Some reactions are said to go to completion because the equilibrium condition is
achieved when practically all the reactants have been converted to products. At
the other extreme, some reactions reach equilibrium immediately with very little
product being formed. These two examples are representative of very large K
values and very small K values, respectively. There are essentially two driving
forces that control the extent of a reaction and determine when equilibrium will
be established. These are the drive to the lowest heat content, or enthalpy, and
the drive to the greatest randomness or disorder, which is called entropy.
Reactions with negative ΔH’s (enthalpy or heat content) are exothermic, and
reactions with positive ΔS’s (entropy or randomness) are proceeding to greater
randomness.
The Second Law of Thermodynamics states that the entropy of the universe
increases for any spontaneous process. This means that the entropy of a system
may increase or decrease but that, if it decreases, then the entropy of the
surroundings must increase to a greater extent so that the overall change in the
universe is positive. In other words,
ΔSuniverse = ΔSsystem + ΔSsurroundingsTIP
When the ΔS is positive for the system, it means greater disorder.The following is a list of conditions in which ΔS is positive for the system:
- When a gas is formed from a solid, for example,
CaCO 3 (s) → CaO(s) + CO 2 (g).- When a gas is evolved from a solution, for example,
Zn(s) + 2H+(aq) → H 2 (g) + Zn2+(aq).- When the number of moles of gaseous product exceeds the moles of
gaseous reactant, for example,
2C 2 H 6 (g) + 7O 2 (g) → 4CO 2 (g) + 6H 2 O(g).- When crystals dissolve in water, for example,
NaCl(s) → Na+(aq) + Cl−(aq).