4.5 Energy changes — 8462 revision notes

4.5.1.1

Energy transfer during exothermic and endothermic reactions

Notes

Evidence from your answers: none yet

Your confidence:

A self-report of how sure you feel. It does not measure mastery. Evidence from your answers reaches secure after the latest Tier 2/3 attempt is correct, with three correct distinct drills across at least two dates and two practice sources.

Explanation

An exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings increases; combustion, many oxidation reactions and neutralisation are examples. An endothermic reaction takes in energy from the surroundings, so the temperature of the surroundings decreases; thermal decomposition and some instant cold packs are examples.
Measure the initial temperature, mix the reactants, stir and record the highest or lowest temperature reached; compare temperature changes while keeping quantities and apparatus controlled.
Energy is conserved: a temperature rise does not mean energy was created.
A common error is to describe the reacting chemicals, rather than the surroundings, as getting hotter or colder.
A fair comparison needs insulated, identical apparatus and consistent reactant amounts so unwanted heat transfer does not dominate the result.

Worked example

A student compares two neutralisation reactions. For each test, the student uses the same cup and the same total volume of solution. Give three other features of the method that should be kept the same or carried out consistently so the temperature changes can be compared fairly.

1.Choose controls that could otherwise alter the measured temperature change: the initial temperature, solution concentrations, mixing or stirring, and the measuring instrument or rule for selecting the maximum temperature. Any three valid consistent features score.

Answer: Use solutions with the same starting temperature and stated concentrations, stir in the same way, and record the maximum temperature using the same thermometer or temperature probe.

Common mistakes

Don't fall into the trap of describing the reacting chemicals, rather than the surroundings, as getting hotter or colder.
Don't fall into the trap of using the final temperature alone instead of calculating and comparing the temperature change.

Exam tip

For required practical 4, state how temperature change is measured and identify the quantities and apparatus kept constant.

Tier 1 · Easy

ORIGINAL

A reaction mixture starts at

19.6\,^\circ\text{C}

and reaches

27.1\,^\circ\text{C}

. State whether the reaction is exothermic or endothermic.

[1 mark]

Total for this question: 1

Tier 2 · Standard

ORIGINAL

An instant cold pack falls from

21.0\,^{\circ}\text{C}

to

8.5\,^{\circ}\text{C}

when its chemicals mix. Classify the process and describe the direction of energy transfer.

[2 marks]

Total for this question: 2

Tier 3 · Hard

ORIGINAL

Two reusable hand warmers are tested from the same room temperature. Warmer P raises the temperature by

18\,^\circ\text{C}

for

11

minutes and costs

\pounds1.70

per use. Warmer Q raises it by

12\,^\circ\text{C}

for

34

minutes and costs

\pounds0.95

per use. Evaluate which warmer is more suitable for a walker who needs gentle heating for a

30

-minute journey.

[4 marks]

Total for this question: 4

4.5.1.2

Reaction profiles

Notes

Evidence from your answers: none yet

Your confidence:

A self-report of how sure you feel. It does not measure mastery. Evidence from your answers reaches secure after the latest Tier 2/3 attempt is correct, with three correct distinct drills across at least two dates and two practice sources.

Explanation

A reaction profile plots energy against progress of reaction; the curved line rises to a maximum before falling or rising to the products' energy level. Activation energy is the minimum energy that colliding particles must have for a reaction to occur, shown from the reactants' energy level to the peak.
Products below reactants indicate an exothermic reaction and a negative overall energy change; products above reactants indicate an endothermic reaction and a positive change.
A common error is to draw activation energy from the vertical axis or from the products.
For the forward reaction, measure it from the reactants' level to the peak.
The sign of the overall energy change follows products minus reactants, whereas the activation-energy arrow always rises to the peak.

An exothermic reaction profile with products below reactants and activation energy measured to the peak.

Worked example

A reaction profile places the reactants at $74\,\text{kJ mol}^{-1}$ , the peak at $139\,\text{kJ mol}^{-1}$ and the products at $102\,\text{kJ mol}^{-1}$ . Calculate the activation energy and the overall energy change, then identify whether the reaction is exothermic or endothermic.

1.The forward activation energy is peak minus reactants: $139-74=65\,\text{kJ mol}^{-1}$ . The overall change is products minus reactants: $102-74=+28\,\text{kJ mol}^{-1}$ . The positive change, with products above reactants, identifies an endothermic reaction.

Answer: Activation energy $=65\,\text{kJ mol}^{-1}$ ; overall energy change $=+28\,\text{kJ mol}^{-1}$ ; the reaction is endothermic.

Common mistakes

Don't fall into the trap of drawing activation energy from the vertical axis or from the products instead of from the reactants' level to the peak.
Don't fall into the trap of reversing exothermic and endothermic profiles by ignoring the relative product and reactant energy levels.

Exam tip

Label reactants, products, activation energy and overall energy change; activation energy starts at the reactant level.

Tier 1 · Easy

ORIGINAL

On a reaction profile, the products are at a lower energy level than the reactants. Identify the type of reaction.

[1 mark]

Total for this question: 1

Tier 2 · Standard

ORIGINAL

Profile X has products below its reactants. Profile Y has products above its reactants. Identify the reaction type shown by each profile.

[2 marks]

Total for this question: 2

Tier 3 · Hard

ORIGINAL

The reactants, peak and products on a reaction profile have relative energies of

181

,

337

and

126\,\text{kJ mol}^{-1}

respectively. Determine the forward activation energy, the reverse activation energy and the overall energy change for the forward reaction.

[5 marks]

Total for this question: 5

4.5.1.3

The energy change of reactions (HT only)

Notes

Evidence from your answers: none yet

Your confidence:

A self-report of how sure you feel. It does not measure mastery. Evidence from your answers reaches secure after the latest Tier 2/3 attempt is correct, with three correct distinct drills across at least two dates and two practice sources.

Explanation

Higher tier: breaking bonds in reactants requires energy, whereas forming bonds in products releases energy. Calculate the overall energy change using $\Delta E=\sum E(\text{bonds broken})-\sum E(\text{bonds formed})$ and include every bond shown by the balanced equation.
If bond formation releases more energy than bond breaking requires, $\Delta E$ is negative and the reaction is exothermic; the reverse balance gives a positive, endothermic change.
A common error is to reverse the subtraction or count molecules instead of bonds.
Multiply each bond energy by the number of that bond broken or formed.
Higher tier: a negative result means more energy is released during bond formation than is supplied for bond breaking.

Worked example

Higher tier: use bond energies H-H $=436$ , Cl-Cl $=243$ and H-Cl $=431\,\text{kJ mol}^{-1}$ to calculate the energy change for $\mathrm{H_2+Cl_2\rightarrow2HCl}$ .

1.Bonds broken: one H-H and one Cl-Cl, so $436+243=679\,\text{kJ mol}^{-1}$ .
2.Bonds formed: two H-Cl, so $2\times431=862\,\text{kJ mol}^{-1}$ .
3.Calculate $\Delta E=679-862=-183\,\text{kJ mol}^{-1}$ .

Answer: $-183\,\text{kJ mol}^{-1}$ ; the reaction is exothermic.

Common mistakes

Don't fall into the trap of reversing the subtraction or counting molecules instead of every bond broken and formed.
Don't fall into the trap of using the number of molecules as the bond count without multiplying by every bond within each molecule.

Exam tip

Higher tier: tabulate every bond broken and formed, total each column, then calculate broken minus formed with the sign.

Tier 1 · Easy

ORIGINAL

State whether energy is taken in or released when a bond is broken.

[1 mark]

Total for this question: 1

Tier 2 · Standard

ORIGINAL

Use the equation

\mathrm{CH_4+2O_2\rightarrow CO_2+2H_2O}

and these bond energies in

\text{kJ mol}^{-1}

:

\mathrm{C-H}=413

,

\mathrm{O=O}=498

,

\mathrm{C=O}

in carbon dioxide

=805

, and

\mathrm{O-H}=464

. Calculate the overall energy change.

[4 marks]

Total for this question: 4

Tier 3 · Hard

ORIGINAL

For

\mathrm{N_2+3H_2\rightarrow2NH_3}

, the overall energy change is

-92\,\text{kJ mol}^{-1}

. The bond energies are

\mathrm{N\equiv N}=945\,\text{kJ mol}^{-1}

and

\mathrm{H-H}=436\,\text{kJ mol}^{-1}

. Calculate the mean

\mathrm{N-H}

bond energy.

[5 marks]

Total for this question: 5

4.5.2.1

Cells and batteries (chemistry only)

Notes

Evidence from your answers: none yet

Your confidence:

A self-report of how sure you feel. It does not measure mastery. Evidence from your answers reaches secure after the latest Tier 2/3 attempt is correct, with three correct distinct drills across at least two dates and two practice sources.

Explanation

A simple cell uses two different metals in contact with an electrolyte; chemical reactions transfer energy electrically and produce a potential difference. The voltage depends on the electrode materials and the electrolyte.
Data about relative metal reactivity can be used to compare or predict cell voltages. Cells connected in series have their voltages added; a battery contains two or more cells connected together in series to provide a greater voltage.
Non-rechargeable cells stop when a reactant is used up.
Rechargeable cells use an external current to reverse the reactions; a common error is to claim that recharging creates new reactants from nothing.
A larger reactivity difference between suitable electrodes generally produces a larger potential difference, but the electrolyte also matters.

Worked example

A cell made from zinc and copper produces $1.08\,\text{V}$ . Three identical cells are connected in series. Calculate the battery voltage and explain why copper-copper electrodes would not make the same cell.

1.Series cell voltages add, so $V=3\times1.08=3.24\,\text{V}$ . A simple cell requires different electrode materials; using copper for both removes that difference and would not reproduce the zinc-copper potential difference.

Answer: $3.24\,\text{V}$ ; identical copper electrodes do not provide the required difference between electrode materials.

Common mistakes

Don't fall into the trap of claiming that recharging creates new reactants from nothing instead of reversing the chemical reactions.
Don't fall into the trap of assuming the more reactive metal must always be labelled positive without using the stated cell arrangement or data.

Exam tip

For a series battery, add cell voltages with their directions; for evaluation, link lifetime and recharging to the chemical reactions.

Tier 1 · Easy

ORIGINAL

State the two essential electrode features needed to make a simple chemical cell with an electrolyte.

[2 marks]

Total for this question: 2

Tier 2 · Standard

ORIGINAL

Simple cells using the same electrolyte give these voltages: zinc-copper

1.10\,\text{V}

, zinc-iron

0.60\,\text{V}

and iron-copper

0.50\,\text{V}

. Choose the electrode pair for the largest voltage and explain what the data suggest about electrode reactivity.

[3 marks]

Total for this question: 3

Tier 3 · Hard

ORIGINAL

A torch needs at least

3.6\,\text{V}

. Cell P is non-rechargeable, gives

1.5\,\text{V}

and costs

\pounds0.60

. Cell Q is rechargeable, gives

1.2\,\text{V}

, costs

\pounds3.50

and can provide

400

evening-use cycles. Evaluate which type is more suitable for a torch used every evening. Assume each new P cell lasts one evening.

[5 marks]

Total for this question: 5

4.5.2.2

Fuel cells (chemistry only)

Notes

Evidence from your answers: none yet

Your confidence:

A self-report of how sure you feel. It does not measure mastery. Evidence from your answers reaches secure after the latest Tier 2/3 attempt is correct, with three correct distinct drills across at least two dates and two practice sources.

Explanation

A fuel cell receives a continuous external supply of fuel and oxygen or air; the fuel is oxidised electrochemically to produce a potential difference. In a hydrogen fuel cell, hydrogen is oxidised and the overall reaction forms water: $\mathrm{2H_2+O_2\rightarrow2H_2O}$ .
Hydrogen fuel cells can operate while reactants are supplied, whereas rechargeable cells store reactants and must be recharged; comparisons should include storage, refuelling, lifetime and environmental effects.
Higher tier: alkaline-cell half-equations may be written $\mathrm{2H_2+4OH^-\rightarrow4H_2O+4e^-}$ and $\mathrm{O_2+2H_2O+4e^-\rightarrow4OH^-}$ ; atoms and charge must balance.
Do not call hydrogen automatically pollution-free without considering its production.
Higher tier: the electrode half-equations must cancel electrons and hydroxide ions to give the overall formation of water.

Worked example

Give two differences between a hydrogen fuel cell and a rechargeable cell when each is used to power a vehicle.

1.Make paired comparisons rather than listing isolated facts. Contrast external fuel supply and refuelling with stored reactants and electrical recharging, then give a valid consequence such as operating emissions, infrastructure, mass or recharge time.

Answer: A fuel cell needs continuing supplies of hydrogen and oxygen and can be refuelled, whereas a rechargeable cell stores its reactants and needs an external current to reverse its reactions. The fuel cell produces water during use, but the overall environmental impact depends on hydrogen production and storage.

Common mistakes

Don't fall into the trap of calling hydrogen automatically pollution-free without considering the source of hydrogen and energy used in its production.
Don't fall into the trap of treating a fuel cell as a rechargeable battery that stores a fixed quantity of reactants inside.

Exam tip

Higher tier: balance atoms and total charge in each alkaline hydrogen-fuel-cell half-equation before combining them.

Tier 1 · Easy

ORIGINAL

Hydrogen and oxygen are supplied to a fuel cell. Name the substance formed.

[1 mark]

Total for this question: 1

Tier 2 · Standard

ORIGINAL

A hydrogen fuel-cell vehicle produces only water at the point of use. Explain why this does not prove that using the vehicle has no environmental impact.

[3 marks]

Total for this question: 3

Tier 3 · Hard

ORIGINAL

A hydrogen system stores

118\,\text{MJ}

per kilogram of hydrogen and delivers

52\%

of this as useful electrical energy. A rechargeable battery stores

0.90\,\text{MJ}

per kilogram and delivers

84\%

usefully. Evaluate the two systems for a long-distance vehicle. Include calculated useful energies per kilogram and one environmental limitation of hydrogen.

[6 marks]

Total for this question: 6

Energy transfer during exothermic and endothermic reactions

Tier 1 · Easy

Tier 2 · Standard

Tier 3 · Hard

Reaction profiles

Tier 1 · Easy

Tier 2 · Standard

Tier 3 · Hard

The energy change of reactions (HT only)

Tier 1 · Easy

Tier 2 · Standard

Tier 3 · Hard

Cells and batteries (chemistry only)

Tier 1 · Easy

Tier 2 · Standard

Tier 3 · Hard

Fuel cells (chemistry only)

Tier 1 · Easy

Tier 2 · Standard

Tier 3 · Hard

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