Question

1) What is enthalpy, and what is ΔH for a reaction, and what does it mean if ΔH is positive or

negative?

2) What is entropy, and what is ΔS for a reaction, and what does it mean if ΔS is positive or

negative?

3) What is Gibbs free energy, and what is ΔG for a reaction, and what does it mean if ΔG is

positive, zero or negative?

Answer 1

1) Enthalpy(H) - It is athermodynamic state function, extensive function.

It is the measure of total heat content of the system.

H = E +PV where e is the internal energy and PV is the product of pressure and volume of system

Delta H of a reaction -

It is the change in enthalpy of the system, when it undergoes a chemical reaction, as specified by the stoichiometric equation.

A -----> B

Delta H rxn = H products - H reactants

It can be positive or negative.

If H products > H reactants , it is positive and is endothermic reaction.

If H products < H reactants, it is negative and is an exothermic reaction.

Q2)

Entropy (S) is the measure of disorder of the system. It depends on the temperature, physicall state of species and the heat absorbed by the sytem .It is an extensive , state thermodynamic function.

It is also understood as the unavailable energy to do the work, which is converted to disorder.

Delta s is the chang ein entropy of the system and mathematically

Delta S = Q_rev/T

Delta S of a reaction is the change in entropy/disorder of the reaction.

A -----> B

Delta S rxn = S products - S reactants

It can be positive or negative.

If S products > Sreactants , it is positive and favored.

If S products < S reactants, it is negative and not favored.   

3) Gibbs free energy G , is an extensive, state function , which is defined as the available energy to do work.

G = H -TS that is the total heat content - unavailable energy .

A -----> B

Delta G rxn = G products - G reactants

It can be positive or negative.

If G products > G reactants , it is positive and non - spontaneous at that temperature.

If G products < G reactants, it is negative and spontaneous reaction.

When delta G = 0 , it indicates that the reaction is at equilibrium.