Chapter: 11th 12th std standard Class Organic Inorganic Physical Chemistry Higher secondary school College Notes
Rate of chemical reactions
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
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