The rate constant or reaction rate constant which is denoted by k or λ determines the rate or speed of a chemical reaction. There are generally zero order reactions, first order reactions and second order reactions. It is very easy to determine the rate constant for the zero order and first order reactions. On the other hand, calculating the rate constant for the second order reactions is a little difficult. The rate constant of a reaction depends upon the concentration of both the reactants. This article on how to calculate rate constant will help you in calculating the rate constant.
For a chemical reaction where a reactant A reacts with the reactant B to produce a product C
aA + bB ——–à cC
The expression of rate constant for a single step reaction is given as:
Rate = k [A]m [B]n
In the above expression
- K is the rate constant of the reaction
- [A] and [B] are the concentrations of the reactants A and B respectively and are given in moles per liter.
- m is the order of reaction with respect to A and n is the order of reaction with respect to B. Hence, the order of reaction is the sum of m and n. These values need to be determined experimentally.
Rate constant of a reaction is mainly used to determine the order of the reaction. The order of reaction is the amount by which the rate of the reaction changes when the concentrations of the reactants are changed.
- If the above equation is given as :
Rate = k [A] 2 [B] 1
Then the reaction is a third order reaction. The order of reaction is 2 for A and the order of reaction is 1 for B.
- The reactions with zero order are those which are independent from the concentrations of one of the reactants.
- First order reaction is the reaction which depends upon the concentration of only one of the reactants. This is also called as unimolar reaction.
- Second order reactions are those whose rate depends upon the concentrations of one second order reactant or two first order reactants.
Determining rate constant of a reaction:
Consider the reaction
2ClO2 (aq) + 2OH-(aq) -> ClO3-(aq) + ClO2-(aq) + H2O
In the above reaction if we take the molar concentrations of the reactants ClO2 as 0.010 and OH- as 0.030. Then the rate constant will be
Rate = [ClO2] [OH-]
Rate= 0.010 × 0.030
Rate 1 = 6.00 × 10 -4
Determining order of reaction experimentally:
If we change the concentration of ClO2 to 0.055, keeping the concentration of OH- ions constant, then the rate of reaction will be
Rate 2= 1.82 × 10-2
To determine the order of reaction, we need to determine the fraction by which the rate of reaction changed when concentration of one of the reactants is changed.
By dividing rate of second reaction with the rate of first reaction we get
Rate 2/ Rate 1 = ([ClO2]3/[ClO2]1)m
1.82 × 10-2/6.00 × 10 -4 = (0.055/0.010)m
30.3 = (5.5) m
Now, from the above expression, square of 5.5 is approximately 30.3. Therefore, value of m is 2 and hence the reaction is a second order reaction.
You can also determine the order of reaction by changing the concentration of OH- ions or by changing the concentration of both the reactants.
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