# Gibbs free energy or reaction enthalphy

If I conduct a reaction say,

$$\ce{CaO(s) + H2O(l) \rightarrow Ca(OH)2 (aq)}$$

let us say , I want to measure the energy generated from the reaction.Will this energy I calculate equal the change in enthalpy of the reaction because it is exothermic or will it be equal to the change in Gibbs free energy because it is the maximum non-expansive work that can be obtained.

Also, suggest a method to calculate the energy say using a coffee-cup calorimeter, etc .

Conditions for the reaction: T = 298 K Case 1:constant volume Case 2:constant pressure

1. If you measure the heat absorbed or released by the process at constant volume, you're measuring $\Delta U$ (the change in internal energy). You'll need to measure the temperature change of the reaction when it's running in a rigid, sealed container---or, better, compute it from $\Delta H$ measured as follows.
2. If you measure the heat absorbed or released by the process at constant pressure, you're measuring $\Delta H$ (the change in enthalpy). You can measure the temperature change of the reaction when it's running in a container open to the atmosphere, e. g. a coffee cup calorimeter.
3. If you're measuring the nonexpansion work reversibly and at constant temperature and pressure, you're measuring $\Delta G$ (the change in Gibbs free energy). You can do your measurements in a container open to the atmosphere immersed in a constant temperature bath. The system must be closed in the sense that you aren't losing any reactants or products.