Class 12 Chemistry Unit 3: Chemical Kinetics – Practice Questions Quiz (2025-26)
Thermodynamics predicts if a reaction occurs, but Chemical Kinetics determines the speed. For Class XII students tackling the 2025-26 syllabus, mastering the “time” variable in chemistry is required for solving numerical problems and interpreting rate graphs correctly.
This guide breaks down the mathematics of reaction rates, comparing Average and Instantaneous speeds while distinguishing experimental Order from theoretical Molecularity. From deriving Integrated Rate Equations for Zero and First Order reactions to calculating Activation Energy using the Arrhenius formula, these sections provide the specific tools needed for board exams.
Explore how concentration, temperature, and catalysts dictate reaction velocity through interactive visualizers designed to clarify the mechanics behind the math.
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Class XII Unit 3: Chemical Kinetics
Chemical Kinetics: The Dimension of Time
Thermodynamics predicts if a reaction is feasible, but Kinetics defines the speed. This guide covers the complete 2025-26 syllabus for Unit 3, identifying how concentration, temperature, and catalysts dictate the pace of chemical change.
Knowledge Check
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Speed & Accuracy Challenge
Ready to test your mastery of Chemical Kinetics? You have 20 questions. The clock starts when you click below.
Rate Concepts & Stoichiometry
Average Rate
Measured over a time interval. Ignores rate fluctuations.
Instantaneous Rate
Rate at a specific moment (\(\Delta t \to 0\)). The slope of the tangent.
Stoichiometric Normalization
For \( aA + bB \to cC \), the unique rate of reaction is found by dividing the rate of change of any species by its stoichiometric coefficient:
Rate = \( -\frac{1}{a}\frac{d[A]}{dt} = -\frac{1}{b}\frac{d[B]}{dt} = \frac{1}{c}\frac{d[C]}{dt} \)
Concept: Pseudo First Order Reaction
A reaction that is theoretically higher order but behaves as first order because one reactant is present in large excess.
Example: Acid hydrolysis of ethyl acetate. Water is in excess, so its concentration remains effectively constant.
\( Rate = k'[Ester][H_2O] \approx k[Ester] \) where \( k = k'[H_2O] \).
Decay Curves
Visualizing Concentration [R] vs Time (t)
Comparison Logic
Rate is constant. Concentration drops linearly. Slope = \(-k\).
\( [R] = -kt + [R]_0 \)
Rate decreases as conc. drops. Exponential decay. Constant Half-Life.
\( \ln[R] = -kt + \ln[R]_0 \)
Half-Life Dynamics
Zero Order: Half-life decreases as concentration drops.
First Order: Half-life remains constant regardless of concentration.
Select a reaction order above.
Arrhenius Equation & Temperature
The Arrhenius equation relates rate constant (\(k\)) to temperature (\(T\)).
\( \ln k = -\frac{E_a}{R}\frac{1}{T} + \ln A \)
Plot of \(\ln k\) vs \(1/T\) (Linear)
Key Parameters
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Slope (\(m\)): Represents \( -E_a/R \). A steeper slope means higher Activation Energy (\(E_a\)).
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Intercept (\(c\)): Represents \( \ln A \). \(A\) is the Frequency Factor.
Temperature Coefficient (\(\mu\))
For most reactions, rate constant nearly doubles for a 10° rise in temperature.
\( \mu = \frac{k_{T+10}}{k_T} \approx 2 \) to \( 3 \).
Collision Theory & Catalysis
Reactions occur when molecules collide. However, not all collisions lead to products. Two barriers must be overcome:
- Energy Barrier: Kinetic energy \(\ge\) Activation Energy (\(E_a\)).
- Orientation Barrier: Molecules must align correctly (Steric factor \(P\)).
Mathematical Form
- \(Z_{AB}\): Collision Frequency
- \(P\): Probability/Steric Factor
- \(e^{-E_a/RT}\): Fraction of energetic collisions
Effect of Catalyst
A catalyst provides an alternate pathway with lower \(E_a\). It does not change the Enthalpy (\(\Delta H\)) or Gibbs Energy (\(\Delta G\)) of the reaction. It simply speeds up the attainment of equilibrium.
Numerical Cheat Sheet
Unit of k: \( s^{-1} \)
Independent of concentration
T in Kelvin, R = 8.314
Unit of k: \( mol L^{-1} s^{-1} \)
Conceptual Doubts (FAQ)
Q: Why does a 10° rise in temperature nearly double the rate?
Collision frequency (\(Z\)) only increases by ~3%. The primary reason is the exponential increase in the fraction of effective collisions. A small temperature rise pushes significantly more molecules over the Activation Energy (\(E_a\)) barrier.
Q: Can Molecularity be zero or fractional?
No. Molecularity refers to the count of reacting species colliding simultaneously in an elementary step. You cannot have zero molecules colliding, nor half a molecule. It must be a positive integer.
Q: Do Zero Order reactions go to completion?
Yes. Unlike First Order reactions (which theoretically take infinite time), Zero Order reactions consume reactants at a constant rate and reach completion in finite time: \( t_{completion} = [R]_0 / k \).
Q: Does a catalyst change the Equilibrium Constant (Kc)?
No. A catalyst lowers the activation energy for both forward and reverse reactions equally. It speeds up the attainment of equilibrium but does not shift the position of equilibrium or change \(\Delta G\).
Q: What is the unit of k for an \(n^{th}\) order reaction?
The general formula is \( (mol \ L^{-1})^{1-n} \ s^{-1} \).
For \(n=0\): \(mol \ L^{-1} \ s^{-1}\).
For \(n=1\): \(s^{-1}\).
For \(n=2\): \(mol^{-1} \ L \ s^{-1}\).
Chapter Summary
We have explored the dynamics of chemical change. From defining rates to analyzing the temperature dependence via Arrhenius, you now possess the tools to calculate when a reaction completes. Remember: Order is experimental, Molecularity is theoretical, and Temperature increases rate by increasing effective collisions, not just total collisions.
