AQA GCSE Chemistry 8462: Energy changes – Study Guide

This guide teaches section 4.5 Energy changes of the AQA GCSE Chemistry (8462) specification, for teaching from September 2016 onwards and exams in 2018 onwards (version 1.1). It covers every point from 4.5.1.1 to 4.5.2.2: exothermic and endothermic reactions, reaction profiles, bond energy calculations, chemical cells and fuel cells. Energy changes is assessed on Paper 1 (topics 1 to 5), which is set at Foundation and Higher Tier. Content the specification marks (HT only) is labelled Higher tier only below. Required practical activity 4 belongs to this topic.

For quick recall, use the Energy changes revision notes. To test yourself, use the Energy changes practice questions. The course hub is AQA GCSE Chemistry, and the printable checklist lists every specification point. Activation energy comes back in topic 6, so read this alongside the rate and extent of chemical change study guide.

What this unit covers

Spec point	What you must be able to do	Tier
4.5.1.1 Energy transfer	Explain that energy is conserved; tell exothermic from endothermic by the temperature change of the surroundings; give examples; evaluate uses	Both
Required practical 4	Investigate the variables that affect temperature changes in reacting solutions	Both
4.5.1.2 Reaction profiles	Draw and use reaction profiles; explain activation energy	Both
4.5.1.3 Energy change of reactions	Explain energy changes in terms of bond breaking and bond making; calculate energy changes from bond energies supplied	Higher tier only
4.5.2.1 Cells and batteries	Describe simple cells, batteries, non-rechargeable and rechargeable cells; interpret reactivity data; evaluate cells	Both
4.5.2.2 Fuel cells	Describe the hydrogen fuel cell; evaluate it against rechargeable cells and batteries	Both
4.5.2.2 Fuel cells	Write the half equations for the electrode reactions in the hydrogen fuel cell	Higher tier only

One limit is worth knowing now. For 4.5.1.1 the specification says you are limited to measuring temperature change: calculating energy changes or ΔH from temperature data is not required.

4.5.1.1 Energy transfer during exothermic and endothermic reactions

Energy is conserved. The total amount of energy in the universe is the same after a reaction as before it. If a reaction transfers energy to the surroundings, the products must have less energy than the reactants, by exactly the amount transferred.

The “surroundings” means everything that is not the reacting chemicals: the water the reaction happens in, the beaker, the thermometer and the air.

	Exothermic	Endothermic
Energy	Transferred to the surroundings	Taken in from the surroundings
Temperature of surroundings	Increases	Decreases
Examples in the spec	Combustion, many oxidation reactions, neutralisation	Thermal decompositions, citric acid + sodium hydrogencarbonate
Everyday uses	Self-heating cans, hand warmers	Some sports injury packs

You decide which type a reaction is only from the temperature change of the surroundings. A thermometer in the solution measures the surroundings, so a rising reading means exothermic.

Worked example – classifying from data

A student records these temperatures.

Reaction	Start (°C)	End (°C)
A	21.0	29.5
B	20.5	16.0

• Reaction A: change = 29.5 − 21.0 = +8.5 °C. The surroundings got hotter, so A is exothermic.
• Reaction B: change = 16.0 − 20.5 = −4.5 °C. The surroundings got colder, so B is endothermic.

Evaluating uses

You may be given information about a product and asked to evaluate it. Weigh up points such as:

• how large the temperature change is and how long it lasts
• whether the product can be used once only or reused
• how safe the chemicals are if the pack leaks
• cost and convenience (no power supply needed)

A good evaluation gives points on both sides and ends with a judgement that follows from them.

Required practical 4 – temperature changes in reacting solutions

The specification asks you to investigate the variables that affect temperature changes in reacting solutions, for example acid plus metals, acid plus carbonates, neutralisations and displacement of metals.

A typical method:

1. Stand a polystyrene cup in a beaker so it does not tip over.
2. Measure a fixed volume of acid (for example 25 cm³) into the cup with a measuring cylinder.
3. Record the starting temperature with a thermometer.
4. Add the second reactant, stir, and record the highest (or lowest) temperature reached.
5. Work out the temperature change. Repeat and calculate a mean.

Variables. The independent variable is the one you change (for example the mass of metal powder or the concentration of acid). The dependent variable is the temperature change. Control variables include the volume of acid, the type of cup and the starting temperature.

Why polystyrene with a lid? Polystyrene is a poor conductor, and a lid cuts energy transfer to the air. Both make the measured temperature change closer to the true value.

Worked example – mean temperature change

Three repeats give temperature rises of 6.8 °C, 7.2 °C and 7.0 °C.

mean = (6.8 + 7.2 + 7.0) / 3 = 21.0 / 3 = 7.0 °C

If one repeat had been far from the others, you would leave it out as anomalous before taking the mean.

4.5.1.2 Reaction profiles

Reactions only happen when particles collide with sufficient energy. The minimum energy that particles must have to react is the activation energy.

A reaction profile (energy level diagram) shows:

• the energy of the reactants and the energy of the products, as horizontal levels
• a curved line rising from the reactants to a peak and falling to the products
• the activation energy: an arrow from the reactant level up to the top of the curve
• the overall energy change: an arrow from the reactant level to the product level

Exothermic                     Endothermic

      /\                              /\
     /  \                            /  \____ products
____/    \                          /
reactants \                  ______/
           \____ products    reactants

• Exothermic: products lower than reactants.
• Endothermic: products higher than reactants.

The activation energy arrow always starts at the reactants, whichever type of reaction it is.

Worked example – reading a profile

On a profile, the reactants are at 120 kJ, the top of the curve is at 310 kJ and the products are at 40 kJ.

• Activation energy = 310 − 120 = 190 kJ
• Overall energy change = 40 − 120 = −80 kJ. The products are lower than the reactants, so the reaction is exothermic.

4.5.1.3 The energy change of reactions (Higher tier only)

During a chemical reaction:

• energy must be supplied to break bonds in the reactants (endothermic step)
• energy is released when bonds form in the products (exothermic step)

overall energy change = energy needed to break bonds − energy released forming bonds

• If more energy is released forming bonds than is needed to break bonds, the reaction is exothermic and the answer is negative.
• If more energy is needed to break bonds than is released forming them, the reaction is endothermic and the answer is positive.

Bond energies are always supplied in the question. You do not need to learn them.

Worked example – hydrogen and chlorine

H–H + Cl–Cl → 2 H–Cl. Bond energies (kJ/mol): H–H = 436, Cl–Cl = 243, H–Cl = 432.

bonds broken: 436 + 243        = 679 kJ/mol
bonds formed: 2 × 432          = 864 kJ/mol
energy change: 679 − 864       = −185 kJ/mol

The answer is −185 kJ/mol. More energy is released forming the two H–Cl bonds than is needed to break the H–H and Cl–Cl bonds, so the reaction is exothermic.

Method in steps

1. Write the balanced equation and draw out every bond (for example CH₄ has four C–H bonds).
2. Multiply each bond energy by the number of those bonds, including the balancing numbers.
3. Add up bonds broken; add up bonds formed.
4. Subtract: broken − formed. Give the sign and the unit, kJ/mol.

4.5.2.1 Cells and batteries

Cells contain chemicals which react to produce electricity. A simple cell is two different metals in contact with an electrolyte. The voltage depends on several factors, including the type of electrode and the electrolyte.

• A battery is two or more cells connected in series, which gives a bigger voltage. Three 1.5 V cells in series give 3 × 1.5 = 4.5 V.
• In non-rechargeable cells and batteries, the reactions stop when one reactant is used up. Alkaline batteries are non-rechargeable.
• Rechargeable cells and batteries can be recharged because the reactions are reversed when an external electrical current is supplied.

Interpreting reactivity data

The further apart two metals are in reactivity, the bigger the voltage of the cell they make. Here is a set of voltages for cells with copper as one electrode.

Metal paired with copper	Voltage (V)
Magnesium	2.7
Zinc	1.1
Iron	0.8
Copper	0.0

Order of reactivity: magnesium > zinc > iron > copper. The more reactive the metal, the bigger the voltage. Two identical metals give 0 V, so a cell needs two different metals.

Predicting a new cell. For a magnesium and zinc cell, take the difference: 2.7 − 1.1 = 1.6 V (approximately). For zinc and iron: 1.1 − 0.8 = 0.3 V.

The specification says you do not need details of cells and batteries beyond those listed here.

4.5.2.2 Fuel cells

A fuel cell is supplied with a fuel from an external source (for example hydrogen) and with oxygen or air. The fuel is oxidised electrochemically inside the cell to produce a potential difference. In a hydrogen fuel cell the overall reaction is the oxidation of hydrogen to make water:

2H₂ + O₂ → 2H₂O

Evaluating hydrogen fuel cells against rechargeable cells

	Hydrogen fuel cell	Rechargeable cell or battery
Product	Water only	No product released while in use
Running out	Keeps working while fuel is supplied	Must be recharged, which takes time
Storage	Hydrogen is a gas, so it needs bulky or pressurised tanks	Stores energy in a compact unit
Lifetime	Not recharged, so no loss from repeated charging	Can only be recharged a limited number of times
Safety	Hydrogen is flammable and can form explosive mixtures with air	Can overheat if damaged
Source of fuel	Hydrogen is often made using fossil fuels or electricity	Charging uses mains electricity, which may come from fossil fuels
Disposal	Less toxic waste	Contains metal compounds that are harder to dispose of

In an evaluation, compare the two directly and give a conclusion that depends on the use (for example a bus depot versus a mobile phone).

Half equations (Higher tier only)

At each electrode, electrons are lost or gained. With an acidic electrolyte:

negative electrode:  2H₂ → 4H⁺ + 4e⁻
positive electrode:  O₂ + 4H⁺ + 4e⁻ → 2H₂O

Add the two and cancel the 4H⁺ and 4e⁻ to get 2H₂ + O₂ → 2H₂O. If the question uses an alkaline electrolyte, the equations are 2H₂ + 4OH⁻ → 4H₂O + 4e⁻ and O₂ + 2H₂O + 4e⁻ → 4OH⁻; the overall equation is the same. Hydrogen loses electrons, so it is oxidised.

Common errors

• Saying the solution “loses heat” in an endothermic reaction without saying the temperature of the surroundings decreases – the spec defines the types by this.
• Calculating ΔH from mass and temperature change. It is not required for 8462; describe the temperature change instead.
• Drawing the activation energy arrow from the products, or stopping it short of the peak.
• Drawing straight lines on a profile. The spec asks for a curved line.
• In bond energy sums, forgetting to multiply by the balancing number or by the number of identical bonds in a molecule (O=O is one bond; H₂O has two O–H bonds).
• Subtracting the wrong way round, then giving the wrong sign.
• Saying “energy is made” or “energy is used up”. Energy is transferred.
• Saying a battery is “one cell”. A battery is two or more cells in series.
• Writing half equations with unbalanced electrons.

Where to go next

• Recall and self-test: Energy changes revision notes
• Exam-style practice: Energy changes practice questions
• Next topic: The rate and extent of chemical change
• Find your weak spots: free 10-minute diagnostics

Official syllabus

AQA GCSE Chemistry (8462) specification, for teaching from September 2016 onwards, exams in 2018 onwards, version 1.1 (4 October 2019), published by AQA – section 4.5 Energy changes.