Kinetics

Rate of Reaction Reaction kinetics is the study of rates of reaction. The rate of a reaction is defined as the change in concentration per unit of time and therefore has units of mol dm -3 s -1. Collision Theory When two substances react their particles must collide. In this topic, the term ‘particles’ refers to atoms, ions or molecules. Not every collision results in a reaction. Particles will only react when they collide with sufficient energy. The minimum amount of energy needed for a reaction to occur is called the activation energy.

Maxwell-Boltzmann Distribution In a sample of a gas or a liquid the particles are constantly colliding with each other and the walls of the container. This results in energy being transferred from one particle to another. The sample will have particles travelling at a variety of speeds and therefore kinetic energies. A plot of the number of particles with a particular energy against that energy is known as the Maxwell-Boltzmann distribution.

Maxwell-Boltzmann Distribution E mp

The effect of temperature on the Maxwell-Boltzmann curve If the temperature of the sample is increased the key features of the new Maxwell-Boltzmann curve will be: The peak of the curve is displaced to a higher energy. The height of the peak (number of particles with the most probable energy) is lower. The curve will only ‘cut’ the original curve at one point. The area under the curve remains constant.

How temperature affects rate of reaction: At higher temperatures there are many more particles that have energy greater than or equal to the activation energy (E ⩾ E a ). The particles move more quickly and there are more collisions in a given time. The more important factor is the higher proportion of collisions that are successful. EaEa The red area represents the number of particles with E ⩾ E a at the lower temperature. The hatched area represents the number of additional particles that have E ⩾ E a at the higher temperature.

The number of particles with energy greater than or equal to the activation energy is very much greater at the higher temperature. Consequently small increases in temperature can result in large increases in rate.

How concentration affects the rate of reaction Increasing the concentration of a reagent increases the number of particles in a given volume and so increases the collision rate. The number of particles with energy above or equal to the activation energy (E E a ) increases. As any reaction proceeds, the concentration of reagents decreases as they get used up. This is why the rate is greatest at the start of a reaction. At higher concentrations, the energies of the particles are not affected, so the number of particles at each energy is higher, but the most probable energy E mp does not change.

How concentration affects the rate of reaction

How surface area affects rate of reaction If one of the reactants in a reaction is a solid, the size of solid pieces will affect the rate. Smaller pieces of solid result in a faster reaction. This is because the same amount of solid has a larger surface area when it is in small pieces. This allows more collisions with particles of other reagents in a given time and, therefore, more collisions with energy greater than or equal to the activation energy (E E a ). The solids with the largest surface area are powders.

How catalysts affect rate of reaction A catalyst is a substance which alters the rate of a reaction without being consumed during the reaction. The term catalyst usually refers to a positive catalyst, which is one that increases the rate of reaction. A catalyst works by changing the reaction mechanism to one which has a lower activation energy (i.e. E cat < E a ). A catalyst does not change the Maxwell-Boltzmann curve, only the position of the activation energy. This results in many more particles being able to react.

How catalysts affect rate of reaction E cat EaEa The red area represents the number of particles with E ⩾ E a without a catalyst. The hatched area represents the number of additional particles that have E ⩾ E cat, the activation energy with a catalyst.

Energy Level Diagrams These are used to illustrate how the chemical energy changes as a reaction proceeds. You must be able to sketch these and label the ΔH, E a and E cat. For an exothermic reaction: Energy

Energy Level Diagrams For an endothermic reaction: Energy Reactants Products ΔH E cat EaEa