Chemical Kinetics

In a zero order reaction, the rate of the reaction is independent of the concentration of the reactants. This means that, although you change the concentration of the reactant, the rate at which the reaction occurs remains constant. One example of a zero order reaction is the decomposition of hydrogen peroxide into water and oxygen gas. This reaction is catalyzed by an enzyme called catalase. Catalase is seen in many living organisms.

The rate equation is simple and does not depend on the concentration of hydrogen peroxide. Rate is equal to k for zero order reaction. k is a rate constant. k depends only on reaction conditions such temperature and pressure. The rate of the reaction is determined solely by the concentration of the catalase enzyme. This means that regardless of the initial concentration of hydrogen peroxide, the reaction will proceed at the same rate as long as there is enough catalase present.

When we plot a graph between the rate of a zero order reaction and concentration, we get a straight-line parallel to x axis. This line shows that although the concentration rises, rate of reaction remains constant. This shows that rate of a zero order reaction is independent of concentration of the reactant.

In a first order reaction, the rate of reaction is directly proportional to the concentration of the reactant. As the concentration of the reactant decreases, the rate of the reaction also decreases proportionally. In the same way, if we increment the concentration of the reactant, the rate of reaction also increases. The rate equation for a first order reaction with respect to a Reactant A is illustrated here.

When we plot a graph between the rate of first order reaction and concentration, we get a straight-line that originates from the origin. As we can see as we increment the concentration, the rate of reaction also increases. This shows that the rate of a first order reaction is directly proportional to the concentration of the reactant.

Let us derive the units for the rate constant of a first order reaction. We shall start with the rate equation for a first order reaction. The unit of rate is molL⁻¹s⁻¹. The concentration of the Reactant A has units of molL⁻¹. Now substitute the units of rate and concentration into the rate equation.

After substituting the values, we shall Isolate 'k' by dividing both sides of the equation by the units of concentration. This is because we we want to take k on one side of equation and units on other side of equation. Finally simplify the units by canceling out molL⁻¹. Now the units of the rate constant for a first order reaction is reciprocal-seconds.

In a second order reaction of two different reactants, the rate of the reaction is directly proportional to the product of the concentrations. In a second order reaction of a single reactant, the rate of the reaction is directly proportional to the square of the concentration. The rate equations for a second order reactions involving single reactant and double reactants are shown here. If the concentrations of both reactants 'A' and 'B' are doubled in a two-reactant second order reaction, the rate of the reaction will rise by a factor of four. Alternatively, in a single-reactant second order reaction, if the concentration of 'A' is doubled, the rate will rise by a factor of four.

Let us plot a graph for a second order reaction involving single reactant. The concentration is taken on the x axis. Rate is plotted on the y axis. We can see that the graph of second order reaction shows a curved line. This curved line shows that the rate of reaction is proportional to the square of the concentration of the reactant. For example, if we increment the concentration of the reactants by two times, the rate of reaction will rise by four times.

Now we shall derive the units of the rate constant for a second order reaction. We shall substitute the units of rate and the concentration into the second order rate equation. After that isolate 'k' by dividing both sides of the equation by the square of units of concentration. Now we shall simplify equation by canceling common units like molL⁻¹. Now the units of the rate constant for a second order reaction is L mol⁻¹s⁻¹.

The overall order of reaction is determined by adding up the individual reaction orders with respect to each reactant. Let us take an example of a reaction in which the Reactant A is a zero order reactant. It means order of Reactant A is zero. The Reactant B is a first order reactant. The order of Reactant B is one. So adding both values gives us a value of 1. This shows that this is a first order reaction. If the sum of order of reactants is 2, then it is a second order reaction.

The instantaneous rate of a chemical reaction refers to the rate of change of reactants or products at a specific moment in time during the course of the reaction. The instantaneous rate can be measured at any time during the progression of a reaction. Instantaneous rate allows us to understand the behavior of reaction at different points in time.

The initial rate of a chemical reaction refers to the rate at which products are formed or reactants are consumed at the beginning of the reaction. The initial rate of a chemical reaction is measured by determining the change in concentration of reactants or products over a very short time interval. This measurement is typically taken at the start of the reaction when the time is almost 0.

The average rate of a chemical reaction refers to the overall rate of the reaction over a specified period of time. To calculate the average rate, we measure the concentration of reactions at two different times. After that we measure the change in concentration and time interval. Then we divide change in concentration by time interval. The average rate gives a broader view of how the reaction is progressing over time.

The half life of a chemical reaction is the time it takes for half of the reactants to be converted into products. For example, if the half life of a reaction is ten seconds, it means that it will take ten seconds for the concentration of a reactant to reduce to half its initial value. Half life of a reaction is represented by t(1/2).