What is Chemical Kinetics?
Chemical Kinetics is a branch of physical chemistry, that deals with the rates of chemical reactions and the mechanism by which they occur.
Rate of Chemical Reaction:
It is defined as the change in concentration of any one of the reactants or products in a particular time interval. Consider a hypothetical reaction-
|A ———-> B|
The rate of reaction can be expressed in either of the following two ways:
- Rate of decrease in concentration of reactants-
|Rate of reaction = Decrease in concentration of A/Time taken = -d[A]/dt|
- Rate of increase in concentration of products-
|Rate of reaction = Increase in concentration of B/Time taken = +d[B]/dt|
[Here in the above expressions, d/dt indicates the change in concentration w.r.t. time and a positive sign indicates the increase in concentration of product and a negative sign indicates the decrease in the concentration of reactant w.r.t. time].
For the reaction involving different stoichiometric coefficients of reactants and products, the rate of reaction is obtained by dividing the rate of reaction defined by any of the reactants or products by the stoichiometric co-efficient of that reactant or products involved in the reaction.
|Example- For reaction, 2N2O5 (g) ———-> 4NO2 (g) + O2 (g)|
Rate of reaction = -(1/2). d[N2O5]/dt = +(1/4) d[NO2]/dt = +d[O2]/dt
However, in an aqueous solution, the rate of reaction is expressed in terms of change in concentration of H2O because the change is very small, i.e., negligible.
What is Average Rate of Reaction?
Average rate of reaction is the rate of reaction over the entire time intervals i.e. Δt. It is denoted by Δx/Δt.
|Thus, average rate of reaction = Total change in concentration/Total time taken = Δx/Δt|
What is Instantaneous Rate of Reaction?
Instantaneous rate of reaction is the rate of change in concentration of any one of the reactants or any one of the products at a given time. So to determine the instantaneous rate of reaction; the time interval is very small, i.e., dt, so that rate remains constant in this interval, i.e., dt —–> 0.
Units of Rate of Reaction:
The units of the rate of reaction are moles Lt-1 sec-1 or moles Lt-1 min-1 or gm Lt-1 sec-1.
In case of gaseous reactants, pressure is used in place of molar concentration. As such units of the rate of a reaction are atm min-1 sec-1.
Experimental Measurement of Reaction Rate:
In order to measure the rate of reaction, the process of the reaction is followed by studying the concentration of any one of the reactants or products at different intervals of time and then a graph is plotted between the concentration of reactants and time.
Calculation of Average Rate of Reaction:
To determine the average rate of reaction, the two time intervals t1 and t2 are marked on the time axis (i.e., x-axis) and corresponding to these two time intervals, the concentration of one of the reactants is determined from the graph. Then,
|Average rate of reaction = Δx/Δt = -(x2 – x1)/(t2 – t1)|
Calculation of Instantaneous Rate of Reaction:
To determine the instantaneous rate of reaction from the progress of the graph, the point “P” corresponding to the time (t) for which the instantaneous rate is to be determined is marked on the curve. A tangent is drawn at point “P” and let it meet both the x-axis and y-axis, then the-
However, it may be noted that in the plot of concentration of reactant versus time, the tangent at any instant of time has a negative sign but if the plot is made between the concentration of product versus time, the tangent has a positive sign i.e.,
|Instantaneous Rate of Reaction |
= – Slope of tangent = – d[R]/dt
= +Slope of tangent = + d[P]/dt
Where [R] is the concentration of reactant and [P] is the concentration of product.