Chapter 3: Chemical Kinetics Short Questions | chemistry Class 12 CBSE

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Chapter 3: Chemical Kinetics Short Questions | chemistry Class 12 CBSE | ByteNotes.in

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Question 1

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Define the rate of a chemical reaction and state its SI units.

The rate of a reaction is defined as the change in concentration of a reactant or product per unit time.
It can be expressed as rate of decrease of reactant concentration: –Δ[R]/Δt, or rate of increase of product concentration: +Δ[P]/Δt.
The SI unit of rate of reaction is mol L⁻¹s⁻¹; for gaseous reactions expressed in partial pressure, the unit is atm s⁻¹.

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Question 2

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Distinguish between average rate and instantaneous rate of a reaction.

Average rate is the change in concentration of a reactant or product over a large, finite time interval Δt, given by –Δ[R]/Δt.
Instantaneous rate is the rate at a specific moment in time, obtained when Δt → 0, expressed as –d[R]/dt; it equals the slope of the tangent to the concentration–time curve at that point.
Average rate is constant for the interval considered and cannot predict the rate at a particular instant, whereas instantaneous rate gives the exact rate at any chosen moment.

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Question 3

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For the reaction 2HI(g) → H₂(g) + I₂(g), write the expression for the rate of reaction in terms of each species involved.

Since two moles of HI decompose to produce one mole each of H₂ and I₂, the stoichiometric coefficients are unequal.
The rate expression is: Rate = –(1/2) Δ[HI]/Δt = Δ[H₂]/Δt = Δ[I₂]/Δt.
Each rate term is divided by its stoichiometric coefficient so that all expressions give the same numerical value for the rate of reaction.

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Question 4

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What is meant by rate law? Why can it not be predicted from the balanced chemical equation?

Rate law is the mathematical expression that relates the rate of a reaction to the molar concentrations of reactants, with each concentration raised to a power determined experimentally.
These powers (order with respect to each reactant) may or may not equal the stoichiometric coefficients in the balanced equation, as seen in reactions like CHCl₃ + Cl₂ → CCl₄ + HCl where Rate = k[CHCl₃][Cl₂]^(1/2).
The balanced equation only gives the overall stoichiometry of reactants and products; the actual pathway and rate-determining steps are not reflected in it, making experimental determination of the rate law essential.

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Question 5

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Define order of a reaction. Give an example of a reaction with fractional order.

The order of a reaction is the sum of the powers of the concentration of the reactants in the rate law expression; it is always determined experimentally.
Order can be 0, 1, 2, 3, or even a fraction. A zero order reaction means the rate is independent of reactant concentration.
Example of fractional order: for the reaction CH₃CHO(g) → CH₄(g) + CO(g), Rate = k[CH₃CHO]^(3/2), giving an overall order of 3/2 (one and a half order).

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Question 6

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What are the units of rate constant for a zero order and a first order reaction?

For a zero order reaction, units of rate constant k are mol L⁻¹s⁻¹ (same as units of rate, since rate = k × (concentration)⁰ = k).
For a first order reaction, units of k are s⁻¹ (or time⁻¹), since rate = k[concentration]¹ gives k = rate/concentration = mol L⁻¹s⁻¹ / mol L⁻¹ = s⁻¹.
The general formula for units of k is: (mol L⁻¹)^(1–n) s⁻¹, where n is the order of the reaction.

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Question 7

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Define molecularity of a reaction. Give one example each of unimolecular and bimolecular reactions.

Molecularity is the number of reacting species (atoms, ions, or molecules) that must collide simultaneously in an elementary reaction for the chemical reaction to occur.
Example of unimolecular reaction: decomposition of ammonium nitrite — NH₄NO₂ → N₂ + 2H₂O, where only one species decomposes.
Example of bimolecular reaction: dissociation of hydrogen iodide — 2HI → H₂ + I₂, which involves simultaneous collision of two HI molecules.

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Question 8

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Distinguish between order and molecularity of a reaction with respect to their applicability.

Order is an experimentally determined quantity; it can be zero, a whole number, or even a fraction, and is applicable to both elementary and complex reactions.
Molecularity is a theoretical concept defined only for elementary reactions; it cannot be zero or a non-integer, and its values are restricted to 1, 2, or 3.
For complex reactions, molecularity has no meaning; the overall order is determined by the slowest (rate-determining) step, and the molecularity of that step equals the overall order.

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Question 9

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Write the integrated rate equation for a zero order reaction and state what graph gives a straight line.

For a zero order reaction R → P, the rate is –d[R]/dt = k, and on integration: [R] = [R]₀ – kt, where [R]₀ is the initial concentration.
A plot of [R] (concentration) against time t gives a straight line with slope = –k and y-intercept = [R]₀.
The rate constant k for zero order reaction is calculated as k = ([R]₀ – [R])/t, with units mol L⁻¹s⁻¹.

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Question 10

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Write the integrated rate equation for a first order reaction and identify the straight line plot used to determine k.

For a first order reaction R → P, the integrated rate equation is: ln[R] = –kt + ln[R]₀, or equivalently [R] = [R]₀e^(–kt).
The rate constant can be calculated using: k = (2.303/t) log([R]₀/[R]).
A plot of ln[R] vs t gives a straight line with slope = –k and intercept = ln[R]₀; alternatively, a plot of log([R]₀/[R]) vs t gives slope = k/2.303.

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Question 11

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Derive the expression for half-life of a first order reaction and show that it is independent of initial concentration.

For a first order reaction: k = (2.303/t) log([R]₀/[R]). At t = t₁/₂, [R] = [R]₀/2.
Substituting: k = (2.303/t₁/₂) log([R]₀/([R]₀/2)) = (2.303/t₁/₂) log 2 = (2.303 × 0.301)/t₁/₂.
Therefore, t₁/₂ = 0.693/k. Since k is a constant at a given temperature and this expression contains no [R]₀ term, the half-life is independent of initial concentration.

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Question 12

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How does the half-life of a zero order reaction differ from that of a first order reaction?

For a zero order reaction, half-life is t₁/₂ = [R]₀/2k, which is directly proportional to the initial concentration [R]₀ and inversely proportional to the rate constant k.
For a first order reaction, half-life is t₁/₂ = 0.693/k, which is entirely independent of the initial concentration of the reactant.
This fundamental difference means that for zero order reactions, successive half-lives decrease (as [R]₀ decreases), while for first order reactions, every half-life is equal regardless of concentration.

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Question 13

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What are pseudo first order reactions? Give one example.

Pseudo first order reactions are those that are of higher order overall, but appear to follow first order kinetics because one reactant is present in such large excess that its concentration remains practically unchanged during the reaction.
Example: Hydrolysis of ethyl acetate — CH₃COOC₂H₅ + H₂O → CH₃COOH + C₂H₅OH. Although it is a second order reaction, water is in large excess (about 1000 times), so its concentration is essentially constant, and the rate depends only on [CH₃COOC₂H₅].
The effective rate law becomes Rate = k[CH₃COOC₂H₅], making it pseudo first order.

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Chapter 3: Chemical Kinetics | Short Questions

Define the rate of a chemical reaction and state its SI units.

Distinguish between average rate and instantaneous rate of a reaction.

For the reaction 2HI(g) → H₂(g) + I₂(g), write the expression for the rate of reaction in terms of each species involved.

What is meant by rate law? Why can it not be predicted from the balanced chemical equation?

Define order of a reaction. Give an example of a reaction with fractional order.

What are the units of rate constant for a zero order and a first order reaction?

Define molecularity of a reaction. Give one example each of unimolecular and bimolecular reactions.

Distinguish between order and molecularity of a reaction with respect to their applicability.

Write the integrated rate equation for a zero order reaction and state what graph gives a straight line.

Write the integrated rate equation for a first order reaction and identify the straight line plot used to determine k.

Derive the expression for half-life of a first order reaction and show that it is independent of initial concentration.

How does the half-life of a zero order reaction differ from that of a first order reaction?

What are pseudo first order reactions? Give one example.

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