Kinetics and Thermodynamics of Simple Chemical Processes 2-1 Chemical thermodynamics: Is concerned with the extent that a reaction goes to completion. Chemical kinetics: Is concerned with the speed that a reaction goes to completion. Thermodynamic Control: The most stable products are formed. Kinetic Control: The fastest product is formed.

Equilibria are governed by the thermodynamics of chemical change. When the concentrations of reactants and products no longer change, the system is said to be at equilibrium. A system at equilibrium is described mathematically: A large value of K indicates that the reaction goes to completion.

The equilibrium constant can be related to the thermodynamic function G o : When G o is negative, the equilibrium constant is greater than 1 and the products of the reaction are favored over the reactants. When G o is positive, the equilibrium constant is less that 1 and the reactants of the reactions are favored over the products.

The free energy change is related to changes in bond strengths and the degree of order in the system. The Gibbs free energy change for a reaction is related to the enthalpy change and the entropy change for the reaction: The enthalpy change, H o, can be estimated: The entropy change, S o, is related to the amount of disorder in the system. The entropy of a substance phase is much larger in the gas than in the liquid phase. In a chemical reaction where all substances are in the same phase, the entropy will increase if there are more product molecules than reactant molecules.

The rate of a chemical reaction depends on the activation energy. The potential energy of the system as a chemical change occurs can be plotted vs. time: The higher the activation energy, E a, the slower the reaction.

Collisions supply the energy to get past the activation-energy barrier. The average kinetic energy of molecules at room temperature is about 0.6 kcal mol -1. The kinetic energies of individual molecules can be plotted as a Boltzmann distribution curve: As can be seen from the curves, there are more molecules having large kinetic energies at high temperature than at low temperature. Since the energy required to reach the transition state in a chemical reaction comes from molecular collisions, the rate of chemical reactions always increases with rising temperatures.

The concentration of reactants can affect reaction rates. The rate of a chemical reaction can be expressed as a rate law. The rate law must be experimentally determined; it cannot be derived directly from the balanced chemical equation.

The Arrhenius equation describes how temperature affects reaction rates. The rate constant, k, depends upon temperature according to the Arrhenius equation: In general, raising the reaction temperature by 10 o C will increase the rate constant by a factor of 2 or 3.