Chemical Kinetics Class 12 One Shot | Grade 12th Chemistry Chapter 3 Revision | CBSE 2026-27 Exam

Main Ideas and Lessons (Chemical Kinetics – Class 12, Chapter 3)

1) What “chemical kinetics” is

Chemical kinetics is the branch of chemistry that studies:

• Reaction rates
• Reaction mechanisms (how reactions occur step-by-step)

2) Reaction rate: meaning and qualitative behavior

Reaction rate describes:

• How quickly reactants are converted to products
• The speed at which products are formed

During a reaction:

• Reactant concentration decreases with time
• Product concentration increases with time

3) Types of rate

• Average rate: calculated over a time interval (longer duration)
• Instantaneous rate: rate at a specific instant (small time span), often found using the slope on a concentration–time graph

4) Average reaction rate (core concept)

For a generic balanced reaction: [ aA + bB \rightarrow cC + dD ]

Average rate is expressed as: [ \text{Rate} = -\frac{1}{a}\frac{\Delta [A]}{\Delta t} = -\frac{1}{b}\frac{\Delta [B]}{\Delta t} = +\frac{1}{c}\frac{\Delta [C]}{\Delta t} = +\frac{1}{d}\frac{\Delta [D]}{\Delta t} ]

Sign convention:

• Negative sign for reactants (concentration falls)
• Positive sign for products (concentration rises)

Stoichiometric coefficients (a, b, c, d) appear in the denominator.

5) Factors affecting reaction rate (general list)

The rate depends on multiple factors, including:

• Concentration of reactants
• Temperature
• Nature of reactants
• Catalyst
• Surface area (greater surface area → faster rate; e.g., powdered sugar vs crystal sugar)
• Presence of sunlight (important for photochemical reactions)
• Physical state of reactants (gases generally react faster due to greater mobility and kinetic energy)

6) Rate law and order of reaction

(a) Rate law

A rate law is a mathematical relationship showing how reaction rate depends on reactant concentrations.

Example: [ \text{Rate} = k[A]^x[B]^y ] where:

• (x, y) are experimental powers
• (k) is the rate constant

(b) Order of reaction

• Order with respect to a reactant = power of that reactant in the rate law (e.g., power of ([A]) is the order w.r.t. A)

• Overall order: [ n = x + y (+ \cdots) ]

Important: The powers (x, y) are not necessarily the same as stoichiometric coefficients from the balanced equation—they must be found by experiments.

7) Units of rate constant (k) and relation to order

Since: [ \text{Rate} = k[A]^x[B]^y ] the unit of (k) depends on the overall order (n). In general, the unit involves concentration units raised to a power and time units inversely, so it changes with order.

8) Zero, first, and second order reactions (conceptual framework)

• Zero order

  • Rate does not depend on reactant concentration.

  • Integrated form: [ a = a_0 - kt ]

  • Units of (k) reflect concentration/time.

  • First order

  • Rate depends linearly on concentration: [ \text{Rate} \propto [A] ]

  • Half-life: [ t_{1/2} = \frac{0.693}{k} ]

• Second order

  • Rate depends on concentration squared (or overall order equals 2).
  • The video emphasizes identifying/using order, half-life, and unit methods.

9) Half-life concept (key use)

• Half-life = time required for 50% completion

Behavior with order:

• Zero order: half-life depends on the initial concentration
• First order: half-life depends only on (k): [ t_{1/2} = \frac{0.693}{k} ]

10) Temperature dependence and Arrhenius equation

Collision theory → why temperature matters

A reaction occurs when collisions are effective, meaning:

• Correct orientation
• Sufficient energy to cross the energy barrier

Key energy terms:

• Activation energy (E_a): minimum energy needed for effective collision/product formation
• Threshold energy: minimum energy requirement for reaction (activation threshold concept)

Arrhenius equation

Relationship between rate constant (k) and temperature (T): [ k = A e^{-E_a/(RT)} ]

Common derived relation: [ \log\left(\frac{k_2}{k_1}\right)=\frac{E_a}{2.303R}\left(\frac{1}{T_1}-\frac{1}{T_2}\right) ]

11) Elementary vs complex reactions; molecularity vs order

Elementary reaction

• Occurs in one step
• For elementary reactions:
  • rate law exponents match stoichiometric coefficients in that step

Complex reaction

• Occurs in two or more steps
• Rate controlled by the slowest (rate-determining) step

Molecularity

• Molecularity = number of reacting species that collide simultaneously in an elementary reaction
• Theoretical values: 1, 2, 3, …
• Higher values are rare
• Molecularity is mechanism-based (theoretical)

Order of reaction

• Order = experimentally determined dependence of rate on concentration from the rate law
• Order can be fractional/zero/integers and is not directly inferred from balanced equations alone

12) “Apparent” first order (false first order idea)

A reaction may appear first order if one reactant is present in very large excess, so its concentration stays nearly constant. Then:

• rate effectively depends on the other reactant only
• the observed rate law changes, while molecularity remains based on the mechanism

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Methodologies / Instruction-like Workflows Included

A) How to compute average rate (for (aA+bB\to cC+dD))

• Identify stoichiometric coefficients (a, b, c, d)
• Measure concentration changes over a time interval:
  • (\Delta[A], \Delta[B], \Delta[C], \Delta[D])
• Use sign convention:
  • Negative for reactants, positive for products
• Divide each concentration change by its coefficient:
  • e.g., divide (\Delta[A]) by (a)
• Use: [ \frac{\Delta(\text{concentration})}{\Delta t} ]

B) How to identify instantaneous rate using graphs

• From a concentration–time graph, instantaneous rate at a point = slope at that point

• Slope idea between two points: [ \frac{y_2-y_1}{x_2-x_1} ]

• Link slope to: [ \frac{\Delta[\text{species}]}{\Delta t} ]

C) How to determine order from rate law and/or units of (k)

• From rate law:
  • order w.r.t. a reactant = exponent of its concentration term
  • overall order = sum of exponents
• From units of (k):
  • units of (k) vary with overall order (n)
  • infer (n) from the given unit (as shown by example-driven inference)

D) How to compute activation energy (E_a) (first-order half-life + Arrhenius/log form)

Workflow:

• Use the first-order relation: [ t_{1/2} = \frac{0.693}{k} ] to determine (k) at each temperature

• Apply: [ \log\left(\frac{k_2}{k_1}\right)=\frac{E_a}{2.303R}\left(\frac{1}{T_1}-\frac{1}{T_2}\right) ]

• Solve for (E_a), including careful unit handling

E) How to solve time for completion in zero/first order

• Zero order: [ a = a_0 - kt ]

• First order:

  • half-life method: [ t_{1/2}=\frac{0.693}{k} ]

  • or integrated log relation: [ t = \frac{2.303}{k}\log\left(\frac{a_0}{a}\right) ]

• If percentages are given:

  • convert completion/remaining into concentration ratio (a_0) and (a)

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Speakers / Sources Featured

• Tapur Omar — “Chemistry Educator” / presenter of the lecture
• No other named sources or speakers are clearly identifiable from the subtitles.