Answer: 350 kJ/mol
Explanation:
1) δh = -250 kj/mol means that the products of the forward reaction are 250 kJ/mol lower in the potential enery diagram.
2) So, the reverse reaction must compensate the 250 kJ/mol and go upper as much as the activation energy of the forward reaction.
This is: Ea reverse = Ea forward + | δh |
You can see this clearer on the attached graph of an exothermic reaction.
Ea reverse = 100 kJ / mol + | -250 kJ/mol | = 100 kJ/mol + 250 kJ/mol = 350 kJ/mol