Rate of a reaction

The speed of a reaction can be understood from the time of completion of the reaction. Thus, at room temperature, the reaction, 2 NO2 + O3 → N2 O5 + O2 is complete within 1/10 seconds whereas the completion of the reaction of the reaction H+ + OH− → H2O

takes place in less than a millisecond. In order to understand whether the reaction takes place fast or slow, the term ’rate of reaction’ is introduced. So we can define the rate of a reaction in terms of change of concentration of substances with respect to time.

Rate of a chemical reaction is the change of concentration of the reactants and products per unit time.

Various factors affect the rate of a reaction. Some of them are discussed below.

Nature of the reactant

Let us do a simple activity. Take a little zinc powder and aluminium powder in two different test tubes. Add about 10 ml dilute hydrochloric acid to each test tube. Aluminium and zinc both react with dilute HCl to evolve hydrogen gas. However, hydrogen bubbles out faster in a test tube containing aluminium than that containing zinc. This shows that aluminium reacts faster than zinc with dilute HCl. Chemically, aluminium is more reactive than zinc.

In general, more the chemical reactivity of the element, more is the rate of the reaction.

Size of the particles

Let us do a simple activity. Take a lump of zinc granule in a test tube. Take a little zinc metal powder in another test tube. Put 10 ml dilute HCl in both the test tubes. Warm both the test tubes for one minute. Zinc reacts with dilute HCl to produce hydrogen gas. However, hydrogen gas bubbles out faster in the test tube containing zinc powder. This shows that smaller particle size offers more surface area for the reaction and hence the rate of the reaction is more.

In general, smaller the particle size of the reacting substances, more the rate of the reaction.

This factor has a direct application in medicine. Medicine if given in a colloidal form rather than in lump form, offer more surface area and are easily assimilated by a human body. So antacids like milk of magnesia, Gelusil are made and given in the colloidal form so that they become more effective.

Concentration

Let us do a simple activity. Place a small piece of marble in two different test tubes. Add little dilute HCl to one test tube and little concentrated HCl to other test tube. Marble is made of calcium carbonate. It reacts with HCl to evolve carbon dioxide gas. The evolution if CO2 gas takes place faster in the test tube containing concentrated HCl. This is seen by the bubbles of CO2 coming out from the solution.

Place a small piece of ferrous sulphide FeS, in one test tube. Place two small pieces of ferrous sulphide in the other test tube. Put about 10 ml concentrated HCl in both the test tubes. Warm both the test tubes for one minute. Ferrous sulphide reacts with HCl to produce H2S gas. However, H2S gas is produced at a faster rate in a test tube containing two pieces of FeS. This shows that if the concentration of the reacting substances is more, then the rate of the reaction is more.

In general, the rate of a chemical reaction is proportional to the concentration of the reacting substances.

Temperature

Let us do a simple activity. Take a small magnesium ribbon. Put it in water. Magnesium does not react with water at room temperature. Now take out the magnesium ribbon. Pass steam over the magnesium ribbon. It decomposes steam evolving hydrogen gas. This shows that magnesium reacts with water at higher temperature. Similarly, marble reacts with dilute HCl at a faster rate when the solution is heated.

Effect of temperature on a chemical reaction is seen in everyday life also. We keep food items in a refrigerator to slow down the rate of their decomposition. Milk is converted to curd faster in summer than in winter.

In general, rate of a chemical reaction is more, if temperature is higher.

Catalyst

Sometimes, the rate of a chemical reaction is influenced by the presence of an entirely foreign substance called catalyst. A substance which increases the speed of the reaction without getting consumed itself is called a catalyst. The catalyst helps to bring about the reaction at a faster rate or at a lower temperature. For example, the decomposition of potassium chlorate and hydrogen peroxide takes place at a faster rate in presence of a catalyst–manganese dioxide.

The opposite of a catalyst is inhibitor which slows down the rate of the reaction. For example, the decomposition of hydrogen peroxide is slowed down by dilute acids.

In general, the rate of some reactions is influenced by the presence of a catalyst.

Industrial applications of catalysts

Catalysts find variety of applications in industries. A number of chemicals are prepared all over the world by catalytic reactions. Petrochemicals, fertilizers, gases, food materials and a number of other chemicals are produced in this way. Different reactions require different catalysts. These days, catalyst design has become a challenging problem to chemists in preparative chemistry.

Some applications of catalysts in industrial processes are given below.

1. In the manufacturing of sulphuric acid by contact process, vanadium pentoxide is used as a catalyst to convert SO2 to SO3.

2. In the manufacture of ammonia by Haber process, finely divided iron is used as a catalyst.

3. In the hydrogenation of oils, i.e. conversion of vegetable oil to vanaspati ghee, finely divided nickel is used as a catalyst.

4. In cracking of petroleum i.e. conversion of large hydrocarbons to small hydrocarbons, aluminium oxide is used as a catalyst.

5. Methyl alcohol i.e. methanol is prepared by passing a mixture of CO and H2 over a catalyst Cu / ZnO at 3000C at a high pressure.

6. Enzyme catalysts are used in the preparation of detergent powders.

7. In the manufacture of nitric acid by Ostwald’s process, platinum and rhodium metals are used as c8. In the catalytic converters of automobiles, a mixture of platinum, palla- dium and rhodium is used as a catalyst to reduce the pollution caused by the exhaust gases to atmosphere.

9. Phenol is converted to cyclohexane by using palladium as a catalyst. The reaction is called hydrogenation.

10. In the Deacon’s process of making chlorine from HCl, CuCl2 is used as a catalyst.