Reaction Energy and Reaction Kinetics

Reaction Energy and Reaction Kinetics Driving Force of Reactions

Objectives • Explain the relationship between enthalpy change and the tendency of a reaction to occur. • Explain the relationship between entropy and the tendency of a reaction to occur. • Define free energy and explain how the quantity is calculated and interpreted. • 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.

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 and Reaction Tendency

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/mol ΔSo = -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/mol ΔSo = +0.215 kJ/mol*K • At room temp. (298K), ΔG is positive. The reaction will not occur.

Problems • Will the following reaction be spontaneous at 298 K? Cu2S(s) + S(s) 2 CuS(s) ΔHo= -26.7 kJ/mol ΔSo = -19.7 J/(mol*K)

Problems • 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?

Reaction Energy and Reaction Kinetics