The
Second Law of Thermodynamics states that
a process is spontaneous in the forward direction (reactants form products)
if the
overall change in entropy (system + surroundings) is
positive.
For spontaneous reactions:
ΔS(total) =
ΔS(system) + ΔS(surroundings) > 0
The
entropy change for a reaction system (Δ
rS°) can be calculated from the absolute entropies of reactants and products.
Δ
rS°=
ΣnS°(products) - ΣnS°(reactants)
where each absolute entropy is multiplied by its coefficient in the balanced equation
Recall that the sign of the entropy change of the system can often be predicted from the reaction equation for reactions involving gases. The entropy change is positive if the reaction has more moles of gas in the products than in the reactants.
The
entropy change for the surroundings is more difficult to determine, but it can be calculated from the enthalpy change for the system if the assumption is made that heat transfer between the system and the surroundings is complete.
| ΔS(surroundings) = | qP | = – | ΔH(system) |
| T | T |