Home / iGCSE Chemistry Multiple Choice (Extended) : 5.1 Exothermic and endothermic reactions: Exam Style Questions Paper 2

iGCSE Chemistry Multiple Choice (Extended) : 5.1 Exothermic and endothermic reactions: Exam Style Questions Paper 2

iGCSE Chemistry MCQ (Extended) Paper 2 - All Chapters
Question

Two reaction pathway diagrams are shown.

Which arrow represents the activation energy for a reaction which releases thermal energy?

▶️ Answer/Explanation
Solution

Ans: A

For an exothermic reaction (which releases thermal energy), the activation energy is represented by the energy difference between the reactants and the transition state (peak of the curve). This is shown by the arrow from reactants to the peak in the first diagram where products are at lower energy than reactants.

The other options represent either the enthalpy change (B) or refer to an endothermic reaction (C and D).

Question

The equation for the formation of ammonia is shown.

N2 + 3H2 → 2NH3

The reaction pathway diagram for the reaction is shown.

What is the enthalpy change for the reaction?

A) −592 kJ/mol
B) −92 kJ/mol
C) +92 kJ/mol
D) +592 kJ/mol

▶️ Answer/Explanation
Solution

Ans: B

The enthalpy change (ΔH) is the difference between the products and reactants:

From the diagram, the total energy released is -342 kJ/mol, which represents the energy difference between the activated complex and the products.

The activation energy is +250 kJ/mol (energy difference between reactants and activated complex).

Therefore, the enthalpy change is:

ΔH = Energy of products – Energy of reactants

We can calculate it as: ΔH = Activation energy – Total energy released

ΔH = 250 kJ/mol – 342 kJ/mol = -92 kJ/mol

This matches option B.

Question

Which row describes a reaction where the overall energy change is exothermic?

▶️ Answer/Explanation
Solution

Ans: D

An exothermic reaction is characterized by:

  1. More energy being released in bond formation than required for bond breaking (net energy release)
  2. An increase in temperature of the surroundings (as energy is released to the environment)

Let’s analyze each option:

Option A: 600 kJ in, 300 kJ out → net absorption of 300 kJ (endothermic)

Option B: 600 kJ in, 1200 kJ out → net release of 600 kJ, but temperature decreases (contradicts exothermic behavior)

Option C: 900 kJ in, 300 kJ out → net absorption of 600 kJ (endothermic)

Option D: 900 kJ in, 1200 kJ out → net release of 300 kJ with temperature increase (correct for exothermic)

Therefore, only option D shows both the correct energy relationship (more energy out than in) and the correct temperature effect (increase) for an exothermic reaction.

Question

 The equation for the reaction between gaseous hydrogen and gaseous iodine to form gaseous hydrogen iodide is shown.

H2(g) + I2(g) → 2HI(g)

The reaction is exothermic.

Which statement explains why the reaction is exothermic?

  1. Energy is released when H–H and I–I bonds are broken.
  2. The bond energies of the reactants are larger than the bond energies of the products.
  3. The products are at a higher energy level than the reactants.
  4. More energy is released when two HI bonds are formed than is used when the H–H and I–I bonds are broken.
Answer/Explanation

Ans:

D

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