CIE iGCSE Co-Ordinated Science C5.1 Exothermic and endothermic reactions Exam Style Questions Paper 2
▶️ Answer/Explanation
Activation energy is the energy difference between the reactants and the highest point (transition state). Reactants at 100 kJ, peak at 350 kJ, so activation energy = 350 – 100 = 250 kJ/mol. Answer C.
✅ Answer: (C)
✅ Answer: (C)
▶️ Answer/Explanation
The reaction mixture becomes warm, indicating that the reaction is exothermic (releases thermal energy to the surroundings). In an exothermic reaction, the products have less energy than the reactants, so the energy level of products is lower on the diagram. The activation energy is the energy barrier shown as the peak. Therefore, diagram C correctly shows reactants at a higher energy level than products with the activation energy peak, representing an exothermic reaction. Diagrams A and B show endothermic reactions (products higher energy), and diagram D incorrectly shows products lower than reactants but without the correct labelling of activation energy.
✅ Answer: (C)
✅ Answer: (C)
▶️ Answer/Explanation
The temperature of the mixture increases, which means the reaction releases thermal energy to the surroundings. This is characteristic of an exothermic reaction. In an exothermic reaction, thermal energy is transferred from the reacting chemicals to the surroundings, causing the temperature to rise. Endothermic reactions absorb energy and cause the temperature to decrease.
✅ Answer: (D)
✅ Answer: (D)
▶️ Answer/Explanation
Burning (combustion) is a chemical change because new substances (\(\text{CO}_2\) and \(\text{H}_2\text{O}\)) are formed and the reaction is irreversible — the original match cannot be restored.
It is also an exothermic reaction because energy (heat and light) is released to the surroundings; therefore the correct row is B (chemical change, exothermic).
✅ Answer: (B)
It is also an exothermic reaction because energy (heat and light) is released to the surroundings; therefore the correct row is B (chemical change, exothermic).
✅ Answer: (B)
▶️ Answer/Explanation
Both starting solutions were cooler (21 °C and 24 °C) than the resulting mixture (31 °C), so the surroundings gained thermal energy.
A temperature rise means the reaction released energy to the surroundings, which is the definition of an exothermic reaction.
In an exothermic reaction, it is the reacting chemicals that lose energy (as it is transferred out to the surroundings).
✅ Answer: (D)
A temperature rise means the reaction released energy to the surroundings, which is the definition of an exothermic reaction.
In an exothermic reaction, it is the reacting chemicals that lose energy (as it is transferred out to the surroundings).
✅ Answer: (D)