Chapter 1: Kinetics

The rate equation

The rate of reaction is the speed at which a chemical reaction takes place and has units’ mol dm-3s-1.

The rate of reaction can be calculated by;

Rate of reaction= (Change in concentration)/Time

The reaction rate in this case is measured in units of mole dm-3 s-1.

Rate of a reaction shows how fast reactants are converted into products. It depends in the concentration of the reactants and a rate constant.

Example:

A+B——->C

Rate = k[A]m[B]n

The constants m and n show the order of the reaction with respect to that species. This means that different species can have more of an effect on the reaction than others.

The total order of reaction for this chemical reaction can be found as the sum if the separate orders.

Total order= m + n

Orders of Reaction

Orders of reaction go from zero to second order. This means that changing the concentration of reactants can have different effects on the whole reaction.

            Zero order- The rate of reaction which doesn’t depend upon concentration of reactant is zero order reaction. Its unit is molL-1-1.

First Order

The concentration of the species and the rate are directly proportional.

Rate = k[A]

Second order Reaction

The rate is proportional to the concentration squared.

Rate = k[A]2

 

Rate Constant (k)

The rate constant for a reaction is constant when the reaction temperature is constant.

Collision Theory

Reactions can only occur when collisions take place between particles have sufficient energy. The energy is usually needed to break the relevant bonds in one or either of the reactant molecules. The minimum energy is called the activation energy.

Measuring Reaction Rate

The rate of reaction is defined as the change in concentration of the substance in unit time. Its unit is mold m-3s-1.

Effect of increasing temperature

At higher temperature the energy of the particles increases. They collide more frequently and more often with energy greater than the activation energy. More collisions result in a reaction.

As the temperature increases, the graph shows that a significantly bigger proportion of the particles have energy greater than the activation energy, so the frequency of successful collision increases.

The Arrhenius Equation

It is an equation that shows how the rate constant k and temperature are related exponentially. It relates three factors, the steric factor, and the collision factor and activation energy, to the rate constant k.

K = Ae-Ea/RT

Where,

K = Rate constant

A= Pre-exponential factor

Ea = Activation Energy

R = gas constant

T = temperature in Kelvin