2nd PUC Chemistry Question Bank Chapter 4 Chemical Kinetics

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Karnataka 2nd PUC Chemistry Question Bank Chapter 4 Chemical Kinetics

Question 1.
Define rate of a reaction.
Answer:
Rate of a reaction is change in concentration of a reactant or product in unit time.

Question 2.
Give the unit of rate of reaction.
Answer:
Unit of rate of reaction molL^-1s^-1 or mol dm^-3 S^-1.

Question 3.
What is average rate of reaction? Give its expression in terms of reactants and products.
Answer:
Rate of a reaction over a range of time is called average rate of a reaction
R → P

Question 4.
What is instantaneous rate of a reaction? Give its expression in terms of reactants and products.
Answer:
Rate of a reaction at a particular moment of a time is called instantaneous rate of a reaction.
R → P

Question 5.
Write the rate of a reaction in terms of reactants and product for the following reactions.
(i) 2HI(g) → H₂(g) + I₂(g)
(ii) Hg(l) + Cl₂(g) → HgCl₂(g)
Answer:

Question 6.
what are the factors which influence rate of a reaction.
Answer:

Concentration of the reactants or pressure of the reactants.
Temperature
Catalyst
Nature of the reactants and their physical states.

Question 7.
What is simple collision theory? Give its significance.
Answer:
According this thoery reactant molecules must colloide themselves in order to form the product.

As concentration of reactants increases, the number of collisions between them increases, hence rate of reaction increases.

Question 8.
Define order of a reaction.
Answer:
Order of a reaction is the sum of the power of the molar concentrations of the reactants in the reactions experimentally determined rate equation.

Question 9.
What is rate law?
Answer:
Experimentally determined rale- expression is called rale law.

Question 10

Answer:

Question 11.
A reaction is first order with respect to the reactant A and second order with respect to the reactant B in a reaction.
A + B → Product.
1. Write the differential rate equation.
2. How is rate of reaction affected on increasing the concentration of B by 2 times.
Answer:
1. \(-\frac{d x}{d t}\) = k[A]¹[B]²
2. \(-\frac{d x}{d t}\) = k[A][2B]²; \(\frac{d x}{d t}\) = 4k[A][B]²
Rate of reaction increases by four times when concentration of B increases by 2 times.

(b) A reaction is second order with respect to a reactant. How is the rate of reaction affected if the concentration of the reactant is
(i) Doubled
(ii) reduced to half?
Answer:
(i) r = k[A]² ⇒ r = k[2A]² ⇒ r = 4k[A]²
Rate increase by four times.

(ii) r = k[A]²
r = k\(\left[\frac{A}{2}\right]^{2}\) ⇒ r = \(\frac { 1 }{ 4 }\)k[A]².
Rate decreases by four times.

(c) A reaction is first order in A and second order in B.
(i) Write the differential rate equation.
(ii) How is rate affected on increasing the concentration of B three times?
(iii) How is rate affected when the concentration of both A and B are doubled?
Answer:
(i) rate = \(\) = k[A]¹ [B]²
(ii) rate = k[A]¹ [3B]² = 9k[A]¹ [B]². Rate increases by 9 times.
(iii) rate = k[2A]¹ [2B]² = 8k[A]¹ [B]². Rate increases by 8 times.

Question 12.
Show that the rate of a first order reaction is doubled when the concentration of the reactant is doubled.
Answer:
Consider first order reaction A → P
r₁ = k[A]¹ ……. (1)
When concentration is doubled
r₂ = k[2A]¹
r₂ = 2k [A] ……. (2)
Comparing equation (1) and (2)
r₂ = 2r

Question 13.
What is zero order reaction? Give two examples.
Answer:
Rate of a reaction which is independent of initial concentration of the reactant is called zero order reaction.
R → P rate = k[R]°
Example:
(1) Thermal decomposition of HI on gold surface.

(2) Decomposition of ammonia on hot platinum surface.

Question 14.
Give an example for zero order reaction.
Answer:
Rate of a reaction which is independent of initial concentration of the reactant is called zero order reaction.
R → P rate = k[R]°
Example:
(1) Thermal decomposition of HI on gold surface.

(2) Decomposition of ammonia on hot platinum surface.

Question 15.
Rate = k[NO]² [O₂]¹. By how many times does the rate of reaction change when the volume of the reaction vessel is reduced to 1/3rd of its original volume? Will there be any change in the order of the reaction.
Answer:
r₁ = k[NO]² [O₂]¹

r₂ = 27r₁
Rate of reaction increases by 27 times
Order of the reaction remains same.

Question 16.
What is first order reaction? Give two examples.
Answer:
Rate of a reaction is proportional to first power of the concentration of the reactant.
R → P
rate = k[R]¹

Question 17.
What is second order reaction? Give two examples.
Answer:
Rate of a reaction is proportional to second power of the concentration of the reactant or reactants.
Example:

Question 18.
Give the unit for zero order first order and second order rate constants.
Answer:

Question 19.
Unit of rate constant of a reaction is same as the unit of rate of reaction. What is the order of the reaction.
Answer:
Zero order reaction.

Question 20.
Identify the reaction order from each of the following rate constants.
1. k = 2.3 × 1.0 3 L mol^-1 s^-1
2. k = 3 × 10^-4 s^-1
Answer:
1. Second order reaction
2. First order reaction.

Question 21.
Rate constant of a reaction is k = 3.14 × 10^-4 mol L^-1s^-1. What is the order of the reaction.
Answer:
Zero order reaction.

Question 22.
What is psuedo first order reaction? Give example.
Answer:
A reaction which seems to be higher order, but follows first order kinetics is called psuedo first order reaction.
Example:

Question 23.
What is molecularity of a reaction?
Answer:
The number of reacting species taking part in an elementory reaction is called molecularity of a reaction.

Question 24.
Write the molecularity for the reaction 2HI ⇌ H₂ + I₂
Answer:
Two.

Question 25.
What is unimolecular reaction? Give an example.
Answer:
When one reacting species involved in an elementary reaction is called unimolecular reaction. Example: NH₄NO₂ → N₂ + 2H₂O

Question 26.
What is bimoleeular reaction? Give an example.
Answer:
Molecular reaction which involves two reacting species in an elementary reaction.
Example: 2HI → H₂ + I₂

Question 27.
What is ter molecular reaction? Give an example.
Answer:
A reaction which involves simultaneous collision between three reacting species.
Example: 2NO + O₂ → 2NO₂

Question 28.
Give four differences between order and molecularity of a reaction.
Answer:

Order	Molecularity
1. Experimentally determined quantity.	1. Theoretical concept.
2. May be whole number fraction or zero.	2. Always a whole number
3. Applicable to elementary as well as complex reactions.	3. Applicable only for elementary reactions.
4. In a complex reaction, order is given to the slowest step.	4. In a complex reactions molecularity is given by each step of the reaction.

Question 29.
Derive an integrated rate equation for rate constant of a zero order reaction.
Answer:
Consider a zero order reaction.
R → P
Rate = \(\frac{-d[R]}{d t}\) = k[R]°
where k is rate constant for zero order reaction.
\(\frac{-d[R]}{d t}\) = k
d[R] = – k. dt
Integrating both sides
∫d[R] = -∫k. dt
[R] = -kt + l …… (1)
I is constant of integration.
At t = 0, [R]₀ = [R]₀ whore [R]₀ is initial concentration of the reactant.
Substituting in equation …… (1)
[R]₀ = -k × o + I; I = [R]₀
Substituting I value in equation (1)
[R] = -kt + [R]₀
k = \(\frac{[\mathrm{R}]_{0}-[R]}{t}\)

Question 30.
Derive an integrated rate equation for rate constant of a first order reaction.
Answer:
Consider a zero order reaction.
R → P
\(\frac{-d[R]}{d t}\) = k[R]¹
where k is velocity constant: of a first order reaction. —tif R]
\(\frac{-d[R]}{d t}\) = -k.dt
Integrating the equation
\(\int\frac{d[R]}{[R]}\) = -∫k. dt
In [R] = -kt + I ….. (1)
I is constant of integration
When t = 0 [R] = [R]₀ where [R]₀ is initial concentration of the reactant.
In [R]₀ = – k × 0 + 1
I = In [R]₀
Substituting I value in equation (i)
In [R] = -kt + In [R]₀
kt = In [R]₀ – In[R]
k = \(\frac{2.303}{t} \log \frac{[R]_{0}}{[R]}\)

Question 31.
For the following graph, identify order of reaction and mention unit of its rate constant.
Answer:

It is a first order reaction and its unit of rate constant is s^-1.

Question 32.
What is half life period of a reaction? Show that half period for a zero order reaction is directly proportional to initial concentration.
Answer:
Half life of a reaction is the time in which the concentration of a reactant is reduced to one half of its initial concentration.

This shows that T_1/2 of a zero order reaction is directly proportional to the initial concentration.

Question 33.
How many times does the t_1/2 of zero order reaction increases if the initial concentration of the reactant is doubled?
Answer:
t_1/2 gets doubled.

Question 34.
In a zero order reaction, the time taken to reduce the concentration of the reactant from 50% to 25% is 30 minutes. What is the time required to reduce the concentration from 25% to 12.5%.
Answer:
15 minutes.

Question 35.
What is half life period of a reaction? Show that half life of a first order reaction is independent of initial concentration.
Answer:
Half life of a reaction is the time in which the concentration of a reactant is reduced to one half of its initial concentration.
For first order reaction.

This shows that half life of a first order reaction is independent of initial concentration.

Question 36.
What happens to the half life period of a first order reaction if the concentration of the reactants is increased?
Answer:
Remains same.

Question 37.
A reaction completes 50% in 2 hours and 75% in 4 hours. What is the order of the reaction. Give reason.
Answer:
t_3/4 = 2 × t_1/2
4 = 2 × 2
Half life period is independent of initial concentration. Hence it is a first order reaction.

Question 38.
What is the effect of temperature on the rate of a reaction?
Answer:
As temperature increases rate of reaction increases. For every 10°C or 10 K increase in temperature rate of reaction gets doubled.

Question 39.
Write Arrhenius equation. Mention the symbols stands for.
Answer:

where k → is rate constant
A → is Arrhenitis factor
E_a → Is energy of activation
R → is gas constant
T → is absolute temperature.

Question 40.
Define temperature coefficient of a reaction. What is general value of it
Answer:
It is the ratio of rate constants of reaction at two different t1emperatures differing by 10°C.
Temperature coefficient = \(\frac{k_{T}+10}{k_{T}}\) = 2
Temperature coefficient of most of the reactions is 2.

Question 41.
Write
(i) Arrhenius equation
(ii) The formula to calculate half life period of zero order reaction.
Answer:
(i) Half life of a reaction is the time in which the concentration of a reactant is reduced to one half of its initial concentration.

This shows that T_1/2 of a zero order reaction is directly proportional to the initial concentration.

(ii)
where k → is rate constant
A → is Arrhenitis factor
E_a → Is energy of activation
R → is gas constant
T → is absolute temperature.

Question 42.
Write the energy distribution curve showing the temperature dependence of rate of a reaction.
Answer:

Question 43.
What is activation energy?
Answer:
Minimum excess energy that the normal reactant molecules must acquire in order their collision leads to chemical reaction.

Question 44.
Give equation to calculate activation energy when rate constants known at two different temperatures.
Answer:

E_a → Energy of activation
k₁ → is rate constant at temperature T₁
R → Gas constant
k₂ → is rate constant at temperature T₂

Question 45.
What is catalyst. Give an example.
Answer:
A catalyst is a substance which increases the rate of a reaction without itself undergoing any permanent chemical change.

In this reaction MnO₂ is a catalyst.

Question 46.
How catalyst increase rate of a reaction.
Answer:

Catalyst provides an alternate pathway by reducing the activation energy and lowering the energy barrier. More molecules enters the threshhold energy state leading to chemical reaction and hence increases the rate.

Question 47.
What happens to the energy of activation of a reaction when positive catalyst is added. [1M]
Answer:
Energy of activation decreases.

Question 48.
Draw a graph of potential energy v/s reaction co-ordinate showing the effect of a catalyst on activation energy.
Answer:

Catalyst provides an alternate pathway by reducing the activation energy and lowering the energy barrier. More molecules enters the threshhold energy state leading to chemical reaction and hence increases the rate.

Question 49.
Explain the effect of catalyst on the activation energy of the reaction with the graph.
Answer:

Catalyst provides an alternate pathway by reducing the activation energy and lowering the energy barrier. More molecules enters the threshhold energy state leading to chemical reaction and hence increases the rate.

Question 50.
Define collision frequency.
Answer:
The number of collisions per second per unit volume of the reaction mixture is known as collision frequency (z).

Question 51.
Give an expression to find out rate of a reaction according to collision theory.
Answer:

Where P is probability factor.
Z_AB collision frequency of reactants A and B
e^-Ea/RT fraction of molecules with energies equal to or greater than E_a.

Question 52.
According to collision theory what are the two factors that lead to the effective collision.
Answer:

Activation energy of reactants
Proper orientation of the molecules.

2nd PUC Chemistry Chemical Kinetics Problems and Solutions

Type – 1 Rate of a reaction

Question 1.
For the reaction R → P, the concentration of a reactant changes from 0.03 M to 0.02 M in 25 minutes. Calculate average rate of reaction.
Answer:
Formula:

Question 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 time interval.
Answer:

Question 3.
The conversion of molecules X to Y follows second order kinetics. If concentration of X is increased to three times how will it affect the rate of formation of Y.
Answer:
r₁ = k[X]²,
r₂ = k[3X]²,
r₂ = 9k[X]²
Rate increases by 9 times.

[Type – 2 ] Rate constant and half life period of first order reaction

Question 4.
The initial concentration of N₂O₅ in the following first order reaction N₂O₅(g) → 2NO₂(g) + \(\frac { 1 }{ 2 }\)O₂(g) was 1.24 × 10^-2 mol L^-1 at 318K. The concentration of N₂O₅ after 60 minutes was 0.2 × 10^-2 molL^-1. Calculate the rate constant of the reaction.
Answer:

Question 5.
The rate constant of a certain first order reaction is 200^-1s^-1. What is the half life period?
Answer:
Data: k = 200^-1 s^-1
t_1/2 = ?

Question 6.
A certain first order reaction is half completed in 46 min. Calculate the rate constant and also time for 75% completion of the reaction.
Answer:

Question 7.
Show that in case of a first order reaction, the time taken for completion of 99.9% reaction is ten times the time required for half change of the reaction.
Answer:

Question 8.
Show that t_99% = 2 × t_99%
Answer:

Question 9.
Rate constant of a first order reaction is 0.0693 min^-1. Calculate the percentage of the reactant remaining at the end of 60 minutes.
Answer:

Question 10.
The rate constant for a first order reaction is 60s^-1. How much time will it take to reduce the initial concentration of the reactant to its 1/16^th value.
Answer:

Question 11.
A first order reaction takes 40 min for 30% decomposition Calculate t_1/2.
Answer:

Question 12.
The half life for 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.
Answer:
k = 1.209 × 10^-4 year^-1
Age → 1848 year.

Question 13.
A first order reaction is found to have a rate constant k = 5.5 × 10^-14 s^-1. Find the half life of the reaction.
Answer:
t_1/2 → 4 1.26 × 10¹³ s

Question 14.
A first order reaction has a rate constant 1.15 × 10^-3 s^-1. How long will 5g of this reactant take to reduce to 3g?
Answer:

Question 15.
Time required to decompose SO₂Cl₂ to half of its initial amount is 60 minutes. If the decomposition is a first order reaction. Calculate the rate constant of the reaction.
Answer:
0.01155 min^-1.

Question 16.
The rate constant of a particular reaction doubles when the temperature changes from 300K to 3 10K, calculate the energy of activation.
Answer:
Data: Rate constant at 300 K is K₁
Rate constant at 310 K is K₂;
K₂ = 2K₁:
R = 8.314 J K^-1 mol^-1

Question 17.
The rate constants of a reaction at 500 K and 700 K are 0.02 s^-1 and 0.07s^-1 respectively. Calculate the value of E_a.
Answer:
18.230 kJ.

Question 18.
The rate of a chemical reaction doubles for an increase of 10K in absolute temperature from 298K. Calculate E_a.
Answer:
52.898 kJ.

Question 19.
The rate of a reaction quadruples when the temperature changes from 293K to 313 K. Calculate the energy of activation of the reaction.
Answer:
50.33 kJ.