4.5 Energy changes — 8462 spec coverage

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 mark

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.

Tier 2 · Standard

3 marks

ORIGINAL

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.

Tier 3 · Hard

4 marks

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.

4.5.1.2

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 mark

ORIGINAL

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

Tier 2 · Standard

4 marks

ORIGINAL

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.

Tier 3 · Hard

5 marks

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.

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 mark

ORIGINAL

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

Tier 2 · Standard

4 marks

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.

Tier 3 · Hard

5 marks

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.

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

2 marks

ORIGINAL

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

Tier 2 · Standard

3 marks

ORIGINAL

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.

Tier 3 · Hard

5 marks

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.

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 mark

ORIGINAL

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

Tier 2 · Standard

4 marks

ORIGINAL

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

Tier 3 · Hard

6 marks

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.