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Rate of chemical reactions
The rate of a reaction tells us how fast the reaction occurs. Let us consider a simple reaction.
A + B -- > C + D
As the reaction proceeds, the concentration of the reactant A and B decreases with time and the concentration of the products C + D increase with time simultaneously. The rate of the reaction is defined as the change in the concentration of any reactant or product in the reaction per unit time.
For the above reaction,
Rate of the reaction
1. Rate of disappearance of A
2. Rate of disappearance of B
3. Rate of appearance of C
4. Rate of appearance of D
During the reaction, changes in the concentration is infinitesimally small even for small changes in time when considered in seconds. Therefore differential form of rate expression is adopted. The negative sign shows the concentration decrease trend and the positive sign shows the concentration increase trend.
Rate = concentration change / time taken = - ∆ [A] / ∆t
= -d[A] / dt = -d[B] / dt = -d[C] / dt = -d[D] / dt
For a general balanced reaction, written with stoichiometries like x,y, for the reactant and l,m for the product, such as
xA + yB -- > lC + mD. The reaction rate is
Rate = - (1/ x) d[A] / dt = - (1/ y) d[B] / dt = + (1/ l) d[C] / dt =
+ (1/ m) d[D] / dt
For example : In the reaction,
H2 + Br2 -- > 2HBr
The overall rate of the reaction is given by
Rate = - d[H2] / dt = - d[Br2] / dt = ½ d[HBr] / dt
Consider the reaction, 2NO + 2H2 -- > N2 + 2H2O
Rate = - 1/2 d[NO] / dt =- 1/2 d[H2] / dt = d[N2] / dt = 1/2 d[H2O] / dt
Units of Rate
Reaction rate has units of concentration divided by time. Since
concentration is expressed in mol lit-1 or mol dm-3 the
unit of the reaction rate is mol lit-1 s-1 or mol dm-3
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