# Enthalpy and activation energy

1. Feb 12, 2015

### brake4country

1. The problem statement, all variables and given/known data
For a particular reversible reaction, the forward process is exothermic and the reverse process is endothermic. Which of the following statements must be true about this reaction?
(A) The forward reaction will be spontaneous under standard conditions
(B) The reverse reaction will be spontaneous under standard conditions
(C) The activation energy will be greater for the forward reaction than for the reverse reaction
(D) The activation energy will be greater for the reverse reaction than for the forward reaction.

2. Relevant equations
NA

3. The attempt at a solution
I eliminated B because if the reverse is endothermic, this will not be spontaneous. I chose the right answer (D) but I wanted to be sure my logic is on the right track. So, I assumed D must be correct because endothermic reactions require much more energy to force a reaction, whereas the forward exothermic reaction would require less.

2. Feb 12, 2015

### Bystander

Let's do them one at a time: why eliminate A?

3. Feb 12, 2015

### brake4country

A is wrong because it usually takes some energy input to make the reactants reach activation energy, which will then lead to products

4. Feb 13, 2015

### Bystander

"A is wrong," is a correct statement, but the activation energy argument doesn't hold water. There are reactions which may be strongly exothermic, but the Gibb's free energy will still be positive due to entropy of the reaction, and which will not be spontaneous (condensation of water vapor, freezing of liquid water, other such things). Enthalpy is a good clue for spontaneity, but is not sufficient of itself to guarantee spontaneity.

"B" you will have deduced can be spontaneous for an argument opposite to what I just gave you for "A," and since we're looking for "must be true," is also discarded.

So, now, what can you tell me about "C?"