, rate of disappearance of any of the reactants is same as   the rate
of appearance of the products But in the following reaction, two moles of
HI decompose to produce one mole each of H2 and I2,
2HI(g)   ®   H2(g)   +   I2(g)
For expressing the rate of such a reaction where stoichiometric
coefficients of reactants or products are not equal to one, rate of
disappearance of any of the reactants or  the rate of appearance of
products is divided by their respective stoichiometric coefficients Since
rate of consumption of HI is twice the rate of formation of H2 or I2, to
make them equal, the term D[HI]  is divided by 2 The rate of this reaction
is given by
Rate  of reaction  
[
]
[
]
[ ]
2
2
H
I
1
HI
2
t
t
t
∆
∆
∆
= −
=
=
∆
∆
∆
Similarly, for the reaction
5 Br- (aq) + BrO3
– (aq) + 6 H+ (aq)  ®  3 Br2 (aq) + 3 H2O (l)
Rate
Br
BrO
H
Br
H
= −
[
] = −

 = −
[
] =
[
] =
−
−
+
51
61
31
1
3
3
2
2
∆
∆
∆
∆
∆
∆
∆
∆
∆
t
t
t
t
OO
[
]
∆t
For a gaseous reaction at constant temperature, concentration is
directly proportional to the partial pressure of a species and hence, rate
can also be expressed as rate of change in partial pressure of the reactant
or the product