For `A + B to C + D, DeltaH = -20 kJ "mole"^(-1)`. The activation energy for the forward reaction is 85 kJ. Then the activation energy for the backward reaction is ___

Options

A. 65 kJ
B. 105 kJ
C. 85 kJ
D. 40 kJ

Explanation

To find the activation energy for the backward reaction, we can use the relationship between the activation energies of the forward and backward reactions and the enthalpy change (ΔH) of the reaction.

### Step-by-Step Solution:

1. **Identify the Given Values**:
- ΔH (enthalpy change) = -20 kJ/mol (indicating an exothermic reaction)
- Activation energy for the forward reaction (Ea, forward) = 85 kJ/mol

2. **Understand the Relationship**:
For an exothermic reaction, the activation energy for the backward reaction (Ea, backward) can be calculated using the formula:
\[ \text{Ea, backward} = \text{Ea, forward} + |\Delta H| \]
Here, |\Delta H| is the absolute value of the enthalpy change.

3. **Calculate the Absolute Value of ΔH**:
Since ΔH is -20 kJ/mol, we take the absolute value:
\[ |\Delta H| = 20 \text{ kJ/mol} \]

4. **Substitute the Values into the Formula**:
Now, substitute the known values into the equation:
\[ \text{Ea, backward} = 85 \text{ kJ/mol} + 20 \text{ kJ/mol} \]

5. **Perform the Calculation**:
\[ \text{Ea, backward} = 85 + 20 = 105 \text{ kJ/mol} \]

6. **Final Answer**:
The activation energy for the backward reaction is **105 kJ/mol**.