reading activation energy (TBR)

[Meredith92]

On page 134 of TBR book 2 for gen chem it says"
"Activation E is read from a free energy diagram, not from a heat energy diagram based on enthalpy"

What does this mean? I get confused about this sometimes- i thought the y axis for an energy diagram could be free energy or enthalpy... arent they both units of energy? why cant we read activation energy from an energy diagram w/ enthalpy on the y axis?

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[milski]

The units are the same but the values and the meaning are different. The easiest example is an endothermic, spontaneous reaction. Think how the energy diagrams in terms of free energy and enthalpy will defer.

 

[Laureatebarrel]

Heat enthalpy is thermodynamic, which only depends on the beginning and end of the reaction. Activation energy is kinetic, which depends on the path of the reaction. So if you read enthalpy table you wont find the activation energy because enthalpy only describes the beginning and end of the rxn. I always relate enthalpy to thermodynamic and activation energy to kinetic to make it easy to remember.

 

[premed8888]

In a given rxn, there the enthalpy only takes into account the heat exchange (whether it is gained or lost). However, in any given rxn, there is a temperature and change in entropy that must be taken into account. This is given by the Gibb's free energy equation which we all know. For a rxn diagram, Gibb's free energy is the best choice - x axis is rxn progress and y is the activation energy. Just think about it this way - a rxn could have a net overall positive enthalpy change, but with the temperature and entropy change taken into account (if they are large enough), the rxn could still be exergonic. Activation energy is simply then the energy needed to get the rxn going.