Chemical Kinetics Questions with Answers
Chemical Kinetics
Que 1. The rate constant of a reaction is 0.005 molL–1s–1. What is the order of this reaction?
Ans 1. Zero-order reaction.
Que 2. In a reaction, 2A → Products, the concentration of A decreases from 0.5 mol L–1 to 0.4 mol L–1 in 10 minutes. Calculate the rate during this interval?
Ans 2: Rate = – change in conc. of A/2 × time interval
= – [0.4 ‐ 0.5]/2 × 10 = 0.005 mol L–1min–1
Que 3. The rate constant for a first-order reaction is 60 s–1. How much time will it take to reduce the initial concentration of the reactant to its 1/16th value?
Ans 3: Given = 60 s–1, [R] =1, [R]=1/16, t =?
t = 2.303/k log[R]o /[R]
= 2.303/60 log 16 = 4.6 × 10–2 s
Que 4. Show that in a first-order reaction, the time required for completion of 99.9% is 10 times of half-life (t1/2) of the reaction.
Ans 4. When reaction is completed 99.9%, [R]n = [R]0 – 0.999[R]0
k = 2.303/t log R0/Rn
= 2.303/t log[R]0 /[R]0 – 0.999 [R]0
= 2.303/t log103
t = 6.909/k
For half‐life of the reaction t1/2 = 0.693/k
t/t1/2 =6.909/0.693 =10
Que 5. The rate constants of a reaction at 500 K and 700 K are 0.02s–1 and 0.07s–1 respectively. Calculate the values of Ea and A.
Ans 5. Log k2/k1=Ea/2.303R log[T2 – T1/T2T1]
Log 0.07/0.02 =Ea/2.303 × 8.314 × log[700‐500/700×500]
Ea = 18230.8 J
log k = log A – Ea/2.303RT
log 0.02 = log A – 18230.8/20303 × 8.314 × 500
A= 1.61
Chemical Kinetics Questions with Answers
Que 6. A first-order reaction takes 40 min for 30% decomposition. Calculate t1/2.
Ans 6. Given t =40 min, [R]o = 100, [R]=100 – 30=70
k = 2.303/t log R0/R
= 2.303/40 log100/70
=0.0575(log100 ‐ log70) = 0.0575(2 ‐ 1.84) = 0.00890 min–1
t1/2 = 0.693/k = 0.693/0.00890 = 77.86 min
Que 7. (a) The half‐life for the radioactive decay of 14C is 5730 years. An archaeological artifact containing wood had only 80% of the 14C found in a living tree. Estimate the age of the sample.
(b) A reaction is first order in A and second order in B.
(i) Write the differential rate equation.
(ii) How is the rate affected by increasing the concentration of B three times?
(iii) How is the rate affected when the concentrations of both A and B are doubled?
Ans 7. (a) Given t1/2= 5730 years, [R]o = 100, [R] = 80
K = 0.693/t = 0.693/5730 = 1.21 × 10–4 year–1
t =2.303/K log[R]o/[R] = 2.303/1.21 × 10 log100/80 = 1.9033 × 10-4(log100–log80)
= 1.9033 × 10–4 (2 ‐ 1.9031) = 1.9033 × 10–4 (0.0969) = 1845 year
(b) (i) Rate = K[A]1 [B]2
(ii) Rate = K[A]1 [3B]2 = 9K[A]1 [B]2 hence, it becomes 9 times.
(iii)Rate = K[2A]1 2[B]2 = 8K[A]1 [B]2 hence, it becomes 8 times.
Chemical Kinetics Questions with Answers
Que 8. MCQS
Que 1. The role of a catalyst is to change ______________.
(i) Gibbs Energy of Reaction.
(ii) Enthalpy of Reaction.
(iii) Activation Energy of Reaction.
(iv) Equilibrium Constant.
Ans 1. (iii) Activation Energy of Reaction.
Que 2. In the presence of a catalyst, the heat evolved or absorbed during the reaction ___________.
(i) Increases.
(ii) Decreases.
(iii) Remains Unchanged.
(iv) May Increase or Decrease
Ans 2. (iii) Remains Unchanged.
Que 3. Which of the following statements is not correct about the order of a reaction.
(i) The order of a reaction can be a fractional number.
(ii) Order of a reaction is experimentally determined quantity.
(iii) The order of a reaction is always equal to the sum of the stoichiometric coefficients of reactants in the balanced chemical equation for a reaction.
(iv) The order of a reaction is the sum of the powers of the molar concentration of the reactants in the rate law expression.
Ans 3. (iii) The order of a reaction is always equal to the sum of the stoichiometric coefficients of reactants in the balanced chemical equation for a reaction.
Chemical Kinetics Questions with Answers
Que 4. Which of the following statements is correct?
(i) The rate of a reaction decreases with the passage of time as the concentration of reactants decreases.
(ii) The rate of a reaction is the same at any time during the reaction.
(iii) The rate of a reaction is independent of temperature change.
(iv) The rate of a reaction decreases with an increase in the concentration of reactant(s).
Ans 4. (i) The rate of a reaction decreases with the passage of time as the concentration of reactants decreases.
Que 5. A first-order reaction is 50% completed in 1.26 × 1014 s. How much time would it take for 100% completion?
(i) 1.26 × 1015s (ii) 2.52 × 1014s (iii) 2.52 × 1028s (iv) Infinite
Ans 5. (iv) Infinite
Explanation: k for the first-order reaction is
Que 6. Rate law for the reaction A + 2B → C is found to be Rate = k [A][B]. The concentration of reactant ‘B’ is doubled, keeping the concentration of ‘A’ constant, the value of rate constant will be______.
(i) The same (ii) Doubled (iii) Quadrupled (iv) Halved
Ans 6. (iii) Quadrupled
Que 7. Which of the following statements is incorrect about the collision theory of chemical reaction?
(i) It considers reacting molecules or atoms to be hard spheres and ignores their structural features.
(ii) Number of effective collisions determines the rate of reaction.
(iii) Collision of atoms or molecules possessing sufficient threshold energy results in the product formation.
(iv) Molecules should collide with sufficient threshold energy and proper orientation for the collision to be effective.
Ans 7. (iii) Collision of atoms or molecules possessing sufficient threshold energy results in the product formation.
Que 8. Which of the following statements are applicable to a balanced chemical equation of an elementary reaction?
(i) Order is the same as molecularity. (ii) Order is less than the molecularity.
(iii) Order is greater than the molecularity. (iv) Molecularity can never be zero.
Ans 8. (i), (iv)
Que 9. In any unimolecular reaction ______________.
(i) Only one reacting species is involved in the rate-determining step.
(ii) The order and the molecularity of the slowest step are equal to one.
(iii) The molecularity of the reaction is one and order is zero.
(iv) Both molecularity and order of the reaction are one.
Ans 9. (i), (ii)
Que 10. For a complex reaction ______________.
(i) Order of overall reaction is the same as molecularity of the slowest step.
(ii) Order of overall reaction is less than the molecularity of the slowest step.
(iii) Order of overall reaction is greater than molecularity of the slowest step.
(iv) Molecularity of the slowest step is never zero or non-integer.
Ans 10. (i), (iv)
Chemical Kinetics Questions with Answers
