# Can the enthalpy change of a reaction be changed, and if so, will this affect the value of the equilibrium constant?

In the case of an arbitrary equilibrium reaction, is it possible to change the absolute enthalpy of the final products, thus the reaction enthalpy change? For example, say pH affects the charge distribution of the product molecule, changing the attractive force between it and the solvent (feel free to come up with a better example).

Given that this does in fact affect $\Delta H$, would the Gibbs free energy change of the reaction $\Delta G=\Delta H-T\Delta S$ be altered as well, or does the entropy change somehow adjust automatically?

If $\Delta G$, changes, I would assume $K$ changes as well through $\Delta G = - RT$ $\mathrm{ln} K$

## 1 Answer

Short answer: No and yes.

Enthalpy is a state function. That means that it depends only in the current "state" - the identity of the substances, their concentrations, pressures, etc. That means that the absolute enthalpy of some substance is independent of how you produced it. However, that substance has different enthalpy in different states: gas versus liquid, pure versus in solution. If you change the state of the product(s), you with change their enthalpy. Same goes with any of the other thermodynamic potentials: entropy, internal energy, free energy.

And that of course will alter the equilibrium situation.

• The equilibrium situation only, or the constant as well? – Marcel Jan 9 '17 at 12:34