OxfordAQA International GCSE Chemistry
OxfordAQA International GCSE Chemistry 3.9 Energy Changes Exam Questions
Free OxfordAQA International GCSE Chemistry revision for 3.9 Energy Changes: what you need to know, common mistakes and exam-style questions with answers. Covers exothermic and endothermic reactions, energy level diagrams, bond energy calculations, calorimetry, chemical cells and fuel cells.
Practise 3.9 Energy Changes with past papers
Once you have learnt this topic, find the 3.9 Energy Changes questions in real OxfordAQA International GCSE Chemistry past papers and mark them with the mark scheme. Doing lots of real questions is the quickest way to learn how examiners ask about each idea.
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About OxfordAQA International GCSE Chemistry 3.9 Energy Changes
Energy Changes in OxfordAQA International GCSE Chemistry covers the energy transferred when reactions happen. You need to classify reactions as exothermic or endothermic, give the sign of the enthalpy change, draw energy level diagrams showing activation energy and the effect of a catalyst, and calculate energy changes from bond dissociation energies. Measuring temperature changes in an insulated container appears for everyone; calorimetry of fuels using Q = mc delta T, energy per gram or per mole, and chemical and hydrogen fuel cells are in the separate Chemistry course. Examiners often set multi-step bond energy calculations and ask you to explain energy changes in terms of bonds broken and made.
What you need to know
- Exothermic reactions Transfer energy to the surroundings, so the temperature rises and delta H is negative. Examples are combustion, many oxidation reactions and neutralisation; uses include self-heating cans and hand warmers.
- Endothermic reactions Take in energy from the surroundings, so the temperature falls and delta H is positive. Thermal decomposition is an example; sports injury packs use endothermic changes. Hydrated copper sulfate (blue) ⇌ anhydrous copper sulfate (white) + water is endothermic forwards and exothermic in reverse.
- Energy level diagrams Show reactants and products at different energy levels with the overall energy change and activation energy marked, plus a curved reaction path. A catalyst lowers the peak of the curve.
- Bond energies Breaking bonds takes in energy; making bonds releases energy. Energy change = energy to break bonds - energy released making bonds. A negative answer means the reaction is exothermic.
- Calorimetry (separate Chemistry) Q = mc delta T, where m is the mass of water heated, c = 4.2 J/g °C and delta T is the temperature change. Divide by the mass or moles of fuel burned to compare fuels in J or kJ per gram or per mole.
- Cells and fuel cells (separate Chemistry) A chemical cell produces a potential difference until a reactant is used up. A hydrogen-oxygen fuel cell reacts hydrogen with oxygen to produce electricity, with water as the only waste product; used in cars and spacecraft. Compare it with burning hydrogen in terms of storage, use and products.
Common mistakes in 3.9 Energy Changes
- Saying bond breaking releases energy. Breaking bonds always requires energy; making bonds releases it.
- Getting the sign wrong. Exothermic reactions have a negative delta H, endothermic a positive one.
- Using the mass of fuel as m in Q = mc delta T. m is the mass of water being heated.
- Drawing the products above the reactants for an exothermic reaction. Products are lower because energy has been released to the surroundings.
- Counting each bond type only once in bond energy questions. Multiply by the number of bonds shown in the balanced equation, for example four C-H bonds in methane.
Try three 3.9 Energy Changes exam-style questions
1. Calculate the energy change for H2 + Cl2 → 2HCl. Bond energies in kJ/mol: H-H 436, Cl-Cl 243, H-Cl 432. (3 marks)
Show the mark scheme answer
Bonds broken = 436 + 243 = 679 kJ/mol; bonds made = 2 x 432 = 864 kJ/mol; 679 - 864 = -185 kJ/mol (exothermic).
2. Burning 0.40 g of a fuel raised the temperature of 100 g of water by 12.0 °C. Calculate the energy released per gram of fuel. (c = 4.2 J/g °C) (3 marks)
Show the mark scheme answer
Q = 100 x 4.2 x 12.0 = 5040 J; 5040 / 0.40 = 12 600 J/g; = 12.6 kJ/g.
3. Explain, in terms of bonds, why a reaction is exothermic. (2 marks)
Show the mark scheme answer
More energy is released when new bonds form in the products; than is taken in to break bonds in the reactants.
3.9 Energy Changes FAQs
What does OxfordAQA International GCSE Chemistry 3.9 Energy Changes cover?
It covers exothermic and endothermic reactions, energy level diagrams and activation energy, bond energy calculations, measuring temperature changes and, in separate Chemistry, calorimetry and chemical and fuel cells.
Which paper is 3.9 Energy Changes examined in?
OxfordAQA International GCSE Chemistry has two equally weighted papers, Paper 1 and Paper 2, each 1 hour 30 minutes and 90 marks. Content from any part of the specification can be assessed in either paper, so revise this topic for both.
How do I avoid mistakes in bond energy calculations?
Draw out every molecule from the balanced equation, list each bond with how many times it appears, then total bonds broken and bonds made separately. Subtract made from broken and keep the sign in your final answer.
