Consider the following information for the reaction A + B → C - VCE - SSCE Chemistry - Question 18 - 2006 - Paper 1

For the reverse reaction, the relationship between the activation energy and the heat of reaction can be expressed as:

$E_a (reverse) = E_a (forward) - \Delta H$

Substituting the given values:

• Activation energy for the forward reaction (A + B → C): +200 kJ mol⁻¹
• Heat of reaction ($\Delta H$): -120 kJ mol⁻¹

Thus, we have:

$E_a (reverse) = 200 - (-120)$

This simplifies to:

$E_a (reverse) = 200 + 120 = 320 kJ mol⁻¹$

Since the activation energy must be positive for the reverse reaction, we consider:

1. The heat of reaction is negative, indicating that the reaction releases energy (exothermic).
2. Therefore, the activation energy for the reverse reaction will be:

$E_a (reverse) = 320 kJ mol⁻¹$

Choosing the correct sign, the final activation energy for the reaction C → A + B is:

-320 kJ mol⁻¹, corresponding to option B.