Contents
Factors Influencing Rates of Chemical Reaction
There are a number of factors which influence the rate of a reaction. Some of the important factors are:
1) Concentration of the reacting species.
2) Temperature of the system
3) Nature of the reactants and products.
4) Presence of a catalyst.
5) Surface area.
6) Exposure to radiation.
1) Concentration of the Reactants
a) The rate of a chemical reaction keeps on decreasing with time. The concentration of the reactants is maximum at the start of the reaction and, therefore, the rate of change of concentration is also maximum.
b) As the concentration of the reactants decreases, it brings about the corresponding decrease in the rate of the reaction. This means that the rate of a reaction is directly proportional to the concentration of the reactants.
For example : The rate of burning of wood depends upon the concentration of oxygen. A piece of wood burns slowly in air (containing about 20 % oxygen) but burns rapidly in pure Oxygen (i.e. 100% oxygen ) because the concentration of oxygen in air is less.
2) Temperature of the System
An increase in the temperature increases the rate of almost all chemical reactions. A decrease in temperature decreases the rate. The reaction rate for most of the chemical reactions becomes almost double, for every 10°C rise in temperature.
3) Nature of reactants and the products
Rates of reactions are influenced by the nature of reactants and products. A chemical reaction involves the breaking of old bonds and formation of new bonds. The reactivity of a substance can be related to the ease with which the specific bonds are broken or formed and the number of such bonds involved.
For example: the oxidation of nitric oxide to nitrogen dioxide takes place fairly rapidly while oxidation of carbon monoxide to carbon dioxide takes place slowly.
2NO2 + O2 ——–> 2 NO2 (Fast)
2CO + O2 ——–> 2 CO2 (slow)
The reacting species appear to be very similar to each other, still they differ in reaction rates.
4) Presence of catalyst
A catalyst is a substance which influences the rate of a reaction without undergoing any chemical change itself.
Many reactions are made to proceed at an increased rate by the presence of certain catalysts.
For example: A mixture of H2 and O2 does not react at room temperature. However, in the presence of a catalyst such as finely divided platinum, the reaction becomes quite vigorous.
5) Surface area
The larger the surface area of the reactants, the faster is rate of reaction. If one of the reactants is a solid, then the rate of the reaction depends upon the state of sub-division of the solid. Finely divided solids react faster than massive substances.
For example: a log of wood burns slowly but if it is cut into small wooden chips, the burning takes place rapidly. The total surface area of the smaller particles is greater than that of large particles and this permits more molecules of the reactants to come in contact and form products.
6) Exposure to radiation
The rate of a chemical reaction is considerably increased by the use of certain radiations. The photons of these radiations having frequencies (ν) possess sufficient energies (E = hv) to break certain bonds in reactants.
For example: Reaction of hydrogen and chlorine takes place very slowly in the absence of light. However, in the presence of light, the reaction takes place very rapidly.
Rate Law Expression
When a chemical reaction proceeds the concentration of reactants decreases while that of products increases.
a) In the beginning of the reaction (i.e. when time = 0), only reactants are present so that the concentration of the products is zero. As the reaction progresses, the concentration of the reactants decreases while the concentration of the products increases.
b) The change in concentration of the various species takes place rapidly in the beginning but very slowly as the reaction approaches the final stage. At the start of the reaction, the rate of the reaction is large and it decreases with passage of time.
The rate of a reaction is directly proportional to the concentration of the reactants. Therefore, to increase the rate of a reaction we should increase the concentration of one or all reactants.
In the gas phase reaction, the increase in pressure of the reactant gases increases the rate of the reaction. With increase in pressure, the number of molecules per unit volume increases and, therefore, rate of reaction increases.
Law of Mass Action
According to this law :
At a given temperature the rate of a chemical reaction is directly proportional to the product of the molar concentrations of the reactants. The molar concentration of the reactant is also called active mass.
A+B ——-> 2AB
According to law of mass action, rate of the reaction may be written as:
Rate = k [A][B]
where [A] and [B] are the molar concentrations of reactants, k is constant of proportionality and is called rate constant.
The rate constant is also called velocity constant and is a measure of rate of the reaction.
Now, if concentration of each of the reactants involved in the reaction is unity ie. [A] = [B] = 1, then substituting these values in above expression, we get
Rate of reaction = k x 1 x 1 = k
Thus, the rate constant of a reaction at a given temperature may be defined as rate of the reaction when the concentration of each of the reactants is unity. The rate constant is also called specific reaction rate.
Characteristics of Rate Constant
Some of the characteristics of rate constant are :
1) Rate constant is a measure of the rate of the reaction. Larger the value of k, faster is the reaction. Similarly, smaller value of k indicates slow reaction.
2) Different reactions have different values of k.
3) At a fixed temperature, the value of k is constant and is characteristic of the reaction. However, it changes with temperature.
4) For a particular reaction, the rate constant is independent of concentration.
| Rate of Reaction | Reaction Rate Constant |
| 1) It is the speed with which reactants are converted into products. It is measured as the rate of decrease concentration of products with time, | It is the proportionality constant in the rate law and is defined as the rate of reaction when the concentration of the reactants is unity. |
| 2) It depends upon the initial concentration of the reactants. | It is independent of the initial concentration of the reactants. |
| 3) Its units are always mol L-1 time-1 | Its units depends upon the order of reaction. |
Rate law Expression
1) 2 NO2 (g) + F2 (g) ——> 2NO2F (g)
The rate of the reaction is proportional to the product of the concentration of nitrogen peroxide and fluorine.
Rate = k[NO2][F2]
This expression is called rate law or rate equation.
2) 2N2O5 (g) ——–>4NO2 (g) + O2 (g)
Rate = k [N2O5]
This expression is called rate law or rate equation.
aA + bB +cC ———> Products
where A , B, C are the reactants and a ,b, c are the stoichiometric coefficients of the balanced equation.
The dependence of the concentration of reactants on the rate of the reaction have been found to be
Rate = k [A]p [B]q [C]r
where p, q, r are powers of the concentration of the reactants.
The rate law or rate equation may be defined as :
the mathematical expression which denotes the rate of a reaction in terms of molar concentration of reactants with each term raised to some power which may or may not be same as the stoichiometric coefficients of the reacting species in a balanced chemical equation.
Rate law expression may differ even for the same reaction depending upon the conditions of the reaction.
1) H2 (g) + I2 (g) ——> 2 HI (g)
Rate = k [H2] [I2]
2) H2 (g) + Br2 (g) ⇔ 2 HBr (g)
Here ka and kb are constant which depend upon the temperature of the reaction. In the absence of HBr, the initial rate of reaction may be given as :
Rate = ka [H2] [Br2]½
3) CHCl3 + Cl2 ——-> CCl4 + HCl
Rate = k [CHCl3] [ Cl2 ]½
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