Unit 8- Chemical Kinetics

Learning Objective The student is able to explain the difference between collisions that convert reactants to products and those that do not in terms of energy distributions and molecular orientation. The student is able to use representations of the energy profile for an elementary reaction (from the reactants, through the transition state, to the products) to make qualitative predictions regarding the relative temperature dependence of the reaction rate.

The change in concentration of a reactant or product per unit time Reaction Rate The change in concentration of a reactant or product per unit time

2 NO2  2 NO + O2

COLLISION THEORY

One of the simplest models to explain reaction rates is collision theory According to collision theory, the rate of reaction is proportional to the number of effective collisions per second among the reacting molecules An effective collision is one that actually gives product molecules

There are two factors that determine whether a collision will result in products being formed: Molecular orientation is important because a collision on the “wrong side” of a reacting species cannot produce any product. Particles must collide with enough energy. The minimum energy required is called the activation energy, Ea.

This increases the rate of reaction for two reasons. The minimum kinetic energy the colliding particles must have is called the activation energy, Ea Activation energies can be large, so only a small fraction of the well-orientated, colliding molecules collide with enough energy. At higher temperatures, the average kinetic energy of the reacting particles is higher. This increases the rate of reaction for two reasons. Collisions happen more often. Collisions happen with more energy.

Kinetic energy distribution for a reaction at two different temperatures. At the higher temperature, a larger fraction of the collisions have sufficient energy for reaction to occur.

Transition state theory explains what happens when reactant particles come together Potential-energy diagrams are used to help visualize the relationship between the activation energy and the development of total potential energy The potential energy is plotted against reaction coordinate or reaction progress

The potential-energy diagram for an exothermic reaction The potential-energy diagram for an exothermic reaction. The extent of reaction is represented as the reaction coordinate.

Potential-energy diagram for an endothermic reaction Potential-energy diagram for an endothermic reaction. The heat of reaction and activation energy are labeled.

Reactions generally have different activation energies in the forward and reverse direction

Learning Objective The student is able to explain the difference between collisions that convert reactants to products and those that do not in terms of energy distributions and molecular orientation. The student is able to use representations of the energy profile for an elementary reaction (from the reactants, through the transition state, to the products) to make qualitative predictions regarding the relative temperature dependence of the reaction rate.