# Entropy and Enthalpy

∆S (Entropy change) is given by ∆H/T (∆H = Enthalpy change , T= absolute temperature). Then how can the Entropy change for a reaction be negative if the Enthalpy change is positive, and vice versa?

• Who says the enthalpy change is always positive? And what about the equation $\Delta G=\Delta H-T\Delta S$? What if $\Delta G$ is not zero between the initial state of the reactants and the final state of the products? – Chet Miller Feb 17 '17 at 12:40

• "Same way" means $dS = dq/T$ @RobChem – Zhe Jan 18 '18 at 14:51
In Chemistry it is usual to conduct experiments at constant pressure and temperature, rather than at constant volume, and so the place to start with many thermodynamic calculations is the (Gibbs) free energy, $\Delta G=\Delta H - T\Delta S$. As the two phases are in equilibrium the free energy change upon vaporisation is $\Delta G_{vap}=\Delta H_{vap}-T\Delta S_{vap} = 0$ and thus $\Delta S_{vap}=\Delta H_{vap}/T$. Only in the special case when $\Delta G = 0$ does this apply.