Eduqas GCSE Chemistry
Eduqas GCSE Chemistry 8 Energy Changes in Chemistry Exam Questions
Free Eduqas GCSE Chemistry revision for 8 Energy Changes in Chemistry: what you need to know, common mistakes and exam-style questions with answers. Covers exothermic and endothermic reactions, reaction profiles, activation energy, bond energy calculations, energy from fuels, chemical cells and hydrogen fuel cells.
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About Eduqas GCSE Chemistry 8 Energy Changes in Chemistry
Energy changes in chemistry looks at why reactions release or take in energy. In Eduqas GCSE Chemistry you classify reactions as exothermic or endothermic using temperature changes, draw and interpret reaction profiles with activation energy, and explain energy changes using bond breaking and bond making. You calculate overall energy changes from bond energies and measure the energy released by a fuel (SP8). The topic also covers simple chemical cells and hydrogen fuel cells, comparing them with batteries. It links to rates (topic 9) and the reactivity series (topic 6). Examiners often set bond energy calculations, reaction profile sketches and evaluations of the fuel experiment, especially sources of heat loss.
What you need to know
- Exothermic and endothermic Exothermic reactions transfer energy to the surroundings so the temperature rises, for example combustion, neutralisation and respiration. Endothermic reactions take in energy so the temperature falls, for example thermal decomposition and dissolving ammonium nitrate.
- Reaction profiles In an exothermic profile the products are lower in energy than the reactants; in an endothermic profile they are higher. Activation energy is the minimum energy particles need to react, shown as the rise from the reactants to the peak.
- Bond energies Breaking bonds is endothermic and making bonds is exothermic. Energy change = energy needed to break bonds - energy released making bonds. A negative answer means the reaction is exothermic.
- SP8: energy from a fuel Burn a fuel in a spirit burner under a copper can of water and measure the temperature rise and mass of fuel used. Energy transferred = mass of water x 4.18 x temperature change (in J). Heat loss to the surroundings gives lower values than expected.
- Chemical cells A simple cell is made from two different metals in an electrolyte; the bigger the difference in reactivity, the larger the voltage. Cells stop when a reactant is used up.
- Fuel cells Hydrogen fuel cells react hydrogen with oxygen to produce electricity, with water as the only product: 2H2 + O2 → 2H2O. Compared with batteries they do not need recharging, but hydrogen is difficult to store and is often made using fossil fuels.
Common mistakes in 8 Energy Changes in Chemistry
- Saying energy is needed to make bonds. Bond making releases energy; bond breaking needs energy.
- Subtracting the wrong way in bond energy calculations. Use bonds broken minus bonds made; a negative result is exothermic.
- Forgetting to count every bond. Multiply each bond energy by the number of molecules in the balanced equation, for example 2 x H-Cl.
- Labelling the overall energy change from the peak of the profile. The overall change is the difference between reactants and products.
- Saying fuel cells produce no pollution at all. Using the cell only produces water, but making the hydrogen may release carbon dioxide.
Try three 8 Energy Changes in Chemistry exam-style questions
1. Calculate the energy change for H2 + Cl2 → 2HCl. Bond energies in kJ/mol: H-H 436, Cl-Cl 242, H-Cl 431. State whether it is exothermic or endothermic. (3 marks)
Show the mark scheme answer
Bonds broken = 436 + 242 = 678 kJ; bonds made = 2 x 431 = 862 kJ; energy change = 678 - 862 = -184 kJ/mol, exothermic.
2. A fuel heated 100 g of water by 25 °C. Calculate the energy transferred to the water in kJ. Specific heat capacity of water = 4.18 J/g °C. (2 marks)
Show the mark scheme answer
Energy = 100 x 4.18 x 25 = 10450 J; = 10.45 kJ (10.5 kJ).
3. Give two advantages and one disadvantage of hydrogen fuel cells compared with rechargeable batteries. (3 marks)
Show the mark scheme answer
Only waste product is water; do not need recharging / do not run down while fuel is supplied (or fewer toxic chemicals to dispose of); disadvantage: hydrogen is a flammable gas that is hard to store / is often made from fossil fuels.
8 Energy Changes in Chemistry FAQs
What does Eduqas GCSE Chemistry 8 Energy Changes in Chemistry cover?
It covers exothermic and endothermic reactions, reaction profiles and activation energy, bond energy calculations, measuring energy from fuels, chemical cells and hydrogen fuel cells.
Which paper is 8 Energy Changes in Chemistry examined in?
It is examined in Component 1: Concepts in Chemistry (2 hours 15 minutes, 120 marks). Component 2: Applications in Chemistry can also test it through practical contexts and an unseen article.
How do I improve the SP8 fuel experiment in Eduqas GCSE Chemistry 8 Energy Changes in Chemistry?
Reduce heat loss by using a lid, a draught shield and insulation around the can, keep the flame close to the can, and use the same mass of water and the same distance each time. Weigh the burner before and after to find the mass of fuel used.
