1. May 7, 2004

### yifan

I've got two questions here.
Why is the enthalpy cahnge for a specific compound differs when temperature, pressure is different?
Can we use the entropy change to deduse whether an reaction can happen naturally?
Thank u

2. May 8, 2004

### GCT

The enthalpy change pertains to the level of the chemical equation and thus it is constant as long as the reaction takes place; since enthalpy changes pertains to bond energies. What you are referring to, I believe is, free energy changes. Free energy changes differ at different temperatures and pressures, which can be due to several reasons. For one, the rate of the unfavorable reaction will be increased relative to the favorable reaction at higher temperatures, thus the enthalpy of the reaction as a whole will be different at different temperatures since different proportions, relating to the chemical equation, of products will be formed (think enthalpy change per mole). Also at higher temperatures the value of TdS becomes significant. In order to calculate the spontaneity of a reaction one needs to know this free energy change, entropy value will not be sufficient.

-------