activation energy
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activation energy
when given the activation energy of the forward and reverse reactions, how can you tell which reaction is spontaneous and endothermic/exothermic?
Re: activation energy
When considering activation energies and determining the sign of deltaH (endo or exothermic), it is a really good idea to think back to the concept of PE charts, or Potential Energy charts. Think of a chemical reaction as moving from one energy state to another, one plateau, up a bump and ending at another plateau, possibly at a different height. Depending on the relative levels of energy of our starts and end states, this determines the potential energy of our reactants and products. When going from a lower energy state, up to a higher activation energy, the reaction must reach a peak of the activation energy before lowering down to the final energy state that is higher than where we started. This means that the reaction is endothermic. Conceptually this makes sense that the activation energy of an endothermic reaction in the forward direction takes more activation energy than the reverse reaction. The reverse is true when going from a higher energy state, up to an activation energy, then down to a lower energy state. This is the definition of an exothermic reaction.
It is important not to confuse the idea of activation energy with spontaneity. Spontaneity is related to Gibbs-Free Energy. Recall the reference equation G = dH -TdS.
In many reactions, you can guess the sign of dS by the number of moles, phase changes, etc. Activation energy gives you an idea of where dH is. This means that exothermic reactions that have high increase in spontaneity will be more likely to be spontaneous.
It is important not to confuse the idea of activation energy with spontaneity. Spontaneity is related to Gibbs-Free Energy. Recall the reference equation G = dH -TdS.
In many reactions, you can guess the sign of dS by the number of moles, phase changes, etc. Activation energy gives you an idea of where dH is. This means that exothermic reactions that have high increase in spontaneity will be more likely to be spontaneous.
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