Chemical kinetics
it is the branch of chemistry Which deals with the study of the rate of chemical reaction the factors affecting the rate of the reaction and the mechanism by which the reaction occurs.
The study of reaction involved three main aspects as-
1 . feasibility of the reaction that means spontaneity of a reaction predicted by thermodynamics On the basis of value of Gibbes energy .When its value is Greater than zero i.e. the value of Gibbs Energy is. Positive. Then reaction will be Non- spontaneous.Or when its value is lesser than zero i.e. the value of Gibbs Energy will be negative,Then the reaction will be spontaneous.
2.If it is spontaneous then to what extent the reaction take place before equilibrium is attained. It can be determined by chemical equilibrium
3.Speed of reaction - i.e. time taken by a reaction to reach equilibrium (predicted by chemical kinetics)
Besides of these aspects some others
4. factor affecting the rate of reaction
5. mechanism by which reaction take place
Rate of a chemical reaction
maybe defined has the change in concentration of reactants or products per unit time it can be expressed by two ways. These are as follow
1. rate of disappearance of reactant- I.e. The rate of decrease in concentration of any one of the reactants
2. Rate of appearance of product-I.e. The rate of increase in concentration of any one of the products
consider a hypothetic reaction
R --------------------> P
At t=t1 time [R]1 [P]1
at t=t1 time [R]2 [P]2
Thus time taken =Δt = t2 - t1 ,
change in concentration = Δ [R]= [R]2 - [R]1 Δ[P] = [P]2 - [P]1
Rate of disappearance of reactant = decrease in concentration of reactant/ time taken
[R]2 - [R]1 / t2 - t1 = - Δ[R] / Δt -------------(1) (here negative sign indicates that the concentration of reactant with time
Rate of appearance of product = increase in concentration of reactant/ time taken
[P]2 - [P]1 / t2 - t1 = + Δ[P] / Δt -------------(1)
Rate of reaction for reaction having different stoichiometry coefficient
Ex decomposition of hydrogen iodide-
2HI(gas) ----------------> H2 (gas) + I2 (gas )
Rate of reaction = Δx /Δt = - Δ[HI] /2 Δt
= + Δ[H2] / Δt = + Δ[I2] / Δt --------(3)
Unit of rate of reaction = concentration of reactant of product/time taken = Mol L-1 s-1
If reactants and products are gaseous form, the the unit is atm s-1 .
Rate of reaction for decomposition of N2O5
2N2O5 ------------------> 4NO2 + O2
Rate of decomposition or disappearance of N2O5 = - Δ(N2O5 )/2Δt
Rate of formation products NO2 Or appearance of product NO2 = + Δ(NO2)/4Δt
Or rate of production of oxygen = +
s.no | Average rate of reaction | instantaneous rate of reaction |
1 | Is the average of the instantaneous reaction rate over a period of time during the reaction | Is the rate at a particular instant of time during the reaction |
2 | Expressed by rav = ΔP/Δt =- ΔR/Δt
| Average rate of reaction may be equal when Δt become small and approaches zero rav = (Δx/Δt)Δt --->0 = dP/dt = - dR/dt = rinst. |
3. | Is equal to the change in concentration divided by the time interval | The instantaneous rate= the slope of the tangent to the curve at a particular time point |