Temperature and Rate

· Most reactions speed up as temperature increases. (E.g., food spoils when not refrigerated.)

· As temperature increases, the rate increases.

· Since the rate law has no temperature term in it, the rate constant must depend on temperature.

The Collision Model

· Goal: develop a model, which explains why rates of reactions increases as concentration and temperature increase.

The collision model: in order for molecules to react they must come into direct contact via collision and must possess a minimum kinetic energy to react

· The greater the number of collisions the faster the rate.

· Complication: not all collisions lead to products. In fact, only a small fraction of collisions leads to product.

Activation Energy

Arrhenius: molecules must possess a minimum amount of energy to react.

Activation energy, E_a, is the minimum energy required to initiate a chemical reaction.

· In order to form products, bonds must be broken in the reactants.

Bond breakage requires energy

Example:

Consider the rearrangement reaction of the conversion of methyl isonitrile CH₃NC to methyl acetonitrile CH₃CN.

· As the temperature increases, the bond bends until the C-N bond breaks and the NC portion is perpendicular to the H₃C portion.

· This structure is called the activated complex or transition state.

The energy required for the above twist and break is the activation energy, E_a.

Once the C-N bond is broken, the NC portion can continue to rotate forming a C-C N bond.

· The change in energy for the reaction is the difference in energy between CH₃N C and CH₃C N.

· The activation energy is the difference in energy between reactants, CH₃NC and transition state.

In order for reaction to occur, the reactant molecules must collide with enough energy in the correct orientation to form products.

· Consider the reaction between Cl and NOCl:

If the Cl collides with the Cl of NOCl then the products are Cl₂ and NO.

If the Cl collided with the O of NOCl then no products are formed.

· We can qualitatively explain the various influence of concentration and temperature on the rate of reactions based on collision model:

Effect of Concentration:

· The more molecules present, the greater the probability of collisions and the faster the rate.

Effect of Temperature

· The higher the temperature, the more energy available to the molecules and the faster the rate.