containing sulfur. Students found 06.5 challenging; 10% scored all the marks in the question. Carbon dioxide was scored more often than the other marks. Many students could not give the equation and many thought the sodium halide was NaCl. 36% of students gained all three marks in 06.6. Many students gave structures with only one lone pair and explanations often stated that the lone pairs and bond pairs repelled each other without stating that the lone pair-lone pair repulsion was greater then the bond pair-bond pair repulsion. Common incorrect bond angles were 120o and 180o. About half the students gave a correct equation in question 06.7; common errors included the wrong formulas for titanium chloride and magnesium chloride and using Mg2 in the equation.

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  This question is about Group 7 elements and their compounds.

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• AQA 7405 · June 2022 · Paper 1 · Q7 · 12 marks

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  43% of students scored both marks in 07.1. More students gained the first mark for stating that light was absorbed; some did not score the second mark since they did not say that yellow light was reflected and a few stated that yellow light was emitted. 07.2 was answered well; some students gave the correct expression but then did not calculate correctly and a few did not rearrange the expression correctly. Answers to 07.3 were generally good with many students gaining 2 or 3 marks. Most students knew the shape in 07.4. In 07.5, 50% of students gave the correct equation; a common error was to have 2 Cl2 as a product. In 07.6, some students did not state that the solution was deep blue and many could not give a correct equation; many students gave complexes with six ammonia ligands and chlorine gas as a product. The correct formula was given, in 07.7, by 53% of students; the most common error was a 2+ charge on the complex.

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  Copper(II) complexes are coloured because d electrons move between split energy levels. In a reaction scheme, [CuCl4]2- reacts with excess water to form P, which reacts with Na4EDTA to form R. Separately, [CuCl4]2- reacts with excess aqueous ammonia to form Q.

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• AQA 7405 · June 2022 · Paper 3 · Q2 · 6 marks

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  This levels of response question was expected to be more accessible than some in the past but a lack of accuracy in describing the tests for the anions meant that Level 3 was rarely attained. The need to warm after adding NaOH when testing for NH4+ was often missing as was the need to acidify the BaCl2 when testing for sulfate. Many students also stopped their description of the test for Al3+ at the stage of forming a white precipitate and failed to mention that it redissolves in excess NaOH. The equations for the test for Al3+ should preferably have been those with the hydrated ion but the simpler versions were allowed from the point of view of indicative content. The equation for the redissolving of the solid was often shown as Al(H2O)3(OH)3 + OH– → Al(OH)4– + 3 H2O which implies a combination of ligand replacement and acid-base behaviour, but the product anion should be shown with two water ligands still present in this case. In stage 1 the empirical formula of NH28AlS2O20 was sometimes not actually shown even though the calculation was done. Some students seemed confused that, in this case, the empirical formula mass is equal to the relative formula mass and they thought that meant that x = 0. It was hoped that students would recognise that 1 × N meant one NH4+ and 2 × S meant two SO42– ions – from which the number of waters could be found.

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  Tschermigite is a hydrated, water-soluble mineral of relative formula mass 453.2, represented by M.xH2O where M contains all ions. Table 4 gives percentage composition by mass: N 3.09%, H 6.18%, Al 5.96%, S 14.16%, O 70.61%. Analysis indicates NH4+, Al3+ and SO4^2- ions.

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• AQA 7405 · June 2022 · Paper 3 · Q5 · 11 marks

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  05.1 was well answered by most students. In 05.2 many students either did not notice that there were six marks available or were not able to work out how to access them as many answers were very brief and generic. The question specifically asked for suitable equations to be included but these were often left out of answers. 05.3 saw the greatest evidence of confusion in the use of correct terminology. One of the key aspects of the topic on electrochemistry and electrode potentials is its reliance on conventional terms of reference. When referring to an electrode potential it is advised that students stick to the conventional form. e.g. Eo S2O82–/SO42– is greater than Eo Co3+/Co2+ Therefore any references to electrode potentials and half-cell equations should follow the convention of showing (or implying) a reduction in the forward direction. A commonly seen example of a lack of clarity was “The electrode potential for Co2+ is less than that for S2O82– but more than for iodide so it can oxidise one and reduce the other.” Apart from the reference to “The electrode potential for Co2+” being incorrect it is not clear from this what is oxidising or reducing what. This is a particular example of a situation where references to ‘it’ are likely to be ambiguous. Many answers were also difficult to mark because of carelessness with CO instead of Co and confusion between subscripts and superscripts e.g. CO3+ Section B – Multiple Choice Some students need to pay more careful attention to the instructions given at the start of Section B about how to complete the answers by shading in the lozenge, crossing out an incorrect answer and then circling a crossed-out answer if it is then decided to be correct. There were quite a few instances of crossed-out and circled answers alongside shaded lozenges – which has to be treated as two answers and so marked incorrect. Questions students found particularly straightforward, with correct answers from at least 80% of the cohort, were B32, B34, B18, B25, B23. Questions that proved to be particularly challenging were B10, B33, B28, B7.

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  This question concerns heterogeneous, auto- and homogeneous catalysis. Table 6 gives standard reduction potentials: S2O8^2- + 2e- -> 2SO4^2-, +2.01 V; Co3+ + e- -> Co2+, +1.82 V; I2 + 2e- -> 2I-, +0.54 V.

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• AQA 7405 · November 2021 · Paper 1 · Q3 · 12 marks

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  Some students omitted the state symbols from the equation or missed out one of the state symbols in an otherwise correct equation in question 03.1; some students incorrectly gave oxygen or sodium oxide as a product. Most students appreciated that the reaction would be more violent and that the glass might shatter. In question 03.2 some students started from P2O5 and some gave H3PO3 as a product. Many gave the pH to be 2 which wasn’t low enough to be given a mark. Answers to question 03.3 were generally quite good. The structure and bonding in silicon dioxide was generally well done but some students discussed forces between silicon dioxide molecules. The structure for phosphorus(V) oxide was less well known; the term molecular was given only in the best answers. Some students did not state that van der Waals’ forces were between molecules. Students found question 03.4 challenging. A range of answers was seen for the oxide; students who gave the correct formula of aluminium oxide often failed to give a correct balanced equation. Question 03.5 was answered well, though common errors included calcium hydroxide or an incorrect formula for magnesium hydroxide. Although many students gave the correct answer in question 03.6, chlorine and argon were common incorrect answers.

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  This question is about Period 3 elements and their compounds.

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• AQA 7405 · November 2021 · Paper 1 · Q4 · 25 marks

  25 marks

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  In question 04.1 most students stated that a heterogeneous catalyst is in a different phase to the reactants or described what a catalyst is or how it works. Students gave better answers for stage 2 than for stage 3; in stage 3 many students knew that active sites were blocked but did not state that this was poisoning and didn’t explain how the efficiency of the catalyst could be improved. Several students confused the efficiency of the catalyst with conditions needed to maximise the yield or to speed up the reaction. In question 04.2 most students stated that ions of the same charge would repel, but fewer stated that the activation energy would be high. The equations were not well known. In question 04.3 many students stated that zinc had a full d sub-shell or had 3d10 configuration but did not explain how that related to the oxidation states of zinc. Some students completed the calculation well in question 04.4; common errors included identifying the limiting reagent as iron and then using the amount of iron (in moles) in the calculation, or stating that hydrochloric acid was the limiting reagent but not realising that this then controlled the amount of hydrogen produced. Some students converted the temperature and pressure incorrectly. In question 04.5 the colour of iron(II) carbonate was well known; however, if iron carbonate was given in words the (II) was often missing. The colour and/or formula were generally known in question 04.6 but the equation was not answered well; if the species were correct the equation was often not balanced. Students found question 04.7 very challenging; students often discussed the charge-to-size ratio of the hydrated ions rather than the iron ions themselves. Students rarely gained the second mark but a few did state that more hydrogen ions were released and so gained the third mark.

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  This question is about iron, catalysis and iron ions in aqueous solution.

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• AQA 7405 · November 2020 · Paper 1 · Q3 · 7 marks

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  Students were about equally divided over whether the second ionisation energy would be higher or lower than that of aluminium in 03.1. There were many different answers in 03.2; argon, sulfur and chlorine were common wrong answers. In the equation the state symbols were often difficult to read and therefore ambiguous, so could not score the mark; sometimes the state symbols were missing. 03.3 was generally well answered. The increase in nuclear charge was well known but the similarity in shielding less so. Answers to 03.4 were well done. In 03.5, many students gave P2O5 instead of P4O10 as the product and some students did not attempt to balance the equation.

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  This question is about Period 3 trends and compounds.

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• AQA 7405 · November 2020 · Paper 1 · Q5 · 10 marks

  10 marks

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  The formula in 05.1 was correctly given by some students; the observation was well known. The equation was correct in only a small proportion of responses; students often failed to balance the equation and some failed to include water as a product. In 05.2, many students did not give the correct formula of the complex ion; common incorrect answers included Al(H2O)3(OH)3 or complex ions with positive charges. Some students incorrectly gave heat, reflux or concentrated alkali as the condition. Students who had the correct formula for the aluminate complex often did not balance the equation correctly or started from Al(H2O)3(OH)3. 05.3 proved challenging for many students. Complex ions were given with a number of EDTA ligands attached to aluminium or a mixture of EDTA ligands and water ligands; the charge was often incorrect even for an otherwise correct complex. Answers to 05.4 were not well done. Many students tried to give an equation involving a base other than water. Many students failed to explain their answers clearly enough to score marks; common errors included stating that aluminium or the aluminium complex has a high charge to size ratio. The bond that was weakened was not clearly stated and there was a failure to state that H+ ions are released which makes the solution acidic.

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  A reaction scheme starts with aqueous [Al(H2O)6]3+: Na2CO3 forms white precipitate B, NaOH forms a white precipitate that dissolves to colourless complex C, and Na4EDTA forms colourless complex A.

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• AQA 7405 · November 2020 · Paper 1 · Q9 · 9 marks

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  Most students knew there would be no reaction and therefore no visible observation in 09.1; the most common incorrect answer was a white precipitate. In 09.2, most students gave a correct observation and the correct equation was often seen; the role of the chloride ion was less well understood. Students found 09.3 more challenging; only a few students gave a correct equation. The explanation was not well done. Although some students gave the oxidation states correctly, they either failed to identify the oxidation or reduction processes or they gave them the wrong way round; many students thought hydrogen was the element being reduced or oxidised. In 09.4, many students incorrectly gave the colour of bromine as a brown solution and some students failed to gain marks by simply stating a colour but omitting to state that a solution formed. Many students were unable to write a correct ionic equation; some students gave an unbalanced ionic equation and some gave a full equation including sodium ions.

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  This question is about reactions of sodium halides.

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• AQA 7405 · November 2020 · Paper 3 · Q2 · 5 marks

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  Some answers to 02.1 were too vague, with alkalinity being explained for example by the statement that Na and Mg are both alkali metals. About two-thirds of students scored the first mark by mentioning hydroxide ions but the second mark for a mention of the difference in solubility of the hydroxides was often missing. It was difficult to understand why a significant minority of students tried to explain the difference in terms of sodium hydroxide having ‘two hydroxide ions’ by comparison with magnesium hydroxide having only one. 02.2 was answered correctly by about 40% of students. A common mistake was to use the wrong formula for phosphorus(V) oxide. Students were expected to use P4O10. In 02.3, the students who could give a correct answer for one of the equations could usually also give a second correct equation.

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  This question concerns the acid-base behaviour of Period 3 oxides and the catalytic oxidation of sulfur dioxide by vanadium(V) oxide.

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• AQA 7405 · November 2020 · Paper 3 · Q3 · 11 marks

  11 marks

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  In 03.1, a majority of students were able to score 2 or more marks but many answers were rather muddled. There was often a failure to appreciate that only certain wavelengths or frequencies of light are absorbed (to excite d sub-shell electrons) and that the colour seen is the result of the remaining unabsorbed wavelengths or frequencies of light being reflected or transmitted. Only 20% of students scored all three marks. The equation needed was often incorrect with E = hcλ seen quite frequently. Many students failed to convert 490 nm to metres (or did so incorrectly). 03.2 proved to be a very discriminating question. Students should now be familiar with the fact that this paper contains a significant proportion of marks related to the description of practical scenarios. The key to success with such questions is to be familiar with a wide range of practical techniques and to be able to write concise, accurate descriptions. For example, in this case, a reference to plotting a graph of absorbance vs concentration needs to be preceded by a reference to taking a series of measurements of the absorbances for differing concentrations and it was not enough to simply repeat the question by saying ‘use the graph to find the unknown concentration’. The examiners wanted to see a description of how the graph should be used. It was encouraging to see many correct answers to 03.4 which proved to be one of the easiest questions on the paper

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  An aqueous transition-metal complex is coloured. Its absorption maximum is at wavelength 490 nm. A calibration graph is used to determine the iron concentration in a 250 cm^3 solution, found to be 4.66 x 10^-3 mol dm^-3. Use h = 6.63 x 10^-34 J s, c = 3.00 x 10^8 m s^-1 and Ar(Fe) = 55.8.

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• AQA 7405 · November 2020 · Paper 3 · Q4 · 15 marks

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  In 04.1, as is often the case when a piece of syllabus recall is tested for the first time, relatively few students were able to recall the name of the process prevented by cisplatin during cell division. Students were expected to answer 04.2 by following the description in the question of a chloride ligand being replaced by a water molecule. In many cases students failed to appreciate that replacement of a negative chloride ion by a neutral water molecule would result in a net charge (of 1+) on the resulting complex. 04.3 proved to be very tricky for many students – perhaps due to the amount of reading needed to pick up on the clues in the question and to appreciate what was being asked. Students are clearly more familiar with questions asking them to show hydrogen bonding between base pairs because that is what many tried to do – and they ignored the need to include the complex in their diagram. Where the complex was included it was sometimes drawn as trans- rather than cis- and often without an indication that the bonds formed are co-ordinate bonds. The experiment in 04.4 was another example of a situation where a description of a (hopefully familiar) practical scenario was needed. Students found this very taxing and over half scored no marks. The question described measurements of concentration being taken at one-minute intervals and then asked for a description of graphical methods. Although there were several alternative possibilities, they all needed to start with a description of the need to plot a graph of these measured concentrations against time. The next stage needed to be some reference to a determination of rate from the concentration-time graph. When referring to a subsequent rate- concentration graph it is not enough to refer to a ‘straight line of constant gradient’ as that could also be a zero order line – a sketch graph was useful to some students here. The idea of constant half-lives is not on the syllabus, so this type of answer was not expected but was credited if seen. Most were able to complete the two simple calculator processes in 04.5 although some students seemed to try and arrive at the answers by using the successive differences between previous values in the table. However they failed to spot that the temperature differences were not all the same. Answers to 04.6 showed an improvement in graph skills with 70% of students scoring at least 3 marks. The negative scale confused some students and some forgot the need for at least 50% of the available space to be taken up by the plotted points. Most students then recognised that the gradient = Ea/R but some tried to solve the whole equation by inventing values for the ln terms. In some cases gradient calculations were the wrong way up or involved a misreading of the scales, e.g. 0.00341 was often seen as the value for ∆x instead of (0.00341 – 0.00310).

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  Cisplatin, cis-[Pt(NH3)2Cl2], is an anticancer drug. In aqueous solution it forms a species that bonds to adjacent guanine bases in DNA. Kinetic data for this process are analysed using concentration-time and Arrhenius plots.

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• AQA 7405 · June 2019 · Paper 1 · Q3 · 7 marks

  7 marks

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  [3.4] Amphoteric was well known, although the spelling was often incorrect.

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  Periodicity and Period 4 chemistry.

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• AQA 7405 · June 2019 · Paper 1 · Q4 · 14 marks

  14 marks

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  [4.3] Very few students could give a correct reducing agent in part 3; some suggested iodide ions but this is not a reagent.

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  Aqueous iron complexes and transition-metal stereoisomerism.

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• AQA 7405 · June 2019 · Paper 1 · Q5 · 13 marks

  13 marks

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  [5.1] Most students gave a correct equation in part 1 but the second mark was gained less frequently, with the role being defined incorrectly.

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  Reactions and tests involving Group 7 compounds.

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• AQA 7405 · June 2019 · Paper 1 · Q6 · 16 marks

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  [6.5] Students found this part challenging; most students did not recognise that the 3d sub-shell is full and therefore were unable to explain that d-d transitions are not possible and visible light is not absorbed.

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  Copper-alloy iodometric titration and related copper/iodine chemistry.

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• AQA 7405 · June 2019 · Paper 3 · Q5 · 6 marks

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  [5.5] Most students knew both these pieces of measuring apparatus.

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  A student determines the iron percentage in 680 mg of steel wire. All iron is converted to Fe2+, the solution is made to 100 cm^3, and 25.0 cm^3 portions are titrated with 0.0200 mol dm^-3 KMnO4. Titre results are 22.90, 22.70 and 22.60 cm^3.

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• AQA 7405 · June 2018 · Paper 1 · Q3 · 11 marks

  11 marks

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  [03.1] Nearly all students (91.1%) gained both marks here.

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  Entropy, Gibbs energy, heterogeneous catalysis and ammonia oxidation.

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• AQA 7405 · June 2018 · Paper 1 · Q5 · 15 marks

  15 marks

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  [05.3] 81.8% of students gave the correct reason here.

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  Strong acids, pH, Kw, buffers and Period 3 oxides.

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• AQA 7405 · June 2018 · Paper 1 · Q7 · 9 marks

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  [07.2] Only a small proportion of students (22.6%) could give a correct equation here; many did not include protonation of water, as implied in the question. A surprisingly high proportion of students (10.8%) failed even to attempt this question.

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  Aqueous aluminium chemistry with carbonate and hydroxide.

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• AQA 7405 · June 2018 · Paper 1 · Q9 · 7 marks

  7 marks

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  Vanadium Compounds and Ions

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  Vanadium redox potentials, complexes, decomposition and catalysis.

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• AQA 7405 · June 2017 · Paper 1 · Q1 · 8 marks

  8 marks

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  [01.3] Many students understood that the lattice was not perfectly ionic and had some covalent character. Linking the covalent character with stronger bonding eluded all but the strongest students.

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  Silver iodide lattice enthalpy, bonding and iodide testing.

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• AQA 7405 · June 2017 · Paper 1 · Q6 · 13 marks

  13 marks

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  [06.3] Most students answered this well, but “simple covalent” was a common incorrect, incomplete answer for sulfur dioxide.

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  Period 3 elements, oxides, structures and acid-base reactions.

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• AQA 7405 · June 2017 · Paper 1 · Q7 · 9 marks

  9 marks

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  [07.6] Students could often state that the electronic configuration was d10, but the effect on electron transitions was less well known. Students often failed to realise the importance of visible light being absorbed. Some students scored the mark by stating the complex was colourless.

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  Aqueous copper complexes and ligand substitution.

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• AQA 7405 · June 2017 · Paper 1 · Q8 · 6 marks

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  [08.2] Good answers to this question were rarely seen. Sodium hydroxide was the most common correct reagent, but students then failed to realise that an excess of sodium hydroxide was needed to distinguish between the two metal ions. A few students suggested sodium carbonate as the reagent but then failed to give full observations.

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  Single-reagent test-tube distinctions between aqueous ions.

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• AQA 7405 · June 2017 · Paper 1 · Q11 · 14 marks

  14 marks

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  [11.3] Many students struggled to draw the displayed formula; those who got close often omitted the carbon atoms from the ligand. Most students knew the bond angle, but some incorrectly gave 120° or 109.5°. The most common incorrect type of isomerism given was stereoisomerism or cis/trans.

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  Ethanedioate mixture titrations and an iron(III) ethanedioate complex.

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• AQA 7405 · sample1 · Paper 1 · Q10 · 10 marks

  10 marks

  10

  
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  Qualitative analysis table. Q: no change with acidified BaCl2, pale-cream precipitate with acidified AgNO3, white precipitates with NaOH and Na2CO3, no change with concentrated HCl. R: no change with acidified BaCl2, white precipitate with acidified AgNO3, white NaOH precipitate dissolving in excess, white carbonate precipitate with gas bubbles, no change with concentrated HCl. S: white precipitate with acidified BaCl2, no change with acidified AgNO3, brown precipitates with NaOH and Na2CO3 (carbonate also gives gas bubbles), and a yellow solution with concentrated HCl.

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