Factors Related to the Magnitude of Keq
The Tendency or Drive Toward Minimum Energy (Enthalpy) 
and Maximum Disorder (Entropy)
Why are some reactions "complete" and others not???
 
You should already know this:
In a closed system all reactions, unless prevented by exceedingly high activation energy barriers (slow rates), spontaneously approach an equilibrium state.
 
Now take a look at:
 
N2(g) + 3 H2(g) <=====>  2 NH3(g) + heat

 
There is a drive towards minimum energy, (to the right), since this is the exothermic reaction. There is an opposing drive to the left since the system at the same time tries to establish maximum randomness (entropy). Entropy is favoured because 4 moles are made out of two. The Keq for this reaction is actually a compromise between these two opposing drives.
 
The relative importance of the drives depends on the temperature. At low temperatures (Low Kinetic Energy), the enthalpy reaction will be favoured. At high temperatures (lots of Kinetic Energy) the drive toward maximum randomness will be favoured.
 
ie.   Equilibrium favours the exothermic reaction at low temperature and the drive towards maximum randomness at high temperatures.
 
The likelihood of a reaction occurring is related to the temperature effect and the relative enthalpy and entropy changes associated with the reaction.
 
Reactions that increase in entropy (products are more random than the reactants) and decrease in enthalpy (endothermic) are spontaneous.
 
Reactions that decrease in entropy (products are more structured than the reactants) and increase in enthalpy (endothermic) are nonspontaneous.
 
Here is a list of conditions in which there is an increase in the entropy of a system.
 
1. When a gas is formed from a solid. The gas is allows to escape from the system in this case so a single arrow is used.
 
CaCO3(s) + heat -----> CaO(S) + CO2(g)
2. When a gas is evolved from a solution and allowed to escape.
 
Zn(s) + 2 H+(aq) -----> H2(g) + Zn2+(aq)
3. When the number of moles of gaseous product exceeds the number of moles of gaseous reactant.
 
2 C2H6(g) + 7 O2 -------> 4 CO2(g) + 6 H2O(g)
4. When any crystal dissolves in water.
 
NaCl(s)  <=====>  Na+(aq) + Cl-(aq)
For each of these processes, predict whether the entropy increases or decreases.
 
a) 2 H2(g) + O2(g) <=====> 2H2O(g)

b) 2 SO3(g) <====> 2 SO2(g) + O2(g)

c) MgCO3(s) + 2 H3O+(aq) <====> Mg2+(aq) + 3 H2O + CO2(g)

d) Ag+(aq) + Cl-(aq)  <====> AgCl(s)

e) Cl2(g) <====> 2 Clo(g)

f) NH4NO3(s)  <=====> NH4+(aq) + NO3-(aq)

g) H2O(l)  <====> H2O(g)

h) Mg(s) + 2 H3O+(aq)  <====>  Mg2+(aq) + H2(g) + 2 H2O

i) 2 C2H2(g) + 5 O2(g)   <====>  4 CO2(g) + 2 H2O(g)

j) NH3(g) + HCl(g)  <====>  NH4Cl(s)