Scope of chemical kinetics
Chemical kinetics is the study of the rates and the mechanism of chemical reactions. Commonly the measure of how fast the products are formed and the reactants consumed is given by the rate values.
The study of chemical kinetics has been highly useful in determining the factors that influence the rate, maximum yield and conversion in industrial processes. The mechanism or the sequence of steps by which the reaction occurs can be known. It is also useful in selecting the optimum conditions for maximum rate and yield of the chemical process.
1. Rate of chemical reactions
The rate of a reaction tells us how fast the reaction occurs. Let us consider a simple reaction.
A + B -- > C + D
As the reaction proceeds, the concentration of the reactant A and B decreases with time and the concentration of the products C + D increase with time simultaneously. The rate of the reaction is defined as the change in the concentration of any reactant or product in the reaction per unit time.
For the above reaction,
Rate of the reaction
1. Rate of disappearance of A
2. Rate of disappearance of B
3. Rate of appearance of C
4. Rate of appearance of D
During the reaction, changes in the concentration is infinitesimally small even for small changes in time when considered in seconds. Therefore differential form of rate expression is adopted. The negative sign shows the concentration decrease trend and the positive sign shows the concentration increase trend.
Rate = concentration change / time taken = - ∆ [A] / ∆t
= -d[A] / dt = -d[B] / dt = -d[C] / dt = -d[D] / dt
For a general balanced reaction, written with stoichiometries like x,y, for the reactant and l,m for the product, such as
xA + yB -- > lC + mD. The reaction rate is
Rate = - (1/ x) d[A] / dt = - (1/ y) d[B] / dt = + (1/ l) d[C] / dt =
+ (1/ m) d[D] / dt
For example : In the reaction,
H2 + Br2 -- > 2HBr
The overall rate of the reaction is given by
Rate = - d[H2] / dt = - d[Br2] / dt = ½ d[HBr] / dt
Consider the reaction, 2NO + 2H2 -- > N2 + 2H2O
Rate = - 1/2 d[NO] / dt =- 1/2 d[H2] / dt = d[N2] / dt = 1/2 d[H2O] / dt
Units of Rate
Reaction rate has units of concentration divided by time. Since concentration is expressed in mol lit-1 or mol dm-3 the unit of the reaction rate is mol lit-1 s-1 or mol dm-3 s-1.
2. Factors influencing reaction rates
There are number of factors which influence the rate of the reaction. These are :
i. Nature of the reactants and products
ii. Concentration of the reacting species
iii. Temperature of the system
iv. Presence of catalyst
v. Surface area of reactants
vi. (vi) Exposure to radiation
(i) Effect of nature of the reactant and product
Each reactant reacts with its own rate. Changing the chemical nature of
any reacting species will change the rate of the reaction. For example, in halogenation reactions, the reactions involving iodine is found to be slower than those involving chlorine.
In case of products, some of them are capable of reacting back to form reactants or some other kind of products. In such cases, the overall rate will be altered depending on the reactivity of the products.
(ii) Effect of reacting species
As the initial concentration of the reactants increase in the reaction mixture, the number of reacting molecules will increase. Since the chemical reaction occurs when the reacting species come close together and collide, the collisions are more frequent when the concentrations are higher. This effect increases the reaction rate.
(iii) Effect of temperature
Increase in temperature of the system increases the rate of the reaction. This is because, as the temperature increases the kinetic energy of the molecules increases, which increases the number of collisions between the molecules. Therefore the overall rate of the reaction increases. This condition is valid only for endothermic reaction. For exothermic reaction the overall rate decreases with increasing temperature.
(iv) Effect of presence of catalyst
A catalyst is a substance that alters the rate of a chemical reaction, while concentration of catalyst remaining the same before and after the reaction. The addition of catalyst generally increases the rate of the reaction at a given temperature. Also, catalyst is specific for a given reaction.
(v) Effect of surface area of reactants
In case of reactions involving solid reactants and in case of heterogeneous reactions, surface area of the reactant has an important role. As the particle size decreases surface area increases for the same mass. More number of molecules at the surface will be exposed to the reaction conditions such that the rate of the reaction increases. Thus the reactants in the powdered form (or) in smaller particles react rapidly than when present in larger particles.
(vi) Effect of radiation
Rates of certain reactions are increased by absorption of photons of energy. Such reactions are known as photochemical reactions. For example, H2 and Cl2 react only in the presence of light. With increase in the intensity of the light (or) radiation, the product yield increases. For photosynthesis light radiation is essential and the process does not proceed in the absence of light.
3. Rate law
According to concepts of chemical kinetics, the rate of the reaction is proportional to the product of the initial concentration of all the reactants with each reactant concentration raised to certain exponential powers.
Consider a general reaction pA + qB cC + dD.
The rate law is given by the expression,
Rate . [A]p [B]q \Rate = k[A]p [B]q
where k is proportionality constant also known as the rate constant or velocity constant of the reactions. p and q represent the order of the reaction with respect to A and B. The values of k, p and q are experimentally determined for a given reaction. Values of p and q need not be same as the stoichiometric coefficients of the reaction.
In the above general equation k represents the rate constant. Rate constant or velocity constant (or) specific reaction rate is defined as the rate of the reaction when the concentration of each of the reactants is unity in the reaction.
When concentration of A and B is unity then, the rate constant is equal to the rate of the reaction. When the temperature of the reaction mixture changes, the value of rate constant changes.
4. Order of the reaction
Order of a reaction is defined as the sum of the exponential powers to which each concentration is raised in the rate expression. For example, if the overall rate is given by the expression
Rate = k[A]p [B]q
Then, the overall order of the reaction is (p+q). The order with respect to A is p. The order with respect to B is q. If p=1; q=0 and vice versa, the order of the reaction is 1, and the reaction is called first order. If p=1, q=1, the order of the reaction is 2 and the reaction is called second order and so on.
A zero order reaction is one where the reaction rate does not depend upon the concentration of the reactant. In this type of reaction, the rate constant is equal to the rate of the reaction.
5. Unit of rate constant
In general, rate expression for the reaction,
pA + qB -- > cC + dD
Rate = k [A]p [B]q
K= Rate / [A]p [B]q
The unit for the rate constant `k' depends upon the rate of the reaction, the concentration of the reactants and the order of the reaction.
In the case of the first order reaction.
K= Rate / [A]1 [B]2
K= mol dm -3 sec-1 / mol dm-3
k = sec-1 for first order reaction.
Similarly unit of k = mol-1 dm3 sec-1 for second order reaction unit of k = mol-(n-1) dm3(n-1) sec-1 for nth order reaction
Following are the important differences between rate and rate constant of a reaction
Rate of reaction
1. It represents the speed at which the reactants are converted into products at any instant.
2. At any instant of time, the rate depends upon the concentration of reactants at that instant.
3. It decreases as the reaction proceeds.
4. Rate of rate determining step determines overall rate value.
Rate constant of reaction
1. It is the constant of proportionality in the rate law expression.
2. It refers to the rate of a reaction at the specific point when concentration of every reacting species is unity.
3. It is constant and does not depend on the progress of the reaction.
4. It is an experimental value. It does not depend on the rate determining step.