Chemical Kinetics

We know that rusting of iron is a chemical reaction. It does not happen quickly. Rusting of iron occurs slowly. However, the burning of petrol is a quick reaction. Have you ever wondered why some reactions happen quickly while others take hours or days? The answer can be found in the concept of rate of reaction.

Rate-of-reaction refers to the speed at which a chemical reaction takes place. The rate of a reaction is typically measured by monitoring changes in concentration of the reactants or products over time. The faster the rate-of-reaction, the shorter the time it takes for the reaction to occur. If the rate-of-reaction is slower then the reaction will take a longer time to complete.

The rate of a chemical reaction can be expressed as change in concentration of a substance involved in the reaction divided by time-interval over which the change in concentration occurs. The units for reaction rates depend on the specific reaction being studied. In a chemical reaction, the concentration of the reactants decrease over time. The concentration of products rises over time.

Let us consider a chemical reaction in which reactant A reacts with reactant B to form product C. With the passage of time the concentrations of reactant A and reactant B will decrease. The rate for this particular reaction in terms of reactant A is expressed as −Δ[A]/Δt. Δ represents the concept of change. The negative-sign is because of the fact that concentration of reactant is decreasing over time.

The rate in terms of reactant B is expressed as −Δ[B]/Δt. The negative-sign here is because of decrease in concentration of reactant B. The rate in terms of product C can be expressed as Δ[C]/Δt. There is no negative-sign here. This is because the concentration of products is increasing with the passage of time.

To calculate the rate of a chemical reaction, you need to determine the change in concentration of a reactant or product over a specific time interval. Let us say Initial concentration of a reactant is expressed as [A]₁. Final concentration of the reactant is expressed as [A]₂. So the change in concentration would be equal to final concentration minus initial concentration. The initial time is expressed as t₁. The final time is expressed as t₂. Time-interval is equal to final time minus initial time.

To calculate the rate of reaction first of all subtract the final concentration of the reactant from the initial concentration of the reactant to obtain the change in concentration of reactant. Similarly, subtract the final time from the initial time to determine the time-interval. Finally, divide the change in concentration of reactant by the time-interval to obtain the rate-of-reaction.

Let us consider an example to calculate the rate of a chemical reaction. Suppose we have a reaction where the initial concentration of reactant A is 0.1M. The final concentration of reactant A is 0.05M. The initial time is 0 seconds and the final time is 30 seconds.

First, we need to calculate the change in concentration of reactant A and the time-interval. The change in concentration of reactant A is -0.05M. Calculated time-interval is 30 seconds. Now we shall divide the change in concentration by time interval. Finally the calculated rate of reaction is 0.00167M/s. The negative-sign indicates that reactants are being consumed. This shows that 00167M of reactant A is being consumed every second during this reaction.

Several factors effect the rate of a chemical reaction. We shall briefly discuss these factors. The concentration of reactants plays a significant role in determining the rate-of-reaction. As the concentration of reactants increases, the frequency of successful collisions between particles increases. Greater number of successful collisions between reactant particles means more reactants will be converted into products in less time. This leads to a higher rate-of-reaction.

Temperature has a direct effect on the rate of a reaction. When the temperature is increased, the kinetic energy of the particles also increases. This results in more frequent and energetic collisions. More frequent and energetic collisions means the reactants will be converted into products faster. Therefore, higher temperature generally leads to higher rate of reaction.

In reactions involving solid reactants, the surface area of the solid plays a role in the reaction rate. By increasing the surface area of a solid reactant, more particles are exposed. This provides a larger area for reactant particles from the other phase such as gas or a liquid to collide. This increased surface area enhances the chances of successful collisions and leads to a higher reaction rate.

Catalysts are substances that speed up chemical reactions without being consumed or permanently altered in the process. Catalysts work by providing an alternative reaction pathway that requires a lower Activation Energy compared to the uncatalyzed reaction. Activation energy is the minimum energy required for a chemical reaction to occur. By lowering the activation energy, catalysts enable reactant particles to more easily overcome the energy barrier and proceed to the formation of products.

Homogeneous-catalysts are catalysts that are in the same phase as the reactants in a reaction. Typically, homogeneous catalysts are used in reactions that occur in solutions or gases. An example of a homogeneous-catalyst is the use of acids or bases in particular chemical reactions. For instance, sulfuric acid can act as a catalyst in the esterification-reaction. In this reaction sulfuric acid helps convert an alcohol and carboxylic acid into an ester and water. Sulfuric acid, alcohol and carboxylic acid are all in the same liquid phase.

Heterogeneous-catalysts are catalysts that exist in a different phase from the reactants. Typically, heterogeneous catalysts are solid substances that facilitate reactions involving gases or liquids. An example of a heterogeneous-catalyst is platinum which is used in catalytic converters in cars. The catalytic converter contains a honeycomb like structure coated with platinum. The platinum catalyst helps convert harmful exhaust gases, such as carbon monoxide and nitrogen oxides, into less harmful substances like carbon dioxide, nitrogen gas, and water.