RATE OF A CHEMICAL REACTION

So far we discussed various types of chemical reactions and the nature of the reactants and products. Let us consider the following reactions:

·                 Rusting of iron

·                 Digestion of food

·                 Burning of petrol

·                 Weathering of rock

How fast is each reaction? Rank them from the slowest to fastest. How will you determine, which is the fastest and which is the slowest? One of the ways to find out how fast a reaction is as follows: Measure the amount of reactants or products before and after a specific period of time. For example, let us assume that 100 g of a substance ‘A’ undergoes a reaction and after an hour 50 g of ‘A’ is left.

A → Product

In another instance, 100 g of substance ‘C’ undergoes a reaction and after an hour, 20 g of ‘C’ is left.

C → Product

Can you say which is the faster reaction? In the first reaction, 50 g of the reactant (A) is converted into products whereas in the second reaction 80 g of the reactant is converted into products in one hour. So, the second reaction is faster. This measurement is called ‘the reaction rate’.

“Rate of a reaction is the change in the amount or concentration of any one of the reactants or products per unit time”.

Consider the following reaction

A → B

The rate of this reaction is given by

Where,

[A]– Concentration of A

[B]– Concentration of B

The negative sign indicates the decrease in the concentration of A with time.

The postive sign indicates the increase in the concentration of B with time.

Note: ‘[ ]’ represents the concentration, ‘d’ represents the infinitesimal change in the concentration.

Why is reaction rate important?

Faster the reaction, more will be the amount of the product in a specified time. So, the rate of a reaction is important for a chemist for designing a process to get a good yield of a product. Rate of reaction is also important for a food processor who hopes to slow down the reactions that cause food to spoil.

 Factors influencing the rate of a reaction

Can the rate of a reaction be changed? The rate of a reaction can be changed. For example, iron gets rusted faster in an acid than in water. Important factors that affect rate of a reaction are

i.                     Nature of the reactants

ii.                     Concentration of the reactants

iii.                     Temperature

iv.                     Catalyst

v.                     Pressure

vi.                     Surface area of the reactants

(i) Nature of the reactants

The reaction of sodium with hydrochloric acid is faster than that with acetic acid. Do you know why? Hydrochloric acid is a stronger acid than acetic acid and thus more reactive. So, the nature of the reactants influence the reaction rate.

2Na_(s) + 2HCl_(aq) → 2NaCl_(aq) + H_{2 (g)} (fast)

2Na_(s) + 2CH₃COOH_(aq) → 2CH₃COONa_(aq) + H_2(g)(slow)

 (ii) Concentration of the reactants

Changing the amount of the reactants also increases the reaction rate. The amount of the substance present in a certain volume of the solution is called ‘concentration’. More the concentration, more particles per volume exist in it and hence faster the reaction. Granulated zinc reacts faster with 2M hydrochloric acid than 1M hydrochloric acid.

 (iii) Temperature

Most of the reactions go faster at higher temperature. Because adding heat to the reactants provides energy to break more bonds and thus speed up the reaction. Calcium carbonate reacts slowly with hydrochloric acid at room temperature. When the reaction mixture is heated the reaction rate increases.

 (iv) Pressure

If the reactants are gases, increasing their pressure increases the reaction rate. This is because, on increasing the pressure the reacting particles come closer and collide frequently.

 (v) Catalyst

A catalyst is a substance which increases the reaction rate without being consumed in the reaction. In certain reactions, adding a substance as catalyst speeds up the reaction. For example, on heating potassium chlorate, it decomposes into potassium chloride and oxygen gas, but at a slower rate. If manganese dioxide is added, it increases the reaction rate.

 (vi) Surface area of the reactants

When solid reactants are involve in a reaction, their powdered form reacts more readily. For example, powdered calcium carbonate reacts more readily with hydrochloric acid than marble chips. Because, powdering of the reactants increases the surface area and more energy is available on collision of the reactant particles. Thus, the reaction rate is increased.

You will study more about reaction rate in you higher classes.