1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Enthalpy and activation energy

  1. Feb 12, 2015 #1
    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. jcsd
  3. Feb 12, 2015 #2

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Let's do them one at a time: why eliminate A?
     
  4. Feb 12, 2015 #3
    A is wrong because it usually takes some energy input to make the reactants reach activation energy, which will then lead to products
     
  5. Feb 13, 2015 #4

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    "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?"
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Enthalpy and activation energy
  1. Activation energy (Replies: 3)

  2. Activation Energy (Replies: 3)

Loading...