Question

What is a good simple definition for standard enthalpy change? Standard enthalpy of formation? And entropy? What about delta S and delta G?

Answer 1

The enthalpy of formation is the standard reaction enthalpy for the formation of the compound from its elements atoms or molecules in their most stable reference states at the chosen temperature and at one bar pressure. The standard enthalpy of formation is measured in units of energy per amount of substance, usually stated in kilojoule per mole.

Complete Step By Step Answer:
Standard enthalpy of formation: The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy during the formation of one mole of the substance from its constituent elements, with all substances in their standard states. The formation reaction is a constant pressure and constant temperature process. Since the pressure of the standard formation reaction is fixed one bar, the standard formation enthalpy or reaction heat is a function of temperature.
Entropy: It is the measure of a system's thermal energy per unit temperature that is unavailable for doing useful work. Because work is obtained from ordered molecular motion, the amount of entropy is also a measure of the molecular disorder, or randomness, of a system.
Gibbs free energy relates enthalpy, entropy and temperature. A spontaneous reaction will always occur when Delta H is negative and Delta S is positive, and a reaction will always be non-spontaneous when Delta H is positive and Delta S is negative

Note:
For many substances, the formation reaction may be considered as the sum of a number of simpler reactions, either real or fictitious. The enthalpy of reaction can then be analyzed by applying Hess's Law, which states that the sum of the enthalpy changes for a number of individual reaction steps equals the enthalpy change of the overall reaction. This is true because enthalpy is a state function, whose value for an overall process depends only on the initial and final states and not on any intermediate states.