Predicting spontaneity

The Second Law of Thermodynamics states that

a process is spontaneous in the forward direction (reactants to products)
if the overall change in entropy (system + surroundings) is positive (1).

(1) ΔS(total)

= ΔS(system) + ΔS(surroundings) > 0

The entropy change for a reaction system can be calculated from the absolute entropies of reactants and products (2).

(2) ΔS(system) =

ΣnS°(products) - ΣnS°(reactants)

where each absolute entropy is multiplied by its coefficient in the balanced equation

Recall that for reactions involving gases, the sign of ΔS(system) can be predicted from the reaction equation. This is positive if the reaction has more moles of gas in the products than in the reactants.

The entropy change for the surroundings depends on the extent to which the heat is absorbed from or released to the surroundings by the reaction. If heat transfer between the system and the surroundings is assumed to be complete, q_p(surroundings) is the negative of the enthalpy change for the system.

(3) ΔS(surroundings) =

q_P(surr)	= –	ΔH(system)
T	= –	T

ΔH(system) can be calculated from enthalpies of formation of all reactants and products.

Δ_rH° = ΔH(system) =

ΣnΔ_fH°(products) - ΣnΔ_fH°(reactants) (2)

where each absolute enthalpy of formation is multiplied by its coefficient in the balanced equation