CCEA GCSE Chemistry
CCEA GCSE Chemistry 2.8 Energy Changes in Chemistry Exam Questions
Free CCEA GCSE Chemistry revision for 2.8 Energy Changes in Chemistry: what you need to know, common mistakes and exam-style questions with answers. Covers exothermic and endothermic reactions, temperature changes, reaction profiles, activation energy, bond breaking and making, and bond energy calculations.
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About CCEA GCSE Chemistry 2.8 Energy Changes in Chemistry
Energy Changes in Chemistry is a Unit 2 topic in CCEA GCSE Chemistry. You need to classify reactions as exothermic or endothermic from temperature changes, give examples, and draw and interpret reaction profiles showing the energy of reactants and products, the overall energy change and the activation energy. The topic explains energy changes in terms of bond breaking (endothermic) and bond making (exothermic), and requires bond energy calculations. It links to rates of reaction (activation energy and catalysts), equilibrium and the combustion of fuels. Examiners regularly ask for labelled reaction profiles, bond energy calculations with the correct sign and explanations of why a reaction is exothermic overall.
What you need to know
- Exothermic reactions Exothermic reactions transfer energy to the surroundings, so the temperature rises. Examples: combustion, neutralisation, respiration and displacement reactions. The energy change, ΔH, is negative.
- Endothermic reactions Endothermic reactions take in energy from the surroundings, so the temperature falls. Examples: thermal decomposition, photosynthesis and dissolving ammonium nitrate in water. ΔH is positive.
- Reaction profiles Plot energy against progress of reaction. For an exothermic reaction the products are lower than the reactants; for endothermic they are higher. Activation energy is the minimum energy needed for a reaction to happen, shown from the reactants up to the peak.
- Bond energies Breaking bonds is endothermic and making bonds is exothermic. ΔH = total energy of bonds broken - total energy of bonds made. A negative answer means the reaction is exothermic.
- Catalysts on profiles A catalyst gives a lower peak (lower activation energy) but does not change the energy of the reactants or products or the overall ΔH.
Common mistakes in 2.8 Energy Changes in Chemistry
- Saying making bonds takes in energy. Bond making releases energy; bond breaking takes it in.
- Subtracting the wrong way round in bond energy calculations. Use bonds broken minus bonds made.
- Forgetting to count all bonds, such as the four C-H bonds in methane or both O=O bonds when there are two oxygen molecules.
- Drawing activation energy from the products to the peak. It is measured from the reactants to the peak.
- Leaving the sign off ΔH. An exothermic change must be given as a negative value.
Try three 2.8 Energy Changes in Chemistry exam-style questions
1. When ammonium nitrate dissolves in water, the temperature falls from 21 °C to 14 °C. State whether the process is exothermic or endothermic and explain. (2 marks)
Show the mark scheme answer
Endothermic; energy is taken in from the surroundings so the temperature decreases (by 7 °C).
2. Use the bond energies H-H 436, Cl-Cl 242 and H-Cl 431 kJ per mol to calculate the energy change for H2 + Cl2 → 2HCl. (3 marks)
Show the mark scheme answer
Bonds broken = 436 + 242 = 678 kJ; bonds made = 2 x 431 = 862 kJ; ΔH = 678 - 862 = -184 kJ per mol.
3. Explain, in terms of bonds, why the reaction in the previous question is exothermic. (2 marks)
Show the mark scheme answer
More energy is released making the new H-Cl bonds; than is taken in breaking the H-H and Cl-Cl bonds.
2.8 Energy Changes in Chemistry FAQs
What does CCEA GCSE Chemistry 2.8 Energy Changes in Chemistry cover?
It covers exothermic and endothermic reactions, temperature changes, reaction profiles, activation energy, and bond energy calculations.
Which unit is 2.8 Energy Changes in Chemistry examined in?
It is examined in the Unit 2 written paper, Further Chemical Reactions, Rates and Equilibrium, Calculations and Organic Chemistry.
How do I avoid mistakes in bond energy calculations?
Draw out the displayed formula of every molecule, count each type of bond and multiply by the number of molecules, then use broken minus made and include the sign and units.
