Chemical Kinetics Class 12 Chemistry Important Questions

Important Questions Class 12

Students can read the important questions given below for Chemical Kinetics Class 12 Chemistry. All Chemical Kinetics Class 12 Notes and questions with solutions have been prepared based on the latest syllabus and examination guidelines issued by CBSE, NCERT and KVS. You should read all notes provided by us and Class 12 Chemistry Important Questions provid ed for all chapters to get better marks in examinations. Chemistry Question Bank Class 12 is available on our website for free download in PDF.

Important Questions of Chemical Kinetics Class 12

Very Short Answer Questions

Question. Express the rate of the following reaction in terms of the formation of ammonia.
N2(g) + 3H2(g) → 2NH3(g)

Question. Define elementary step in a reaction.
Answer.  Elementary step : Each step of a complex reaction is called an elementary step.

41. For a chemical reaction R → P, the variation in the concentration
[R] vs. time (t) plot is given as

(i) Predict the order of the reaction.
(ii) What is the slope of the curve?
Answer. (i) The reaction is of zero order.
(ii) Slope of the straight line graph gives ‘k’

Question. For a first order reaction, show that time required for 99% completion is twice the time required for the completion of 90% of reaction.

Question. Why does the rate of a reaction not remain constant throughout the reaction process?
Answer. The rate of a reaction does not remain constant throughout the reaction process because the rate of the reaction depends upon concentration of reactants which keeps on decreasing.

Question. Define the specific rate of reaction.
Answer. At a given temperature, rate is equal to the rate constant of reaction when concentration of the reactant in unity. Thus rate constant is also known as specific reaction rate.
In the case of two reactants, the reaction may be written as :

where all the terms have their usual meaning as :
if CA = CB = 1 then r = k.

Question. For a reaction A + B → P, the rate law is given by,
r = k[A]1/2 [B]2
What is the order of this reaction?
Answer. A + B → Product
Rate law, r = k[A]1/2 [B]2
Order of reaction is sum of the powers of concentration terms,

Question. Define the rate constant.
Answer. Rate constant is the proportionality factor in the rate law expression for a chemical reaction. It is defined as the rate of a chemical reaction for which the concentration of each of the reacting species is unity.

Question. Define the following term :
Pseudo first order reaction
Answer. A reaction of second order which obeys the first order rate law is known as pseudo first order reaction.
e.g., Inversion of cane sugar :

Question. Define the following :
Energy of activation of reaction.
Answer. The minimum extra energy which must be supplied to the reactants to enable them to cross over the potential energy barrier between reactants and products is called activation energy. It is denoted by Ea. Ea = ET – ER

Question. In some cases it is found that a large number of colliding molecules have energy more than threshold energy, yet the reaction is slow, Why?
Answer. It is due to improper orientation. Energy more than threshold energy and proper orientation are the two main factors which are responsible for a reaction to occur.

Question. For a reaction :

Rate = k
(i) Write the order and molecularity of this reaction.
(ii) Write the unit of k.

Answer. (i) The decomposition of gaseous ammonia on a hot platinum surface is a zero order reaction at high pressure. In this reaction, platinum metal acts as a catalyst. At high pressure, the metal surface gets saturated with gas molecules. So, a further change in reaction conditions is unable to alter the amount of ammonia on the surface of the catalyst making rate of the reaction independent of its concentration. However, two molecules of ammonia react to give products thus, the molecularity is two.
(ii) For a zero order reaction, unit of rate constant is mol L–1 sec–1.

Short Answer Questions

Question. With the help of diagram, explain the physical significance of energy of activation (Ea) in chemical reactions.
Answer. The energy required to form activated complex is called activation energy. It is the difference between the threshold energy and the average energy possessed by the reacting molecules.

Activation energy (Ea) = Threshold energy – Average energy possessed by reacting molecules.

Question. What is meant by rate of reaction? Differentiate between average rate and instantaneous rate of reaction.
Answer. Change in concentration i.e., either (decrease in concentration of reactant or increase in concentration of product) per unit time is called rate of reaction.

The ratio of change of concentration of reactants to the time consumed in that change is called average rate of reaction.

The rate of reaction at a particular instant (time) is called instantaneous rate of reaction.

Question. A reaction is second order in A and first order in B.
(i) Write the differential rate equation.
(ii) How is the rate affected on increasing the concentration of A three times?
(iii) How is the rate affected when the concentration of both A and B are doubled?

Answer. Since the reaction is second order in A and first order in B
(i) Differential rate equation is
Rate = k[A]2[B]
(ii) The new concentration of A = [3A]
New rate = k[3A]2[B] = 9k[A]2[B]
New rate = 9 times of the original rate
(iii) New concentration of A = [2A]
New concentration of B = [2B]
New rate = k[2A]2[2B] = 8k[A]2[B]
New rate = 8 times of the original rate

Question. A first order decomposition reaction takes 40 minutes for 30% decomposition. Calculate its t1/2 value.
Answer. Here t = 40 min, t1/2 = ?
Let a = 100 x = 30% of 100 = 30
Using the formula,

Question. For a chemical reaction R → P, the variation in the concentration, ln [R] vs. time (s) plot is given as

(i) Predict the order of the reaction.
(ii) What is the slope of the curve?
(iii) Write the unit of the rate constant for this reaction.
Answer. (i) the reaction is of 1st order.
(ii) For first order reaction
ln[R] = –kt + ln [R]0
comparing eqn. y = m × x + c
we get a straight line with slope = –k and intercept
equal to ln[R]0.
(iii) Unit of rate constant for first order reaction

Question. For the first order thermal decomposition reaction, the following data were obtained :

Calculate the rate constant.
(Given : log 2 = 0.301, log 3 = 0.4771, log 4 = 0.6021)

Answer. The given reaction is

Total pressure = 0.30 – x + x + x = 0.50
or 0.30 + x = 0.50
x = 0.50 – 0.30 = 0.20
Initial pressure, P0 = 0.30 atm
Pressure of C2H5Cl after 300 sec, Pt = 0.30 – 0.20 = 0.10 atm
Using formula for first order reaction,

Question. 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 1/10th value?

Question. (i) A reaction is first order in A and second order in B.
(a) Write differential rate equation.
(b) How is rate affected when concentration of B is tripled?
(c) How is rate affected when concentration of both A and B is doubled?
(ii) What is molecularity of a reaction?
Answer. (i) (a) Differential rate equation of reaction is

When conc. of B is tripled, it means conc. of B becomes [3 × B]

i.e., the rate of reaction will become 9 times.
(i) When conc. of A is doubled and that of B is also doubled, then conc. of A becomes [2A] and that of B becomes [2B].

i.e., the rate of reaction will become 8 times.
(ii) Molecularity of a reaction is the number of reacting particles which collide simultaneously to bring about the chemical change. It is a theoretical concept.

Question. The following data were obtained during the first order thermal decomposition of SO2Cl2 at a constant volume :

Calculate the rate constant.
(Given : log 4 = 0.6021, log 2 = 0.3010)

Answer. The given reaction is

Total pressure at time t will be
PT = (0.4 – x) + x + x = 0.4 + x
x = (PT – 0.4)
Pressure of SO2Cl2 at time t will be
pSO2Cl2 = 0.4 – x = 0.4 – (PT – 0.4) = 0.8 – PT
At time t = 100 s, PT = 0.7 atm
pSO2Cl2 = 0.8 – 0.7 = 0.1 atm
According to first order kinetic equation

Question. For a decomposition reaction, the values of k at two different temperatures are given below :
k1 = 2.15 × 10–8 L/(mol.s) at 650 K
k2 = 2.39 × 10–8 L/(mol.s) at 700 K
Calculate the value of Ea for the reaction.
(R = 8.314 J K–1 mol–1)

Question. Rate constant ‘k’ of a reaction varies with temperature ‘T’ according to the equation :

slope of – 4250 K is obtained. Calculate ‘Ea’ for the reaction.
(R = 8.314 J K–1 mol–1).

Answer. Given : Slope, m = – 4250 K,
R = 8.314 J K–1 mol–1
Ea = ?
Using formula,

Question. Hydrogen peroxide, H2O2(aq) decomposes to H2O(l) and O2(g) in a reaction that is first order in H2O2 and has a rate constant k = 1.06 × 10–3 min–1.
(i) How long will it take for 15% of a sample of H2O2 to decompose?
(ii) How long will it take for 85% of the sample to decompose?

Question. A first order reaction takes 30 minutes for 50% completion. Calculate the time required for 90% completion of this reaction. (log 2 = 0.3010)
Answer. t50% or t1/2 = 30 minutes

Question. A reactant has a half-life of 10 minutes.
(i) Calculate the rate constant for the first order reaction.
(ii) What fraction of the reactant will be left after an hour of the reaction has occurred?

Question. The decomposition of a compound is found to follow a first order rate law. If it takes 15 minutes for 20 per cent of original material to react, calculate
(i) the rate constant.
(ii) the time at which 10% of the original material remains unreacted.
Answer. 20% decomposition means that x = 20% of a = 0.20 a

Question. A first order reaction takes 10 minutes for 25% decomposition. Calculate t1/2 for the reaction.
(Given : log 2 = 0.3010, log 3 = 0.4771,

log 4 = 0.6021)
Answer. For a first order reaction,

Question. A + 2B → 3C + 2D. The rate of disappearance of B is 1 × 10–2 mol/L/s.
What will be (i) rate of the reaction (ii) rate of change in concentration of A and C ?


Question. The data given below is for the reaction,
2N2O5(g) → 4NO2(g) + O2(g) at 298 K

Determine for this reaction :
(i) order of reaction
(ii) rate constant
(iii) rate law.

Answer. (i) Let rate law for the disappearance of N2O5 is
Rate = k[N2O5]a

Question. List the factors on which the rate of a chemical reaction depends. 
Answer. Factors affecting the rate of a chemical reaction :
(i) Concentration of reactants : In general the rate of a reaction increases when concentration of reactants is increased.
Rate = kCn
(ii) Temperature : Most of the chemical reactions are accelerated by increase in temperature.
(iii) Catalyst : Rate of a reaction increases in presence of a catalyst.
(iv) Nature of reactants : Reactions involving ionic reactants are fast as compared to those involving covalent reactants.
(v) Surface area of the reactants : In case of solid reactants, the rate of reaction increases with the surface area of the particles of the reactants.
(vi) Light : In case of photochemical reactions, the rate of reaction increases with increasing the intensity of light.

Question. The first order reaction take 100 minutes for completion of 60% of the reaction. Find the time when 90% of the reaction will be completed.
Answer. For the first order reaction,

Question. A first order reaction is 15% completed in 20 minutes. How long will it take to complete 60% of the reaction? 

Long Answer Questions

Question. For the hydrolysis of methyl acetate in aqueous solution, the following results were obtained :

(i) Show that it follows pseudo first order reaction, as the concentration of water remains constant.
(ii) Calculate the average rate of reaction between the time interval 30 to 60 seconds.
(Given : log 2 = 0.3010, log 4 = 0.6021)

Answer. (i) For a first order reaction,

As the value of k is constant at different time intervals, the reaction is first order w.r.t. ester when [H2O] is constant. Hence, it is pseudo first order reaction.

Question. For the reaction :
2NO(g) + Cl2(g) 2NOCl(g)
The following data were collected. All the measurements were taken at 263 K.

(i) Write the expression for rate law.
(ii) Calculate the value of rate constant and specify its units.
(iii) What is the initial rate of disappearance of Cl2 in exp. 4?

Answer. Given reaction :
2NO(g) + Cl2(g) → 2NOCl(g)

Question. the following results have been obtained during the kinetic studies of the reaction :
2A + B → C + D

Calculate the rate of formation of D when
[A] = 0.5 mol L–1 and [B] = 0.2 mol L–1

Answer. Suppose order with respect to A is m and with respect to B is n. Then the rate law will be
Rate = k[A]m[B]n
Substituting the value of experiments 1 to 4, we get
Expt. 1 : Rate = 6.0 × 10–3 = k (0.1)m (0.1)n               …(i)
Expt. 2 : Rate = 7.2 × 10–2 = k (0.3)m (0.2)n               …(ii)
Expt. 3 : Rate = 2.88 × 10–1 = k (0.3)m (0.4)n             …(iii)
Expt. 4 : Rate = 2.4 × 10–2 = k (0.4)m (0.1)n               …(iv)
Comparing equation (i) and equation (iv),

Rate law expression is : Rate = k[A][B]2
The rate constant can be calculated from the given data of any experiment using expression :

= 6 × 5 × 4 × 10–3
= 1.2 × 10–1 mol L–1 min–1