Energy Changes: Exam Practice

This lesson brings together all the key concepts from the AQA GCSE Chemistry Energy Changes topic and prepares you for the exam. You will review the main content, practise applying your knowledge to exam-style questions, learn command words, and develop strategies for answering different question types effectively.

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Key Concepts Review

Before tackling exam questions, make sure you are confident with all the core content:

Topic	Key Points
Exothermic reactions	Transfer energy to surroundings, temperature rises. Examples: combustion, neutralisation, oxidation
Endothermic reactions	Take in energy from surroundings, temperature falls. Examples: thermal decomposition, dissolving ammonium nitrate, photosynthesis
Reaction profiles	Diagrams showing energy of reactants and products; exothermic = products lower, endothermic = products higher
Activation energy (Ea)	Minimum energy needed for a reaction to occur; shown as height to peak on reaction profile
Catalysts	Lower activation energy, provide alternative pathway, not used up, do not change overall energy change
Bond energy calculations [H]	Energy change = bonds broken - bonds formed; negative = exothermic, positive = endothermic
Cells and batteries	Chemical cells produce voltage from two different metals in electrolyte; batteries = cells in series
Fuel cells [H]	Convert hydrogen and oxygen to water and electricity; clean at point of use
Required practical	Measuring temperature changes in neutralisation; polystyrene cup, variables, sources of error

Exam Tip: Make a revision checklist from the table above. Tick off each topic only when you can explain it without looking at your notes. Any topic you cannot explain needs more revision.

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AQA Command Words

Understanding what the exam question is asking is just as important as knowing the content. AQA uses specific command words:

Command Word	What It Means	What You Should Do
State	Give a short, factual answer	No explanation needed — just the fact
Name	Identify something	Give the correct name — one or two words
Describe	Say what happens	Give a detailed account of what occurs, step by step
Explain	Say why something happens	Give reasons — link cause to effect using scientific knowledge
Suggest	Apply your knowledge to an unfamiliar context	There may be more than one correct answer — use logic and science
Compare	Identify similarities and/or differences	Always mention BOTH things being compared
Evaluate	Consider all factors and reach a judgement	Give advantages AND disadvantages, then a conclusion
Calculate	Work out a numerical answer	Show all working — even if the final answer is wrong, method marks are available
Draw	Produce a diagram or graph	Label clearly and accurately; use a ruler for straight lines
Determine	Use data to reach a conclusion	Analyse the given data and state what it shows

Exam Tip: Underlining the command word in each question is a simple but powerful exam technique. It focuses your mind on exactly what type of answer is required and prevents you from writing too much or too little.

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Common Question Types and How to Answer Them

Type 1: Identify Exothermic or Endothermic

Question pattern: "A student mixes two chemicals and the temperature of the solution decreases. Is this reaction exothermic or endothermic? Explain your answer."

Model answer structure:

1. State the type: "The reaction is endothermic."
2. Define the term: "An endothermic reaction takes in energy from the surroundings."
3. Link to evidence: "The temperature decrease shows that energy has been absorbed from the solution, causing it to cool down."

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Type 2: Draw and Label a Reaction Profile

Question pattern: "Draw and label a reaction profile for an exothermic reaction."

Model answer steps:

1. Draw axes (Energy vs Progress of reaction).
2. Draw reactants line on the left (higher).
3. Draw products line on the right (lower).
4. Draw a smooth curve rising to a peak and falling to products.
5. Label: Reactants, Products, Activation energy (Ea), Overall energy change.

graph TD
    subgraph "Exothermic Reaction Profile Checklist"
        A["1. Axes labelled correctly"]
        B["2. Reactants labelled (left, higher)"]
        C["3. Products labelled (right, lower)"]
        D["4. Smooth curve with peak"]
        E["5. Ea arrow from reactants to peak"]
        F["6. Energy change arrow from reactants to products (downward)"]
    end

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Type 3: Bond Energy Calculation [H]

Question pattern: "Use the bond energies provided to calculate the overall energy change for the reaction. State whether the reaction is exothermic or endothermic."

Model answer structure:

1. List all bonds broken in reactants and their energies.
2. Calculate total energy to break bonds.
3. List all bonds formed in products and their energies.
4. Calculate total energy released forming bonds.
5. Calculate: Energy change = bonds broken - bonds formed.
6. State whether exothermic (negative) or endothermic (positive).

Example:

H2 + Cl2 --> 2HCl

Bond energies: H-H = 436 kJ/mol, Cl-Cl = 242 kJ/mol, H-Cl = 431 kJ/mol

Bonds broken: 1 x H-H + 1 x Cl-Cl = 436 + 242 = 678 kJ

Bonds formed: 2 x H-Cl = 2 x 431 = 862 kJ

Energy change = 678 - 862 = -184 kJ/mol

The reaction is exothermic because the value is negative (more energy is released forming bonds than is needed to break bonds).

Exam Tip: In bond energy calculations, show EVERY step of your working. Even if your arithmetic is wrong, you will gain marks for the correct method. Never skip steps.

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Type 4: Required Practical Questions

Question pattern: "Describe how you would investigate the temperature change when hydrochloric acid is added to sodium hydroxide solution."

Model answer must include:

1. Apparatus: polystyrene cup, thermometer, measuring cylinder.
2. Method: measure fixed volume of NaOH, record initial temperature, add measured portions of HCl, stir, record temperature.
3. Variables: independent (volume of acid), dependent (temperature change), control (concentration, volume of alkali, starting temperature).
4. Key details: use a lid, stir to mix, record maximum temperature.

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Type 5: Evaluate (6-mark Extended Answer)

Question pattern: "Evaluate the use of hydrogen fuel cells as an alternative to petrol engines for powering cars."

Model answer structure:

Paragraph 1 — Advantages:

• Only product is water — no CO2 emissions at point of use.
• More efficient than combustion engines.
• Quiet operation.

Paragraph 2 — Disadvantages:

• Hydrogen must be produced (steam reforming produces CO2; electrolysis is expensive).
• Storage is difficult (high pressure or low temperature required).
• Very few refuelling stations exist.
• Fuel cells use expensive platinum catalysts.

Paragraph 3 — Conclusion:

• Hydrogen fuel cells offer significant environmental benefits at point of use, but challenges remain in production, storage, and infrastructure. They are a promising long-term solution, particularly if hydrogen can be produced from renewable sources.

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Marks Distribution on the Exam

Understanding where marks come from helps you allocate your time: