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This MCQ module is based on: Rate of Reaction
TOPIC 9 OF 23
Rate of Reaction
🎓 Class 12
Chemistry
CBSE
Theory
Ch 3 – Chemical Kinetics
⏱ ~14 min
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Rate of Reaction
3.1 What Is Chemical Kinetics?
Chemical reactions happen at vastly different speeds. Some reactions, such as the combustion of hydrogen in oxygen or the precipitation of silver chloride, are over in a fraction of a second. Others, like the rusting of iron or the conversion of diamond into graphite, are so slow that they appear not to happen at all. Between these extremes lie reactions of moderate speed — neutralisation of an acid by a base or the inversion of cane sugar.
Chemical kinetics is the branch of chemistry which deals with the study of rates of chemical reactions, the factors affecting these rates, and the mechanism by which the reactions proceed.
Why study kinetics? Thermodynamics tells us whether a reaction is feasible (negative ΔG), but not how fast it will go. Both diamond → graphite and Fe + O₂ → Fe₂O₃ are thermodynamically spontaneous, yet diamonds last forever and iron rusts slowly. Kinetics fills this gap.
3.2 Rate of a Chemical Reaction
The rate of a chemical reaction is defined as the change in the concentration of any one of the reactants or products in unit time. For a hypothetical reaction
\[ R \;\longrightarrow\; P \]
if \([R]_1\) and \([P]_1\) are the concentrations of R and P at time \(t_1\), and \([R]_2\) and \([P]_2\) are the concentrations at time \(t_2\), then
\[ \Delta[R] = [R]_2 - [R]_1, \qquad \Delta[P] = [P]_2 - [P]_1, \qquad \Delta t = t_2 - t_1 \]
The average rate of the reaction is
\[ r_{\text{av}} \;=\; -\frac{\Delta[R]}{\Delta t} \;=\; +\frac{\Delta[P]}{\Delta t} \]
Square brackets denote molar concentration. The negative sign in front of \(\Delta[R]\) makes the rate positive (since \([R]\) decreases, \(\Delta[R]\) is negative).
Average Rate vs Instantaneous Rate
The average rate depends on the time interval over which it is measured. As the reaction proceeds, reactant concentration falls, so the rate keeps decreasing. To know the rate at a particular instant we shrink \(\Delta t \to 0\), giving the instantaneous rate:
\[ r_{\text{inst}} \;=\; \lim_{\Delta t \to 0} \frac{-\Delta[R]}{\Delta t} \;=\; \frac{-\,d[R]}{dt} \;=\; \frac{d[P]}{dt} \]
Graphically, the instantaneous rate is the slope of the tangent drawn to the concentration-time curve at the chosen instant.
Rate Expression for a General Reaction
For a reaction with stoichiometric coefficients
\[ a\,A \;+\; b\,B \;\longrightarrow\; c\,C \;+\; d\,D \]
concentrations change at different numerical speeds depending on the coefficients. To define a unique rate, we divide each rate by the coefficient:
\[ \text{Rate} \;=\; -\frac{1}{a}\frac{d[A]}{dt} \;=\; -\frac{1}{b}\frac{d[B]}{dt} \;=\; +\frac{1}{c}\frac{d[C]}{dt} \;=\; +\frac{1}{d}\frac{d[D]}{dt} \]
Example: For \(2\,N_2O_5 \to 4\,NO_2 + O_2\), the rate is
\(\text{Rate} = -\dfrac{1}{2}\dfrac{d[N_2O_5]}{dt} = +\dfrac{1}{4}\dfrac{d[NO_2]}{dt} = +\dfrac{d[O_2]}{dt}\).
\(\text{Rate} = -\dfrac{1}{2}\dfrac{d[N_2O_5]}{dt} = +\dfrac{1}{4}\dfrac{d[NO_2]}{dt} = +\dfrac{d[O_2]}{dt}\).
Units of Rate
Concentration is in mol L⁻¹ and time in seconds, so the unit of rate is mol L⁻¹ s⁻¹. For a gaseous reaction at constant volume the concentration of a gas is proportional to its partial pressure, and rate may be given in atm s⁻¹.
Sample Data: Hydrolysis of Butyl Chloride
The reaction \( C_4H_9Cl + H_2O \to C_4H_9OH + HCl \) was followed by measuring \([C_4H_9Cl]\) at intervals of 50 s. The average rate is computed for each interval.
| \([C_4H_9Cl]_{t_1}\) mol L⁻¹ | \([C_4H_9Cl]_{t_2}\) mol L⁻¹ | \(t_1\)/s | \(t_2\)/s | \(r_{\text{av}} \times 10^4\) / mol L⁻¹ s⁻¹ |
|---|---|---|---|---|
| 0.100 | 0.0905 | 0 | 50 | 1.90 |
| 0.0905 | 0.0820 | 50 | 100 | 1.70 |
| 0.0820 | 0.0741 | 100 | 150 | 1.58 |
| 0.0741 | 0.0671 | 150 | 200 | 1.40 |
| 0.0671 | 0.0549 | 200 | 300 | 1.22 |
| 0.0549 | 0.0439 | 300 | 400 | 1.10 |
| 0.0439 | 0.0335 | 400 | 500 | 1.04 |
| 0.0210 | 0.0170 | 700 | 800 | 0.40 |
Notice that the rate decreases as the reaction proceeds — a classic feature of most reactions, because reactant concentration is falling.
Interactive: Average Rate Calculator
Pick two concentrations of reactant R and the corresponding times to compute the average rate.
Average rate \(= -\dfrac{\Delta[R]}{\Delta t} =\) 2.00 × 10⁻⁴ mol L⁻¹ s⁻¹
Worked Example 3.1 L3 Apply
From the concentration of butyl chloride remaining after various times, calculate the average rate between t = 50 s and t = 100 s.
\(\text{Average rate} = -\dfrac{\Delta[C_4H_9Cl]}{\Delta t} = -\dfrac{(0.0820 - 0.0905)}{100 - 50} = -\dfrac{-0.0085}{50}\)
\( = 1.7 \times 10^{-4}\) mol L⁻¹ s⁻¹.
Worked Example 3.2 L3 Apply
The decomposition of \(N_2O_5\) in CCl₄ at 318 K has been studied by monitoring the concentration of \(N_2O_5\) in the solution. Initially, \([N_2O_5] = 2.33\) mol L⁻¹ and after 184 minutes it falls to \(2.08\) mol L⁻¹. Calculate the average rate during this interval in mol L⁻¹ s⁻¹.
\(\text{Average rate} = -\dfrac{1}{2}\dfrac{\Delta[N_2O_5]}{\Delta t}\) (note the stoichiometric coefficient 2).
\(= -\dfrac{1}{2} \cdot \dfrac{(2.08 - 2.33)}{184 \times 60} = \dfrac{0.125}{11040}\)
\( = 1.13 \times 10^{-5}\) mol L⁻¹ s⁻¹.
3.3 Factors Influencing the Rate of a Reaction
The rate of a chemical reaction depends on the experimental conditions under which it is studied. The principal factors are:
(i) Concentration of Reactants
By the law of mass action, the rate is proportional to a product of reactant concentrations raised to suitable powers. Increasing concentration usually increases rate.
(ii) Temperature
An increase in temperature accelerates almost every reaction. As a rough rule, the rate roughly doubles for every 10 K rise. The quantitative form is the Arrhenius equation studied later.
(iii) Catalyst
A catalyst alters the rate without itself being consumed by providing an alternative pathway with lower activation energy.
(iv) Surface Area (for heterogeneous systems)
Powdered solids react faster than lumps because of greater contact area. Coal dust burns explosively whereas a coal lump burns steadily.
(v) Pressure (for gaseous reactions)
Increasing pressure compresses gases, raises concentration, and speeds up the reaction.
Activity 3.1 — Effect of Concentration on Rate
Setup: Take three test tubes labelled A, B, C. Add 5 mL of 0.5 M, 0.25 M and 0.10 M sodium thiosulphate solutions respectively. Place a paper marked with a dark cross under each tube. Add 2 mL of 0.5 M HCl to each at the same time and start a stopwatch.
Predict: In which test tube will the cross disappear first as colloidal sulphur clouds the solution?
The cross disappears fastest in tube A (highest thiosulphate concentration) and slowest in tube C. The reaction is
\( Na_2S_2O_3 + 2\,HCl \to 2\,NaCl + SO_2 + S\downarrow + H_2O \)
The colloidal sulphur formed makes the solution opaque. Higher concentration of \(S_2O_3^{2-}\) means more frequent productive collisions per second → faster rate. This is a direct demonstration of the law of mass action.
Worked Example 3.3 L4 Analyse
Express the rate of the following reaction in terms of disappearance of hydrogen and formation of ammonia:
\( N_2(g) + 3\,H_2(g) \to 2\,NH_3(g) \).
The unique rate is
\(\text{Rate} = -\dfrac{d[N_2]}{dt} = -\dfrac{1}{3}\dfrac{d[H_2]}{dt} = +\dfrac{1}{2}\dfrac{d[NH_3]}{dt}\)
Therefore the rate of disappearance of \(H_2\) and rate of formation of \(NH_3\) are related as
\(-\dfrac{d[H_2]}{dt} = \dfrac{3}{2}\dfrac{d[NH_3]}{dt}\)
i.e. \(H_2\) is consumed 1.5 times faster than \(NH_3\) is produced.
Competency-Based Questions
Q1. The average rate of a reaction \( A \to B \) over the interval 0–10 s is reported as \(2.5 \times 10^{-3}\) mol L⁻¹ s⁻¹. Which of the following statements is most accurate? L4
Ans (c). Average rate is the arithmetic average over the interval; instantaneous rate is generally largest at the start (when [A] is highest) and falls thereafter, so neither end of the interval need equal the average.
Q2. For the reaction \(2\,SO_2 + O_2 \to 2\,SO_3\), if the rate of disappearance of \(O_2\) is \(2.5 \times 10^{-4}\) mol L⁻¹ s⁻¹, what is the rate of formation of \(SO_3\)? L3
Rate \(= -\dfrac{d[O_2]}{dt} = +\dfrac{1}{2}\dfrac{d[SO_3]}{dt}\). So \(\dfrac{d[SO_3]}{dt} = 2 \times 2.5 \times 10^{-4} = 5.0 \times 10^{-4}\) mol L⁻¹ s⁻¹.
Q3. (Short answer) Why does powdered limestone react faster with dilute HCl than a lump of the same mass? L2
Powdered limestone has a far larger total surface area exposed to the acid. More \(CaCO_3\) particles can collide with \(H^+\) ions per unit time, so the rate of effervescence is greater.
Q4. (True/False) The unit of average rate of a reaction is always mol L⁻¹ s⁻¹. L2
False. For gaseous reactions where partial pressures are used, the unit may be atm s⁻¹ or Pa s⁻¹. Time may also be in min or h.
Q5. (Long answer) A graph of [R] vs t is concave-up (rate decreasing). On the same axes, sketch [P] vs t and explain how the slope of the tangent at any instant relates to instantaneous rate of reaction. L5
The [P]-vs-t curve is the mirror image: starts at zero, rises steeply, then flattens. At any instant t*, the magnitudes of the slopes of the tangents to the [R] and [P] curves are equal (for 1:1 stoichiometry). The slope of the [P]-tangent equals \(+d[P]/dt\) = instantaneous rate; the slope of the [R]-tangent is \(-d[R]/dt\), also equal to the instantaneous rate.
Assertion–Reason Questions
(A) Both A and R true and R is correct explanation of A. (B) Both true but R is not correct explanation. (C) A true, R false. (D) A false, R true.
Assertion: A negative sign is used while writing the average rate in terms of reactant concentration.
Reason: The concentration of the reactant decreases with time, so \(\Delta[R]\) is negative; the negative sign makes the rate a positive quantity.
(A) Both true and R correctly explains A.
Assertion: The instantaneous rate of a reaction equals the average rate when \(\Delta t \to 0\).
Reason: The slope of the chord becomes the slope of the tangent in the limit.
(A) Both true; reason is the geometrical interpretation of the derivative.
Assertion: The rate of a reaction usually decreases with time.
Reason: The activation energy of a reaction increases as the reaction proceeds.
(C) Assertion is true (because reactant concentration falls). Reason is false — activation energy is fixed for a given reaction, independent of how much has reacted.
Frequently Asked Questions - Rate of Reaction
What is the main concept covered in Rate of Reaction?
In NCERT Class 12 Chemistry Chapter 3 (Chemical Kinetics), "Rate of Reaction" covers the core chemistry principles and reactions students need for board exam success. The MyAiSchool lesson explains the topic with definitions, structural diagrams, reaction mechanisms, worked examples, and interactive simulations. Key reactions, IUPAC names, and chemical reasoning are highlighted throughout aligned with CBSE 2025-26 syllabus.
How is Rate of Reaction useful in real-life or applied chemistry?
Real-life applications of "Rate of Reaction" from NCERT Class 12 Chemistry Chapter 3 include drug design, polymer industry, food chemistry, electrochemical cells, fuel cells, dyes/pigments, agrochemicals, and biochemistry. The MyAiSchool lesson links every concept to a tangible industrial or biological example so students see chemistry as a problem-solving framework for the molecular world.
What are the key reactions students should memorize for Rate of Reaction?
Key reactions in "Rate of Reaction" (NCERT Class 12 Chemistry Chapter 3 Chemical Kinetics) are tabulated in the MyAiSchool reaction map. Students should memorize each reaction with its reagent, conditions, mechanism class (SN1/SN2/E1/E2/electrophilic addition/etc), product, and stereochemistry. The Summary section provides a quick-reference reaction chart for last-minute revision.
How does this part connect to other parts of Chapter 3?
NCERT Class 12 Chemistry Chapter 3 (Chemical Kinetics) is structured so each part builds chemical understanding sequentially. "Rate of Reaction" connects to neighbouring parts via shared functional groups, reaction mechanisms, and structural concepts. The MyAiSchool lesson cross-references related concepts with internal links so students can navigate the whole chapter as one connected story rather than disconnected fragments.
What types of CBSE board questions come from Rate of Reaction?
CBSE board questions from "Rate of Reaction" typically include: (1) 1-mark MCQs on definitions and IUPAC naming, (2) 2-mark short-answer reactions/products, (3) 3-mark mechanism questions, (4) 5-mark long-answer combining mechanism + product + stereochemistry + application. The MyAiSchool lesson tags each Competency-Based Question (CBQ) with Bloom level (L1-L6) so students know how to study for each weight.
How can students use the interactive simulation effectively?
The interactive simulation in the "Rate of Reaction" lesson allows students to explore reaction outcomes, predict products, or compare reaction conditions, with live visual feedback. To use it effectively: (1) try every option/configuration, (2) compare with the analytical reasoning, (3) check IUPAC names and structural correctness, (4) test edge cases from worked examples. The simulation reinforces conceptual intuition that pure mechanism memorisation cannot provide.
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