According to the following figure, the magnitude of the enthalpy change of the reaction
A + B → M + N in kJ mol⁻ is equal to ___________. (Integer answer)

Solution

Chemistry ma’am · 3 steps

Enthalpy change is only the height gap from the reactant line to the product line, so ignore the hump.

1. Read the labels. x = 20 kJ mol⁻¹ runs from the A + B level up to the peak, so it is the activation energy. y = 45 kJ mol⁻¹ runs from the A + B level down to the M + N level. z = 15 kJ mol⁻¹ is only the height of M + N above the axis.
2. Enthalpy is a gap. Δ H is the energy of the products minus the energy of the reactants. It does not depend on the path, so the peak height x plays no part. The baseline height z plays no part either.

   Δ H = H_(M + N) - H_(A + B) = -y = -45 kJ mol⁻¹

3. So. The products lie lower, so the reaction is exothermic with Δ H = -45 kJ mol⁻¹. The magnitude is 45.

The common slip: 65. Adding x and y gives 65, but that is the activation energy of the reverse reaction, not the enthalpy change.

The idea to keep. On an energy profile, Δ H is the vertical gap from the reactant level to the product level. The peak only tells you the activation energy.

Answer from the NTA final answer key. Solution by PrepWiser.