4.5 Energy changes — printable coverage pack

4.5.1.1 · Energy transfer during exothermic and endothermic reactions

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.

Tier 1 · Easy

1. 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]
Answer
- Exothermic.
Method: The surroundings have warmed by $27.1-19.6=7.5\,^\circ\text{C}$ . A reaction that transfers energy to the surroundings is exothermic.

Tier 2 · Standard

1. 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.[3 marks]
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.
Method: 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.

Tier 3 · Hard

1. 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]
Answer
- Warmer Q is more suitable because its heating lasts beyond $30$ minutes and it costs less per use, although P gives the larger temperature rise.
Method: Use every relevant comparison. P heats more strongly because $18>12$ , but its $11$ -minute duration is too short. Q lasts $34$ minutes, which covers the journey, and saves $\pounds1.70-\pounds0.95=\pounds0.75$ per use. Therefore Q best matches the stated need, while the lower temperature rise is its disadvantage.

4.5.1.2 · Reaction profiles

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.

Tier 1 · Easy

1. On a reaction profile, the products are at a lower energy level than the reactants. Identify the type of reaction.[1 mark]
Answer
- Exothermic.
Method: Products with less energy than the reactants mean that energy has been transferred to the surroundings, so the reaction is exothermic.

Tier 2 · Standard

1. 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.[4 marks]
Answer
- Activation energy $=65\,\text{kJ mol}^{-1}$ ; overall energy change $=+28\,\text{kJ mol}^{-1}$ ; the reaction is endothermic.
Method: 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.

Tier 3 · Hard

1. 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]
Answer
- $E_{\text{a, forward}}=156\,\text{kJ mol}^{-1}$
- $E_{\text{a, reverse}}=211\,\text{kJ mol}^{-1}$
- Overall energy change $=-55\,\text{kJ mol}^{-1}$
Method: For the forward reaction, $E_{\text{a}}=337-181=156\,\text{kJ mol}^{-1}$ . For the reverse reaction, start from the products: $E_{\text{a}}=337-126=211\,\text{kJ mol}^{-1}$ . The forward overall change is $126-181=-55\,\text{kJ mol}^{-1}$ , so the forward reaction is exothermic.

4.5.1.3 · The energy change of reactions (HT only)

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.

Tier 1 · Easy

1. State whether energy is taken in or released when a bond is broken.[1 mark]
Answer
- Energy is taken in when a bond is broken.
Method: Bond breaking requires an input of energy. Bond formation, not bond breaking, releases energy.

Tier 2 · Standard

1. 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]
Answer
- $-818\,\text{kJ mol}^{-1}$
Method: Breaking four $\mathrm{C-H}$ bonds and two $\mathrm{O=O}$ bonds requires $4(413)+2(498)=2648\,\text{kJ mol}^{-1}$ . Forming two $\mathrm{C=O}$ bonds and four $\mathrm{O-H}$ bonds releases $2(805)+4(464)=3466\,\text{kJ mol}^{-1}$ . Therefore $\Delta E=2648-3466=-818\,\text{kJ mol}^{-1}$ .

Tier 3 · Hard

1. 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]
Answer
- $391\,\text{kJ mol}^{-1}$ to three significant figures
Method: Breaking one $\mathrm{N\equiv N}$ bond and three $\mathrm{H-H}$ bonds requires $945+3(436)=2253\,\text{kJ mol}^{-1}$ . Six $\mathrm{N-H}$ bonds form; let their mean energy be $x$ . Then $-92=2253-6x$ , so $6x=2345$ and $x=390.833\ldots\,\text{kJ mol}^{-1}$ . To three significant figures, the mean bond energy is $391\,\text{kJ mol}^{-1}$ .

4.5.2.1 · Cells and batteries (chemistry only)

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.

Tier 1 · Easy

1. State the two essential electrode features needed to make a simple chemical cell with an electrolyte.[2 marks]
Answer
- Two electrodes made from different metals, both in contact with the electrolyte.
Method: Award one idea for using two metal electrodes and one for the metals being different. The electrolyte provides the ionic contact needed by the cell.

Tier 2 · Standard

1. 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.[3 marks]
Answer
- $3.24\,\text{V}$ ; identical copper electrodes do not provide the required difference between electrode materials.
Method: 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.

Tier 3 · Hard

1. 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]
Answer
- Three P cells give $4.5\,\text{V}$ and cost $\pounds1.80$ per replacement; three Q cells give exactly $3.6\,\text{V}$ and cost $\pounds10.50$ initially but can be reused for up to $400$ evenings. Q is more suitable for frequent long-term use, provided charging is available.
Method: P needs three cells because $3(1.5)=4.5\,\text{V}$ , costing $3(0.60)=\pounds1.80$ each evening. Q also needs three because $3(1.2)=3.6\,\text{V}$ , with an initial cost of $3(3.50)=\pounds10.50$ . Repeated use makes Q much cheaper per evening and creates less discarded-cell waste, but P has the advantage of no charger and a larger voltage margin. For daily use, Q is the justified choice.

4.5.2.2 · Fuel cells (chemistry only)

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.
For 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.

Tier 1 · Easy

1. Hydrogen and oxygen are supplied to a fuel cell. Name the substance formed.[1 mark]
Answer
- Water.
Method: Hydrogen is oxidised by oxygen in the fuel cell, giving water as the overall reaction product.

Tier 2 · Standard

1. Give two differences between a hydrogen fuel cell and a rechargeable cell when each is used to power a vehicle.[4 marks]
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.
Method: 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.

Tier 3 · Hard

1. 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]
Answer
- Hydrogen delivers about $61\,\text{MJ kg}^{-1}$ and the battery about $0.76\,\text{MJ kg}^{-1}$ . Hydrogen offers much greater useful energy per kilogram, but storage is difficult and its production may cause emissions or require substantial energy. A justified choice depends on range, storage and the hydrogen source.
Method: For hydrogen, useful energy is $118\times0.52=61.36\,\text{MJ kg}^{-1}$ . For the battery it is $0.90\times0.84=0.756\,\text{MJ kg}^{-1}$ . Hydrogen therefore has the large mass-specific energy advantage needed for long range. Balance this against bulky or high-pressure storage, refuelling infrastructure and the fact that producing hydrogen can use fossil fuels or considerable electrical energy. State a conclusion linked to those data rather than claiming either system is always best.