The rate of reaction refers to the speed at which a chemical reaction takes place, meaning how quickly reactants are converted into products. It involves the breaking of old chemical bonds and the formation of new bonds, leading to the rearrangement of atoms in molecules. The rate of a Chemical Reaction is directly proportional to the increase in the concentration of a product per unit of time and to the decrease in the concentration of a reactant per unit of time.
Rate of Reaction Meaning
The Rate of Reaction is defined as the change in the concentration of any one of the reactants or products per unit of time.
Rate of Reaction Formula
The Rate of Reaction is proportional to the increase in the concentration of a product per unit time and to the decrease in the concentration of a reactant per unit time and can be defined as,
- Rate of Reaction = Decrease in the concentration of a reactant R / Time Interval
- Rate of Reaction = Increase in the concentration of a product P / Time interval
If we consider, [R1] and [P1] are the molar concentrations of the reactant and the product respectively at any time t1 and [R2] and [P2] are the concentrations of reactant and product at time t2, then changes in concentrations of the reactant and product will be d [R] = R2 - R1 and d [P] = P2 - P1 and time interval is dt = t2 - t1 and the rate of reaction in terms of reactant or product is given by
\text{Rate of reaction} = -\frac{R_2 - R_1}{t_2 - t_1} = +\frac{P_2 - P_1}{t_2 - t_1} \text{Rate of reaction} = -\frac{\Delta [R]}{\Delta t} = +\frac{\Delta [P]}{\Delta t}
Significance of Negative and Positive Signs
The sign in the rate of reaction tells about the increase and decrease in the concentration.
- Negative sign indicates that the concentration of the reactant is decreasing.
- Positive sign indicates that the concentration of the product is increasing.
Expressing the Rate of Reaction in terms of different Reactants and Products
Let us consider a reaction ⇒ a A + b B ⇢ x X + y Y
Then the Rate of Reaction is given by
\text{Rate} = -\frac{1}{a}\frac{d[A]}{dt} = -\frac{1}{b}\frac{d[B]}{dt} = +\frac{1}{x}\frac{d[X]}{dt} = +\frac{1}{y}\frac{d[Y]}{dt} where
- d[A], d[B] represent small decrease in the concentrations of A and B respectively
- d[X] and d[Y] represent small increase in the concentrations of X and Y respectively in the small interval of time dt
Average Rate of Reaction
From beginning to end the rate of reaction does not remain the same, it can vary from time to time. Therefore, the rate of reaction is defined as the 'Average Rate of Reaction'.
Mathematically, the Average rate of reaction is given by,
r_{av} = -\frac{\Delta [R]}{\Delta t} = +\frac{\Delta [P]}{\Delta t}
Instantaneous Rate of Reaction
The Rate of Reaction at any instant of time is the rate of change of concentration of any one of the reactants or products at that particular instant of time. From the Average Rate of Reaction, we can understand it as when the change in the time interval is very less i.e. Δt→0 then the Rate of Reaction is termed as Instantaneous Rate of Reaction. Let us suppose that the small change in concentration is given dx in the small interval of time dt. Then the rate of reaction at that instant is given by dx/dt. It is given by the tangent to the curve of Concentration of Reactant/Product vs Time.
Mathematically, the Instantaneous Rate of Reaction is given by,
\text{Rate} = \frac{dx}{dt}
r_{\text{inst}} = -\frac{\Delta [R]}{\Delta t}\ \text{as}\ \Delta t \to 0 = -\frac{d[R]}{dt} = -\text{slope}
r_{\text{inst}} = +\frac{\Delta [P]}{\Delta t}\ \text{as}\ \Delta t \to 0 = +\frac{d[P]}{dt} = +\text{slope}
Unit of Rate of Reaction
The Rate of Reaction, in general, can be measured as a change in concentration of reactant or product where concentration is denoted in moles/liter and time in seconds or minutes.
So the Unit of Rate of Reaction = mol L-1 s-1 or mol L-1 min-1