Factors Affecting The Rate Of A Chemical Reaction
Chemical Kinetics of Class 12
It may be observed that the increase in the total number of collision per unit volume per unit time (collision frequency) is not so much responsible for the higher reaction rate as is the increase in the fraction of effective collisions. Let us, for eg, calculate the increase in collision frequency when temperature increases from 298 to 308 K. As we know that collision frequency is directly proportional to the square root of absolute temperature, therefore, the rate of collision frequencies at these temperatures follows as
From the above ratio it is clear that there is a insignificant increase in the collision frequency. Hence it can not explain the observed increase in the ratio of the reaction with increase in temperature.
Let us now consider the effect of increase in temperature on the number of effective collisions.
Now, as we know that the rise in temperature increases the kinetic energy of the molecules. therefore the energy distribution curve gets flattened and shifts towards higher energy region. A close revels examination of the curves in the graph clearly reveals that the fraction of molecules possessing higher kinetic energy i.e. energy greater than threshold energy, as indicated by shaded portion becomes almost double and therefore, the rate of reaction almost doubles for 10° rise in temperature. Thus, increase in the rate of reaction with increase in temperature is mainly due to increase in no. of collisions which are energetically effective.
The reaction rate dependence on temperature can also explained by Vant Hoff’s equation.
Nature of reacting substance
The nature of reacting substances affect, the rates significantly. For e.g. the oxidation of ferrous (Fe+2) by KMnO4 in acidic medium is practically instaneous. On the other hand, oxidation of oxalate ions by KMnO4 in acidic medium is comparatively much slower.
A catalyst is a substance, which increases the rate of a reaction without itself being consumed at the end of the reaction, and the phenomenon is called catalysis. There are some catalysts which decrease the rate of reaction and such catalysts are called negative catalyst. Obviously, the catalyst accelerating the rate will be positive catalyst. However, the term positive is seldom used and catalyst itself implies positive catalyst.
Catalyst are generally foreign substances but sometimes one of the product formed may act as a catalyst and such catalyst is called “auto catalyst” and the phenomenon is called auto catalysis. Thermal decomposition of KClO3 is found to be accelerated by the presence of MnO2. Here MnO2 (foreign substance) acts as a catalyst.
2KClO3 + [MnO2] → 2KCl + 3O2↑ + [MnO2]
MnO2 can be received in the same composition and mass at the end of the reaction. In the permanganate titration of oxalic acid in the presence of bench H2SO4 (acid medium), it is found that the titration in the beginning there is slow discharge of the colour of permanganate solution but after sometime the discharge of the colour become faster. This is due to the formation of MnSO4 during the reaction which acts as a catalyst for the same reaction. Thus, MnSO4 is an “auto catalyst” for this reaction. This is an example of auto catalyst.
2KMnO4 + 3H2SO4 + 5H2C2O4 → K2SO4 + 8H2O + 10CO2 + 2MnSO4
General characteristics of catalyst
- A catalyst does not initiate the reaction. It simply fastens it.
- Only a small amount of catalyst can catalyse the reaction.
- A catalyst does not alter the position of equilibrium i.e. magnitude of equilibrium constant and hence ΔG0. It simply lowers the time needed to attain equilibrium. This means if a reversible reaction in absence of catalyst completes to go to the extent of 75% till attainment of equilibrium, and this state of equilibrium is attained in 20 minutes then in presence of a catalyst also the reaction will go to 75% of completion before the attainment of equilibrium but the time needed for this will be less than 20 minutes.
A catalyst drives the reaction through a different route for which energy barrier is of shortest height and hence Ea is of lower magnitude. That is, the function of the catalyst is to lower down the activation.
Ea = Energy of activation in absence of catalyst.
E′a = Energy of activation in presence of catalyst.
Ea – E′a = lowering of activation energy by catalyst.
If k and kcat be the rate constant of a reaction at a given temperature T, and Ea and E′a are the activation energies of the reaction in absence and presence of catalyst, respectively, the
Since Ea > Ea′ so kcat • k. the ratio gives the number of times the rate of reaction will increase by the use of catalyst at a given temperature and this depends upon Ea -. Greater the value of Ea –, more number of times kcat is greater than k.
The rate of reaction in the presence of catalyst at any temperature T1 may be made equal to the rate of reaction in absence of catalyst but for this sake we will have to raise the temperature. Let this temperature be T2 then