(a) Explain the following terms :
(i) Rate of a reaction
(ii) Activation energy of a reaction
(b) The decomposition of phosphine, PH3, proceeds according to the following equation:
4PH3 (g) P4(g) + 6 H2(g)
It is found that the reaction follows the following rate equation:
Rate = K[PH3].
The half - life of PH3 is 37.9 s at C.
(i) How much time is required for 3/4 th of PH3 to decompose?
(ii) What fraction of the original sample of PH3 remains behind after 1 minute?
(a) (i) Rate of a reaction: The change in the concentration of any one of the reactants or products per unit time is called rate of a reaction.
(ii) The minimum extra amount of energy absorbed by the reactant molecules to form the activated complex is called activation energy.
The activation energy of the reaction decreases by the use of catalyst.
(b) (i) According to the formula :
K = .......(1)
and K = ........(2)
On comparing both equations
K = =
or t =
Let the initial concentration = a i.e.
then t =
or t =
or t = = 75.8
Time required to decompose, t =75.8 seconds
(ii) K =
= antilog 0.4764 = 2.995
If a=1, then (a- ) i.e. fraction left may be calculated as:
Fraction of original sample,
(a- ) = = 0.334