Engineering Chemistry S Datta

REACTION DYNAMICS/CHEMICAL KINETICS 133
Q. 10. What is a first order reaction?
Ans. If the rate of a chemical reaction varies to the first power of concentration of the
single reactant, it is known as first order reaction.
A ⎯⎯→ Products.
∴ Rate = – d dtA = k . [A] = rate equation for a first order reaction.
Q. 11. Give examples of first order reaction.
Ans. 1. Radioactive disintegration process.
2. Rate of growth of bacterial culture.
Q. 12. What is a second order reaction?
Ans. A second order reaction is a reaction, the rate of which is directly proportional to
the square of the concentration of a single reactant or equal to concentration of two different
reactants. For a reaction:
A + B ⎯⎯→ Products
Rate = k . [A] [B] or Rate = k . [A] 2 = Rate equation for second order reaction.
Q. 13. Comment on half-life of a first order and a second order reaction.
Ans. t 1/2
for a first order reaction = 0. 693 . So, it is a constant quantity.
k
1
t 1/2
for a second order reaction =
ak .
So, half-life in this case is inversely proportional to the initial concentration.
Q. 14. Is a first order or a second order reaction completed?
Ans. Integrated form for a first order rate equation is:
c = c 0
e –kt
The plot of concentration vs. time will be asymptotic plot. Hence, concentration will be
zero at infinite time. So the reaction is not completed.
For a second order reaction the integrated form of the rate equation is:
k = 1 t
x
aa ( – x)
when, x = a, t = ∞
So, a second order reaction is also an incomplete reaction.
Q. 15. For a first order reaction one hour is needed for a change of concentration
c to c/2. What is the time for the change from c/2 to c/4?
Ans. One hour.
Q. 16. For a second order reaction one hour is needed for a change of
concentration a to a/2. What will be the time for the change from a/2 to a/4?