Different reactions take place with different rates. The branch of physical chemistry that deals with the study of rate of chemical reaction is called chemical kinetics.
During chemical reaction, concentration of reactants decreases while that of product increases. Thus, the rate of reaction may be defined as the decrease in the concentration of reactant or the increase in the concentration of product per unit time.
Let us consider a reaction,
A → B
When time (t) = t1, (A)1 → (B)1
When time (t) = t2, (A)2 → (B)2
If t2 > t1,
Rate of reaction = – (A)2 – (A)1/t2 – t1 = +(B)2 – (B)1/t2 – t1
Here, the negative sign indicates that the concentration of reactants decreases with time. The rate of reaction is measured in molL-1sec-1.
Factors affecting rate of reaction
There are many factors affecting the rate of reaction. Some of them are described below:
i) Size of particle: Rate of chemical reaction increases with the increase in the surface area or decrease in size of reactant particles. For example, when calcium carbonate is reacted with dilute hydrochloric acid, calcium chloride and water are formed with liberation of carbondioxide gas as:
CaCO3 + 2HCl → CaCl2 + H2O + CO2
Rate of reaction can be monitored by the evolution of carbondioxide gas. Evolution of carbondioxide gas is more, when powdered CaCO3 is used rather than lump of CaCO3.
ii) Concentration: Rate of chemical reaction is directly proportional to concentration of the reactants. When concentration is increased, the frequency of collisions between reactant molecules increases. This increases the rate of reaction. In the above given reaction, the evolution of carbondioxide is more rapid when concentrated hydrochloric acid is used rather than dilute hydrochloric acid.
iii) Temperature: The rate of reaction also depends upon temperature. Rate of reaction increases with the increase in the temperature. When temperature is increased, the kinetic energy of reactant molecules increases and their collisions become more effective. It has been found that the rate of reaction increases by two to three times upon each 10oC rise in temperature.
iv) Catalyst: Catalyst increases the rate of reaction by providing alternate path for the reaction having lower activation energy. For example, iron acts as a catalyst during the manufacture of ammonia by Haber’s process. Similarly, the decomposition of potassium chlorate is catalysed by using manganese dioxide.
Average and Instantaneous Rate
Concentration of reactants decreases with time and hence rate of chemical reaction decreases with the propagation of reaction. The rate of reaction at certain interval of time is called average rate of reaction.
To determine average rate, a graph between concentration of reactant or product is plotted against time and concentrations at given time intervals are noted and average rate is determined.
Let us consider a reaction,
A → B
Average rate = – (A)2 – (A)1/t2 – t1 = + (B)2 – (B)1/t2 – t1
or, Average rate = –Δ(A)/Δt = +Δ(B)/Δt
Similarly, the rate of chemical reaction at a particular instant of time is called instantaneous rate of reaction. To determine the instantaneous rate, a graph between concentration of reactant or product is plotted against time. Then a perpendicular is drawn from the curve to the instant of time where the rate is to be determined. Finally, the slope of the tangent at the corresponding point on the curve gives the instantaneous rate.
Instantaneous rate = – (A)2 – (A)1/t2 – t1 = + (B)2 – (B)1/t2 – t1
or, Instantaneous rate = –dA/dt = +dB/dt
The average rate becomes instantaneous rate of reaction when Δt → 0.
∴ Instantaneous rate = Δt lim→ 0 Average rate
or, Instantaneous rate = Δt lim→ 0 – Δ A/Δ B
or, Instantaneous rate = –dA/dB
