Chemical changes

Explanation

• A metal reacting with oxygen forms a metal oxide; the metal is oxidised because it gains oxygen.
• Reduction is the loss of oxygen from a substance, while oxidation is the gain of oxygen.
• For example, heating copper in oxygen forms copper oxide: $2\mathrm{Cu}+\mathrm{O_2}\rightarrow2\mathrm{CuO}$.
• Do not decide oxidation from the presence of oxygen alone: track whether oxygen is gained or lost by the named substance.
• Exam answers should link the observed product or property to the relevant particles, electron transfer or position in the reactivity series.

Explanation

• A useful order is potassium, sodium, lithium, calcium, magnesium, carbon, zinc, iron, hydrogen and copper, from more reactive to less reactive. Metal reactivity is linked to how readily its atoms form positive ions; more reactive metals form positive ions more readily.
• A more reactive metal displaces a less reactive metal from its compound, so displacement evidence can establish an order.
• At room temperature, potassium, sodium and lithium react rapidly with cold water, calcium less vigorously and magnesium very slowly; zinc, iron and copper do not.
• Metals above hydrogen react with dilute acids, increasingly slowly down to iron, while copper does not; reactions with steam are outside this point.
• Exam answers should link the observed product or property to the relevant particles, electron transfer or position in the reactivity series.

Explanation

• Very unreactive metals can occur native, but most metals occur as compounds and must be extracted by chemical reactions.
• A metal below carbon in the reactivity series can be extracted from its oxide by reduction with carbon.
• In oxygen-transfer language, the metal oxide is reduced because it loses oxygen; carbon is oxidised because it gains oxygen.
• Use supplied evidence to evaluate an extraction route; detailed industrial processes beyond reduction of oxides with carbon are not required here.
• Exam answers should link the observed product or property to the relevant particles, electron transfer or position in the reactivity series.

Explanation

• Higher tier: oxidation is loss of electrons and reduction is gain of electrons: OIL RIG. A half equation shows electrons explicitly and must balance both atoms and total charge.
• Add state symbols when the question asks for them; they can be part of the mark for a complete ionic equation.
• In a metal displacement, the more reactive metal loses electrons while the displaced metal ions gain electrons.
• Do not identify redox from charge signs alone: compare the same species before and after the reaction and track electron transfer.
• Exam answers should link the observed product or property to the relevant particles, electron transfer or position in the reactivity series.

Explanation

• Acids react with magnesium, zinc and iron to produce a salt and hydrogen gas. Hydrochloric acid forms chloride salts, while sulfuric acid forms sulfate salts.
• For example, $\mathrm{Zn}+2\mathrm{HCl}\rightarrow\mathrm{ZnCl_2}+\mathrm{H_2}$; the metal has replaced hydrogen from the acid.
• Do not add water as a product: acid plus metal gives salt plus hydrogen.
• Higher tier: metal atoms lose electrons and are oxidised, while hydrogen ions gain electrons and are reduced; explain these redox reactions in terms of electron transfer and identify both species.
• Exam answers should link the observed product or property to the relevant particles, electron transfer or position in the reactivity series.

Explanation

• An acid and an alkali or base form a salt and water; an acid and a metal carbonate form a salt, water and carbon dioxide.
• Hydrochloric acid makes chlorides, nitric acid makes nitrates and sulfuric acid makes sulfates.
• The positive ion in the alkali, base or carbonate supplies the metal part of the salt.
• A common error is to choose the salt only from the acid: both the acid's negative ion and the other reactant's positive ion are needed.
• Exam answers should link the observed product or property to the relevant particles, electron transfer or position in the reactivity series.

Explanation

• Warm dilute acid gently, then add an insoluble metal oxide or carbonate in small portions until some solid remains unreacted.
• Filter to remove the excess insoluble solid; the filtrate is the salt solution.
• Use a water bath or electric heater to evaporate some water, allow the concentrated solution to cool and crystallise, then separate and dry the crystals.
• Do not evaporate all the water with strong heating, because this can spit, lose product or leave an impure powder rather than good crystals.
• Exam answers should link the observed product or property to the relevant particles, electron transfer or position in the reactivity series.

Explanation

• The pH scale runs from $0$ to $14$: acids have pH below $7$, neutral solutions have pH $7$, and alkalis have pH above $7$. Universal or wide-range indicator gives an approximate pH from colour; a calibrated pH probe gives a numerical reading.
• Acids in water produce $\mathrm{H^+}$ ions, while aqueous alkalis contain $\mathrm{OH^-}$ ions.
• Neutralisation is $\mathrm{H^+(aq)}+\mathrm{OH^-(aq)}\rightarrow\mathrm{H_2O(l)}$; include these state symbols when the stem requests them.
• Equal volumes are not necessarily neutral if the reacting amounts differ.
• Exam answers should link the observed product or property to the relevant particles, electron transfer or position in the reactivity series.

Explanation

• Use a volumetric pipette to transfer a fixed volume to a conical flask and a burette to deliver the other solution accurately.
• Add a suitable indicator, approach the end point dropwise while swirling, and record the burette difference as the titre.
• Repeat until concordant titres are obtained, then calculate a mean from concordant results rather than including a rough value.
• Higher tier: use the balanced equation and volumes in $\mathrm{dm^3}$ to calculate chemical quantities and determine concentrations in $\mathrm{mol\,dm^{-3}}$ and $\mathrm{g\,dm^{-3}}$ from titration data.
• Exam answers should link the observed product or property to the relevant particles, electron transfer or position in the reactivity series.

Explanation

• Higher tier: a strong acid ionises completely in water, whereas a weak acid ionises only partially.
• Hydrochloric, nitric and sulfuric acids are strong; ethanoic, citric and carbonic acids are weak.
• Strength describes degree of ionisation, while concentration describes the amount of solute per volume; either a strong or weak acid may be dilute.
• For whole-number pH values, a decrease of one pH unit means a tenfold increase in hydrogen ion concentration, not an increase of one unit.
• Exam answers should link the observed product or property to the relevant particles, electron transfer or position in the reactivity series.

Explanation

• An electrolyte is a molten ionic compound or ionic solution whose mobile ions allow it to conduct electricity. Positive ions move to the negative cathode, while negative ions move to the positive anode.
• When ions are discharged at the electrodes, they form elements; the electric current drives this decomposition process.
• In the electrolyte, charge is carried by moving ions rather than free electrons; a solid ionic compound does not conduct because its ions are fixed.
• Higher tier: write, complete and balance half equations for electrode reactions.
• Exam answers should link the observed product or property to the relevant particles, electron transfer or position in the reactivity series.

Explanation

• A molten binary ionic compound contains only its metal ions and non-metal ions, so its products are predictable from those ions. The metal forms at the negative cathode and the non-metal forms at the positive anode when inert electrodes are used.
• For molten lead bromide, lead forms at the cathode and bromine forms at the anode.
• Do not apply the aqueous-solution rules to a molten compound: there is no water supplying competing hydrogen or hydroxide ions.
• Higher tier: write, complete and balance half equations for electrode reactions.
• Exam answers should link the observed product or property to the relevant particles, electron transfer or position in the reactivity series.

Explanation

• Electrolysis extracts metals that are too reactive for carbon reduction or that react with carbon. Extraction uses much energy because the ionic compound must be molten and an electric current must be maintained.
• Aluminium is extracted from a molten mixture of aluminium oxide and cryolite; the mixture has a lower melting point than pure aluminium oxide.
• Oxygen produced at the carbon anode reacts with carbon to form carbon dioxide, so the positive electrodes wear away and must be replaced.
• Higher tier: write, complete and balance half equations for electrode reactions.
• Exam answers should link the observed product or property to the relevant particles, electron transfer or position in the reactivity series.

Explanation

• An aqueous electrolyte contains ions from the dissolved compound as well as $\mathrm{H^+}$ and $\mathrm{OH^-}$ originating from water. At the cathode, hydrogen forms if the metal is more reactive than hydrogen; otherwise the metal forms.
• At the anode, a halide ion forms its halogen; if no halide is present, oxygen forms.
• Apply both electrode rules separately and assume inert electrodes unless told otherwise; do not simply name the elements in the solute.
• Higher tier: write, complete and balance half equations for electrode reactions.
• Exam answers should link the observed product or property to the relevant particles, electron transfer or position in the reactivity series.

Explanation

• Higher tier: at the cathode, positive ions gain electrons, so cathode reactions are reductions.
• At the anode, negative ions lose electrons, so anode reactions are oxidations.
• Balance a half equation by conserving atoms and total charge; electrons go on the side needed to balance charge.
• Check electron direction: electrons are reactants in a reduction half equation and products in an oxidation half equation.
• Exam answers should link the observed product or property to the relevant particles, electron transfer or position in the reactivity series.