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# Chemical Kinetics Class 12 Chemistry Important Questions

Please refer to Chemical Kinetics Class 12 Chemistry Important Questions with answers below. These solved questions for Chapter 4 Chemical Kinetics in NCERT Book for Class 12 Chemistry have been prepared based on the latest syllabus and examination guidelines issued by CBSE, NCERT, and KVS. Students should learn these solved problems properly as these will help them to get better marks in your class tests and examinations. You will also be able to understand how to write answers properly. Revise these questions and answers regularly. We have provided Notes for Class 12 Chemistry for all chapters in your textbooks.

## Important Questions Class 12 Chemistry Chapter 4 Chemical Kinetics

All Chemical Kinetics Class 12 Chemistry Important Questions provided below have been prepared by expert teachers of Standard 12 Chemistry. Please learn them and let us know if you have any questions.

Question. Th rate of most of the reactions becomes double when their temperature is raised from 298 K to 308 K. Calculate their activation energy.
[Given R = 8.314 J mol–1 K–1

Question. Define the term ‘order of reaction’ for chemical reactions.
Answer : It is defined as “the sum of the powers or exponents to which the concentration terms are raised in the rate law expression.”
If rate = k[A]m [B]n, then order = m + n.

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,
∴ Order of reaction = 1/2 + 2 = 5/2 = 2.5

Question. Distinguish between ‘rate expression’ and ‘rate constant’ of a reaction.
Answer : Rate expression is a way of expressing rate of reaction in terms of concentration of reactants, e.g., for a general reaction, aA + bB → cC + dD
Rate = k[A]x [B]y
Rate constant (k) is equal to the rate of reaction when molar concentration of reactant is unity. Its units depends upon the order of reaction.

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

Question. Define half-life of a reaction. Write the expression of half-life for
(i) zero order reaction and
(ii) first order reaction.
Answer : The time taken for half of the reaction to complete, i.e., the time in which the concentration of a reactant is reduced to half of its original value Is called half-life period of the reaction.
t = t R1/2 when [R] = [R0]/2
(i) For zero order reaction rate constant is given by :

In first order reaction, t1/2 is independent of initial concentration.

Question. How does a change in temperature affect the rate of a reaction? How can this effect on the rate constant of reaction be represented quantitatively?
Answer : The rate constant is nearly doubled with a rise in temperature by 10° for a chemical reaction.
The temperature effect on the rate constant can be represented quantitatively by Arrherius equation :
k = Ae–Ea/RT
where  k → Rate constant
A → Arrhenius factor
R → Gas constant
T → Temperature
Ea → Energy of activation for the reaction.

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 :
A + B → Products
r = dx/dt = k CACB
where all the terms have their usual meaning as :
if CA = CB = 1 then r = k.

Question. Th 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

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. For a reaction R → P, half-life (t1/2) is observed to be independent of the initial concentration of reactants. What is the order of reaction?
Answer : Half-life of first order reaction is independent of the initial concentration of reactants.
t1/2 = 0.693/k

Question. Following data are obtained for the reaction:

(a) Show that it follows first order reaction.
(b) Calculate the half-life.
(Given : log 2 = 0.3010, log 4 = 0.6021)
Answer : (a) The formula of rate constant for first order reaction is

Question. 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’
= − k = d[R]/dt

Question. Define “order of a reaction” and “activation energy of a reaction”.
Answer : (i) Order of a reaction :  It is defined as “the sum of the powers or exponents to which the concentration terms are raised in the rate law expression.”
If rate = k[A]m [B]n, then order = m + n.
(ii) Activation energy of a reaction : The rate constant is nearly doubled with a rise in temperature by 10° for a chemical reaction.
The temperature effect on the rate constant can be represented quantitatively by Arrherius equation :
k = Ae–Ea/RT
where  k → Rate constant
A → Arrhenius factor
R → Gas constant
T → Temperature
Ea → Energy of activation for the reaction.

Question. Write two differences between ‘order of reaction’ and ‘molecularity of reaction’.
Answer : Distinction between order and molecularity of a reaction :

Generally, in a complex reaction the order of reaction is equal to the molecularity of the slowest step.

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.
Rate of reaction = C2 − C1/t2 − t1 = ΔC/Δt
The ratio of change of concentration of reactants to the time consumed in that change is called average rate of reaction.
rav = Δx/Δt = − C2 − C1/t2 − t1
The rate of reaction at a particular instant (time) is called instantaneous rate of reaction.
rins dt = dx/dt
dx = small change in concentration
dt = small time interval

Question. For a reaction : 2NH3(g)pt N2(g) + 3H2(g) 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.

Question. Write units of rate constants for zero order and for the second order reactions if the concentration is expressed in mol L–1 and time in second.
Answer : Unit of rate constant (k), for zero order reaction.
Rate = k[A]0 ⇒ k = mol L–1 s–1
Unit of rate constant (k), for second order reaction
Rate = k[A]2 ⇒ k = mol–1 s–1

Question. Explain the term ‘order of reaction’. Derive the unit for first order rate constant.
Answer : Order of a reaction : It is defined as “the sum of the powers or exponents to which the concentration terms are raised in the rate law expression.”
If rate = k[A]m [B]n, then order = m + n.
Unit for first order rate constant :
For first order reaction,
Rate = dx/dt = k[A]
mol L–1/sec = k .mol L–1 ⇒ k 1/sec = sec–1 .

Question. Th rate constant of a first order reaction increases from 2 × 10–2 to 4 × 10–2 when the temperature changes from 300 K to 310 K.
Calculate the energy of activation (Ea).
(log 2 = 0.301, log 3 = 0.4771, log 4 = 0.6021)

Question. Th rate of a reaction becomes four times when the temperature changes from 293 K to 313 K.
Calculate the energy of activation (Ea) of the reaction assuming that it does not change with temperature.
[R = 8.314 J K–1 mol–1, log 4 = 0.6021]
Answer : Since the rate of a reaction quadruples when temperature changes from 293 K to 313 K.

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. Th thermal decomposition of HCO2H is a first order reaction with a rate constant of 2.4 × 10–3 s–1 at a certain temperature. Calculate how long will it take for three-fourth of initial quantity of HCO2H to decompose. (log 0.25 = – 0.6021)
Answer : For a first order reaction,

Question. Th decomposition of A into products has a value of k as 4.5 × 103 s–1 at 10°C and energy of activation is 60 kJ mol–1. At what temperature would k be 1.5 × 104 s–1

Question. A first order reaction is 20% complete in 5 minutes. Calculate the time taken for the reaction to be 60% complete.
Answer : For the first order reaction :

Question. What is meant by the ‘rate constant, k’ of a reaction? If the concentration be expressed in mol L–1 units and time in seconds. What would be the units for k (i) for a zero order reaction and (ii) for a first order reaction?
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.
Unit for k :
(i) For a zero order reaction : mol L–1 s–1
(ii) For a first order reaction : s–1

Question. In a first order reaction, the concentration of the reactant is reduced from 0.6 mol L–1 to 0.2 mol L–1 in 5 minutes. Calculate the rate constant of the reaction.
Answer : a = 0.6 mol/L, (a – x) = 0.4 mol/L
t = 5 min
k = 2.303/5 min log 0.6/0.4 = 0.0811 min–1

Question. A reaction is of second order with respect to its reactant. How will its reaction rate be affected if the concentration of the reactant is (i) doubled (ii) reduced to half ?
Answer : Let the concentration of the reactant [A] = a Order of reaction = 2 so that
Rate of reaction = k [A]2                                                                            …(1)
= ka2
(i) Given that concentration of the reactant is doubled So, that [A] = 2a,
Putting the value in equation (1) we get
New rate of reaction, R1 = k(2a)2 = 4ka2
Hence, rate of reaction will increased to 4 times.
(ii) Given that concentration of the reactant is reduced to half
So that [A] = (1/2)a
Putting the value in equation (1), we get
New rate of reaction R2 = k((1/2)a)2
= (1/4)ka2
Hence, rate of reaction will reduced to 1/4.

Question. Th rate constant for the first order decomposition of H2O2 is given by the following equation :
log k = 14.2 − 1.0 X 104/T K
Calculate Ea for this reaction and rate constant k if its half-life period be 200 minutes.
(Given : R = 8.314 J K–1 mol–1)

Question. Show that for a first order reaction, the time required for half the change (half-life period) is independent of initial concentration.
Answer : The time taken for half of the reaction to complete, i.e., the time in which the concentration of a reactant is reduced to half of its original value Is called half-life period of the reaction.
t = t R1/2 when [R] = [R0]/2
(i) For zero order reaction rate constant is given by :

In first order reaction, t1/2 is independent of initial concentration.

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