Introduction to Chemical Kinetics

Chemical kinetics is a branch of physical chemistry that involves the study of the rate of chemical reactions.

There are countless chemical reactions, and they all occur at different rates governed by the laws of nature. Chemical kinetics is an approach that enables us to develop mathematical models to study and analyze the characteristics of chemical reactions. Hence, it is also called reaction kinetics.

A sister branch to chemical kinetics is thermodynamics, which tells us the direction of chemical processes. Thermodynamics gives us information about the spontaneity of a reaction and equilibrium. In simple words, thermodynamics tells us whether a reaction will happen or not, while kinetics will tell how fast a reaction will take place. Both kinetics and thermodynamics are essential subjects of reaction engineering; they complement one another.

Rate of chemical reaction

What do we mean by the rate of chemical reaction?

The rate of chemical reaction is the speed at which the ingredients of a reaction are consumed or produced.

How do we measure it? We measure the reaction rate in the same way we measure the speed of moving objects in physics: distance divided by time. In the case of reaction rate, we replace distance with the change in the amount of reactants or products during the reaction. For example, in a reaction, R → P, the rate of reaction is the rate at which R is consumed or the rate at which P is produced.

Some reactions are super fast and instantaneous, e.g., neutralization, while some take months or even decades to complete, e.g., rusting of iron, weathering of rocks.

Factoring affecting rate of chemical reaction

As chemists and engineers, we cannot wait for chemical reactions to finish at their natural pace. We would like to have more control over them. Having controllability gives many advantages, such as increasing the yield of the desired product and minimizing the undesired byproducts. In this section, we will cover the list of factors that affect the rate of chemical reaction.

Nature of reactants

The reaction rate is dependent on the nature of reactants. It includes both physical and chemical forms of reactants. When a chemical reaction takes place, the chemical bonds within every individual reactant are broken, and new chemical bonds are formed to give products.

Hence, if a reactant is held by weaker and unstable bonds, it would be easy to break. Consequently, the reaction will finish faster. On the other hand, if a reactant is held by strong and stable bonds, it will take a great amount of energy to break them. And the reaction will be slow.

Reactants’ physical nature also decides the rate. It covers the phase of reactants (solid, liquid, or gas) and the particle size of reactants in the case of solid.

The rate of reaction is directly proportional to the surface area of reactants. The more the surface area, the more the reactants are exposed to one another. For example, gas or liquid reactants can be easily swirled to increase the contact area. In the case of a solid reactant, we can crush it into a fine powder, which speeds up the reaction rate.

As an example, a solid lump of limestone (calcium carbonate) dissolves in acid solution at a much slower rate than crushed powder of limestone to give calcium salt. Because crushed powder increases the surface area and so the rate. You can watch a demonstration of this on YouTube.

Concentration

Concentrations of reactants also affect the rate of reaction. Imagine yourself running in a crowded street. It is likely you will hit someone. Why? Because a crowded street has more people in a given area.

Concentration is very similar to crowdedness. When the concentration of reactants is high, individual reactant molecules are more likely to come in contact and hit one another. It, in turn, increases the collision frequency, which promotes the reaction.

This explanation will be more evident when you write the rate equation for a given reaction. You will see the direct dependence of the rate on concentration.

When the concentration of reactants is very low, it has the reverse effect on the reaction rate.

A person who walks mindlessly on a path with almost no people or traffic will be less likely to encounter a fatal accident. In the same vein, the collision frequency of reactants is small when the concentration of reactants is low. So, a reaction mixture having a low concentration of reactants has a low rate of reaction.

In the previous example, if we dissolve calcium carbonate (CaCO3) in dilute acid, the reaction would be very slow and perhaps not even noticeable by the naked eye if the solution is very dilute.

Temperature

Temperature is another very important factor that plays a critical role in chemical kinetics. In fact, the rate constant is a function of temperature.

As we increase the temperature of the reacting mixture, two things happen. First, the kinetic energy associated with reacting molecules increases, which magnifies the number of collisions. Second, the thermal energy of molecules also increases. At higher temperatures, it is easy to break chemical bonds of reactants. As a result, the rate of reaction increases with temperature.

The effects of temperature on the rate can be described with the help of the Arrhenius equation.

There is a general rule of thumb that says every 10°C rise in temperature, the rate of reaction twofold. However, this rule may not always work. 

Pressure

Pressure is the equivalent of concentration when we are dealing with gaseous reactants. When the partial pressure of reactants is high, the molecule density of reactants is high. As a result, the collision is high, and the rate of reaction is improved.

Catalyst

Catalyst is a non-reactive component in a chemical reaction. It remains chemically unaltered at the end of the reaction. Reactants and intermediate products may interact with the catalyst to find an alternative path to complete the reaction. The new path has lower activation energy than the earlier. Thus, we can say the catalyst offers a new reaction mechanism with lower activation energy, which promotes the rate of reaction.

Example

Before concluding the article, let’s take a simple chemical reaction: decomposition of N2O5.

The decomposition of N2O5 is a gaseous reaction; both reactant and products are in the gas phase. The reaction is as follows:

2N2O5(g) → 4NO2(g) + O2(g)

The concentration profile is presented below.

The data source of the above graph is from prenhall; the data was extracted using plotdigitizer from the source and replotted using matplotlib.

From the above graph, the concentration of the reactant (N2O5) decreases with time, and the concentration of products (O2 and NO2) increases. The rate of reaction is the change in concentration of the reactant (N2O5) per unit time, which is nothing but the slope at any point on the N2O5 concentration profile.

We can see the slope on the N2O5 becomes less steep as the reaction proceeds. In other words, the rate of reaction decreases with time since the concentration of the reactant decreases.