Factors Affecting the Reaction Rate

In a chemical reaction, reactants are converted into products. In this process, the concentration of the reactant decreases and the concentration of the products increases. The rate of a reaction is defined according to the change in concentration of either reactants or products within a unit time period.

If the products are gases, it is easy to measure the change in pressure instead of measuring concentration.

The reaction rate can be measured in two ways.

• The first way is to measure the time taken to get a desired amount of products.
• The other way is to measure the amount of products produced in a determined period of time.  

The reaction rate tells us how fast the reactants turn into products. The rate of a reaction depends on multiple factors. By changing these several factors, the reaction rate can be changed.

The factors affecting the reaction rate are,

1. Physical nature of the reactants
2. Concentration
3. Temperature
4. Catalysts
5. Pressure (gaseous systems)
6. Light and radiation
7. Current that flows

01. Effect of the physical nature of reactants on reaction rate

Physical nature of the reactants refers to the surface area of the reactants. If the surface area is high, the number of collisions between the reactants is also high. It leads to an increase in the number of collisions that occur at the correct orientation. Therefore, it increases the fraction of reactants that are higher than the activation energy. Thus, the reaction rate increases.

The effect of the physical nature of the reactants can be studied using the following experiment. When calcium carbonate is added to a dilute acid like HCl, it will react, evolving Carbon dioxide gas.

Procedure

1. Two samples of CaCO₃ with the same masses are taken, and one sample is crushed into a powder while the other sample remains as larger crystals.
2. Both samples are separately added to two boiling tubes, and they are both placed in the same water bath to provide the same temperature for both samples.
3. Then the samples are added similar volumes of 0.01 M HCl solution, and observe the rate of the evolution of the air bubbles.

In the above experiment it can be observed that rate of the evolution of the air bubbles in the boiling tube with crushed CaCO₃ is high. In crushed CaCO₃, the surface area of the reactants is higher than the crystalls. Therefore, the reaction rate is high.

02. Effect of concentration of reactants on the reaction rate

The concentration of a reactant is the number of reactants that exist in a unit volume. When the concentration is increased, it increases the number of collisions between reactants. It leads to an increase in the number of collisions that occur at the correct orientation. Therefore, it increases the fraction of reactants that are higher than the activation energy. Thus, the reaction rate increases.

The effect of concentration on the rate of the reaction can be observed from the following experiment. The Magnesium metal reacts with dilute acids and liberates H₂ gas as follows.

Procedure

1. Two identical strips of cleaned Mg metal, with 2 cm are taken to two different boiling tubes
2. Both boiling tubes are filled with 10 cm³ of distilled water, followed by 0.1 M HCl. One sample is added 2 drops of 0.1 M HCl, and the other sample is added 4 drops of 0.1 M HCl separately.
3.  Two boiling tubes are placed in the same water bath to provide the same temperature for both samples. And it observes the rate of evolution of gas bubbles in each tube.

In the above experiment, it can be observed that the rate of evolution of the air bubbles in the boiling tube that was added 4 drops of HCl is high. This is because the concentration of HCl is high in that boiling tube. Therefore, the reaction rate is high.

03. Effect of temperature on the reaction rate

Temperature is directly proportional to the kinetic energy. When the temperature is increased, the kinetic energy of the molecules is also increased. The relation between the kinetic energy and the temperature can be given as follows.

Where

• K = Average kinetic energy (J)
• R = Universal gas constant (8.314 Jmol^-1K^-1)
• T = Temperature (K)
• N_A = Avogadro constant (6.022 × 10 23 mol^-1)

As the kinetic energy of the molecules increases when the temperature is increased, it leads to an increase in the collisions between the molecules. Thus, it increases the collisions occurring in the correct orientation. Also, since the molecules have a higher average kinetic energy, the fraction of molecules that exceed the activation energy of the reaction is also high. Therefore, when the temperature increases, the reaction rate also increases.

The temperature effect on reaction rate can be observed using the following experiment. In acidic medium, Oxalate ion (C₂O₄^2-) is reduced to Carbon dioxide (CO₂) gas by permanganate ion (MnO₄^-). In this reaction purple color MnO₄^- ion is reduced to Mn²⁺, which is colorless in acidic medium. So, in this reaction, it can be observed that the color of the reaction medium turns from purple to colorless, and air bubbles are evolved.

Procedure

1. Two identical samples are prepared by adding 10 cm³ of 0.01 M KMnO₄ solution, followed by 25 cm³ of 0.01 M Na₂C₂O₄ solution and 10 cm³ of 1M H₂SO₄ solution to two flasks.
2. One sample is placed in a cold-water bath, which is maintained at room temperature (25 °C), and the other sample is placed in a hot water bath, which is maintained above 60 °C.

It can be observed that the purple color of the sample placed in the hot water bath turns colorless faster than the sample placed in the cold water bath. This experiment explains that when the temperature is increased, the reaction rate increases

04. Effect of catalysts on the reaction rate

Catalysts are added to a reaction medium to increase the rate of the reaction. Catalysts introduce for a reaction an alternative path that has lower activation energy. But it does not affect the overall chemical change of the reaction.

The effect of catalysts can be observed in the following experiment. Hydrogen peroxide (H₂O₂) will dissociate into H₂O and O₂ gas. This reaction can be catalyzed by introducing NaOH.

Procedure

1. Two samples of 10 cm³ H₂O₂ are prepared in two boiling tubes.
2. One sample is added 10 cm³ of 0.1 M NaOH, and the other sample is added 10 cm³ of distilled water.

It can be observed that the rate of the liberation of the air bubbles is higher in the sample that was added NaOH than in the sample that was added distilled water.

05. Effect of pressure on the reaction rate

Changing the pressure in a liquid or a solid system does not affect the reaction rate. But the change in pressure in a gaseous system affects the reaction rate. When the pressure increases, the reaction rate also increases. The increase in pressure can be done in two ways.

1. Adding more reactants to the system
2. Decreasing the volume of the system

Either way, it increases the concentration of the reactants. Therefore, it increases the number of molecules in a unit volume. It leads to more collisions that occur in the correct orientation. So, it increases the number of collisions that surpass the activation energy. Thus, the reaction rate increases. Mathematically, this phenomenon can be explained using the ideal gas equation.

Where,

• P = pressure
• V = volume of the system
• n = number of mol
• R = universal gas constant
• T = absolute temperature
• C = concentration

The above equation shows that the pressure of a gas is directly proportional to the pressure of the gas. When adding more reactants to the system, it increases the number of moles and leads to an increase in concentration. When decreasing the volume of the system, it also increases the concentration. Since the reaction rate is directly proportional to the concentration, the rate increases.

In the manufacturing of ammonia from the Harbor process, it uses high pressure is order to increase the reaction rate.

06. Effect of radiation on the reaction rate

The rate of the reaction is affected by the radiation its intensity. Electromagnetic radiation, like UV, gamma, and X-rays, supplies energy to the molecules, which helps to cleave chemical bonds. In other words, radiation increases the kinetic energy of the molecules, or excites the molecules.  The energy of the radiation helps the reactants to surpass the activation energy barrier more easily. Thu, as the reaction rate increases.

Not every reaction is affected or induced by radiation. For a reaction that is induced by radiation, it only occurs within a certain frequency range because chemical bonds respond only to that range. As an example, we can take photosynthesis. That reaction occurs within the visible range of the electromagnetic spectrum.

When this requirement of the reaction is fulfilled and the reaction is initiated, the reaction rate of that reaction is based on the intensity of the radiation.

Methane chlorination (Free radical chain reaction of alkanes) is an example of this type of creation. When the intensity of light increases, that increases the rate of chlorination.

Photosynthesis is a well-known example of the radiation-induced reaction. This reaction is induced by the visible light.

07. Effect of current flow on the reaction rate

Current flow is directly proportional to the energy. When it increases the current flow, it increases the energy of the reactants. Therefore, it will help to surpass the activation energy of the reaction. It leads increase in the number of molecules that have higher energy than the activation energy. Thus, it increases the rate of the reaction. Electrolysis of water is an example of this.