1. Gibbs and the Law of Entropy
In chemical reactions, reactants and products will always try to reach their lowest energy state. This low energy state relates to the chemical's minimum potential energy.
Free energy (DG) is the measure of the energy stored in a substance and is representative of the work that can be done by that substance.
The enthalpy (DH) is the numerical representation of the potential energy of the of the products compared to the potential energy of the reactants.

2. Enthalpy Enthalpy DH for an endothermic reaction is positive
DH for an exothermic reaction is negative
After the activation energy has been generated, most spontaneous reactions are exothermic. The energy required to move the reaction forward is provided by the formation of the new bonds.
reactions that generate a large amount of heat are always spontaneous
C3H8(g) + 5O2(g) g 3CO2 (g) + 4H2O(g) DH = -2043kJ
Not ALL spontaneous reactions are exothermic
H2O(s) g H2O (l) DH = +6.01kJ

3. Entropy
In spontaneous reactions there is a tendency for things to become less ordered. This increasing amount of disorder is referred to as Entropy.
entropy is the quantitative measure of disorder (randomness) in a system.
entropy is given the symbol S
DS = Sproducts - Sreactants
when Sproducts > Sreactants the entropy is positive and the amount of disorder increases in the reaction as the products are more disordered than the reactants
Melting ice makes a perfect example of entropy. As ice the individual molecules are fixed and ordered. As ice melts the molecules become free to move therefore becoming disordered. As the water is then heated to become gas, the molecules are then free to move independently through space.

4. Examples of Entropy
Here are some examples of reactions that would have positive entropy.
Formation of gases
CO2(s) g CO2(g)
H2O(l) g H2O(g)
Formation of ions in solution
NaCl(s) g Na+(aq) + Cl-(aq)
More molecules of product than reactants
2NH3(g) g N2(g) + 3H2(g)
NH4Cl(s) g NH3(g) + 3HCl(g)
The temperature of a substance is increased.
H2O(l, 25oC) g H2O(l, 50oC)
The second law of thermodynamics is that ALL things lead to entropy. What this means is that with all spontaneous reactions, entropy increases.
Within you, your community, country, world, solar, system, and universe things are speeding toward increasing disorder (absolute chaos!).
It requires a constant input of energy to counteract Entropy.

5. Gibbs Free Energy
Because not all exothermic reactions are spontaneous, an American mathematician named J. Willard Gibbs related enthalpy and entropy to determine whether reactions would be spontaneous or not. The information was related in the Gibbs Free Energy equation.
DG = DH - TDS
If DG is negative
the reaction is spontaneous (favors the formation of products)
energy is released
If DG is positive
the reaction is not spontaneous and requires additional energy to proceed
energy is absorbed
If DG is zero, the reaction is at equilibrium