Chapter 4 Chemical Kinetics Solutions
Question - 11 : - From the rate expression for thefollowing reactions, determine their order of reaction and the dimensions ofthe rate constants.
Answer - 11 : -
(ii) H2O2 (aq) + 3 I− (aq)+ 2 H+ → 2 H2O (l) + 
Rate= k[H2O2][I−]
(iii) CH3CHO(g)→ CH4(g)+ CO(g) Rate = k [CH3CHO]3/2
(iv) C2H5Cl(g) → C2H4(g) + HCl(g) Rate = k [C2H5Cl]
Answer
Question - 12 : - For the reaction:
2A + B → A_2B
Answer - 12 : -
the rate = k[A][B]2 with k =2.0 × 10−6 mol−2 L2 s−1.Calculate the initial rate of the reaction when [A] = 0.1 mol L−1, [B] = 0.2 mol L−1.Calculate the rate of reaction after [A] is reduced to 0.06 mol L−1.
Answer
The initial rate of the reaction is
Rate = k [A][B]2
= (2.0 × 10−6 mol−2 L2 s−1) (0.1 mol L−1)(0.2 mol L−1)2
= 8.0 × 10−9 mol−2 L2 s−1
When [A] is reduced from 0.1 mol L−1 to0.06 mol−1, the concentration of A reacted = (0.1 − 0.06)mol L−1 = 0.04 mol L−1
Therefore,concentration of B reacted 
= 0.02 mol L−1
Then, concentration ofB available, [B] = (0.2 − 0.02) mol L−1
= 0.18 mol L−1
After [A] is reduced to 0.06 mol L−1,the rate of the reaction is given by,
Rate = k [A][B]2
= (2.0 × 10−6 mol−2 L2 s−1) (0.06 mol L−1)(0.18 mol L−1)2
= 3.89 mol L−1 s−1
Question - 13 : - The decomposition of NH3 on platinum surface is zero orderreaction. What are the rates of production of N2 and H2 if k =2.5 × 10−4 mol−1 Ls−1?
Answer - 13 : -
The decomposition of NH3 on platinum surface is represented by thefollowing equation.
Therefore,
However, it is given that the reaction is of zeroorder.
Therefore,
Therefore, the rate of productionof N2 is
Question - 14 : - The decomposition of dimethyl ether leads to the formation of CH_4, H_2 and CO and the reaction rate is given by
Answer - 14 : -
Rate = k [CH3OCH3]3/2
The rateof reaction is followed by increase in pressure in a closed vessel, so the ratecan also be expressed in terms of the partial pressure of dimethyl ether, i.e.,
If the pressure is measured inbar andtime in minutes, then whatare the units of rate and rate constants?
Answer
If pressure is measured in bar and time in minutes,then
Unit of rate = bar min−1
Question - 15 : - Mention the factors that affectthe rate of a chemical reaction.
Answer - 15 : -
The factors that affect the rateof a reaction are as follows.
(i) Concentration of reactants (pressure in case of gases)
(ii) Temperature
(iii) Presence of a catalyst
Question - 16 : - A reaction is second order withrespect to a reactant.
Answer - 16 : -
How is the rate of reaction affected if the concentration of the reactant is
(i) doubled (ii) reduced to half?
Answer
Letthe concentration of thereactant be [A] = a
Rate of reaction, R = k [A]2
= ka2
(i)If theconcentration of the reactant is doubled, i.e. [A] = 2a, then the rate of thereaction would be
What is the effect of temperatureon the rate constant of a reaction? How can this temperature effect on rateconstant be represented quantitatively?
Answer - 17 : -
The rate constant is nearlydoubled with a rise intemperature by 10° for a chemical reaction.
Thetemperature effect on the rate constant can be represented quantitatively byArrhenius equation,
where, k isthe rate constant,
A is theArrhenius factor or the frequency factor,
R is the gas constant,
T is the temperature, and
Ea is the energy of activation for thereaction
Question - 18 : - In a pseudo first orderhydrolysis of ester in water, the following results were obtained:
Answer - 18 : -
| t/s | 0 | 30 | 60 | 90 |
| [Ester]mol L−1 | 0.55 | 0.31 | 0.17 | 0.085 |
(i) Calculate the average rate of reaction between the time interval30 to 60 seconds.
(ii) Calculate the pseudo first order rate constant for the hydrolysisof ester.
Answer
Answer - 19 : -
(i) Write the differential rate equation.
(ii) How is the rate affected on increasing the concentration of Bthree times?
(iii) How is the rate affected when the concentrationsof both A and B are doubled?
Answer
(i) The differential rate equation will be
(ii) If the concentration of B is increased three times, then
Therefore, the rate of reactionwill increase 9 times.
(iii) When the concentrations of both A and B aredoubled,
Therefore, the rate of reactionwill increase 8 times.
In a reaction between A and B,the initial rate of reaction (r0) was measured for different initialconcentrations of A and B as given below:
Answer - 20 : -
| A/ mol L−1 | 0.20 | 0.20 | 0.40 |
| B/ mol L−1 | 0.30 | 0.10 | 0.05 |
| r0/ mol L−1 s−1 | 5.07 × 10−5 | 5.07 × 10−5 | 1.43 × 10−4 |
Whatis the order of the reaction with respect to A and B?
Answer
Let the order of the reactionwith respect to A be x andwith respect to B be y.
Therefore,
