Reaction Energy and Reaction

Kinetics

Driving Force of Reactions

Objectives1. Explain the relationship between enthalpy change

and the tendency of a reaction to occur.2. Explain the relationship between entropy and the

tendency of a reaction to occur.3. Define free energy and explain how the quantity is

calculated and interpreted.4. Describe the use of free energy change to determine

the tendency of a reaction to occur.

Enthalpy and Reaction Tendency

• A majority of the reactions that occur in nature are exothermic.

• Reactions tend to proceed in a direction of a lower energy state.

• However, some endothermic reactions occur spontaneously.

Entropy and Reaction Tendency• Melting is an endothermic process that happens

spontaneously.

2 NH4NO3 (s) 2 N2(g) + 4 H2O(l) + O2(g)

• Processes tend to occur in nature in a direction of increasing disorder.

• Entropy(S) – measure of the degree of randomness of the particles of a system.

Entropy

Entropy• In general, entropy increases going from solid to

liquid to gas.• Solution formation is also results in an increase in

entropy.• At absolute zero, the particles of a solid have an

entropy of 0 kJ/mol*K. • Entropy increases with temperature.• Change in entropy (ΔS) = entropy of the products -

entropy of the reactants

Entropy

• Increase in entropy (+ΔS)

• Decrease in entropy (-ΔS)

Free Energy• Processes occur in nature in the direction of

decreasing enthalpy and/or increasing entropy.

Free energy (G) – a function combining the entropy and enthalpy of a system

Free energy change (ΔG) – Difference between the change in enthalpy and the product of the Kelvin temperature and entropy change.

ΔGo = ΔHo - TΔSo

Free Energy• If ΔG is negative, a reaction can occur spontaneously.• If ΔG is positive, a reaction will not occur naturally.

C2H4(g)+ H2(g) C2H6(g)

ΔHo= -136.9 kJ/molΔSo = -0.1207 kJ/mol*K

• At room temperature, ΔG is negative spontaneous

Free Energy

CH4(g)+ H2O(g) CO(g) + 3 H2(g)

ΔHo= +206.1 kJ/molΔSo = +0.215 kJ/mol*K

• At room temp. (298K), ΔG is positive. The reaction will not occur.

Problems1. Will the following reaction be spontaneous at 298

K?Cu2S(s) + S(s) 2 CuS(s)

ΔHo= -26.7 kJ/molΔSo = -19.7 J/(mol*K)

Problems2. For the vaporization of Bromine, ΔHo = 31.0 kJ/mol

and ΔSo = 93.0 J/(mol*K). At what temperature will this process be spontaneous?