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Chemical Kinetics
Chemical kinetics studies how fast a chemical reaction occurs and which factors control that speed. In Class 12 Chemistry, this chapter connects concentration changes, time, temperature, catalysts, and reaction mechanism with measurable reaction rates. The chapter is important for numerical questions because rate expressions, integrated rate equations, half-life relations, and Arrhenius calculations require correct formula selection, units, substitution, and interpretation. A reaction may be fast or slow depending on concentration, temperature, surface area, and catalyst. Chemical kinetics does not only ask what products form; it asks how quickly reactants are converted into products and what pathway the reaction may follow. CBSE questions commonly test rate law, order, molecularity, graphical identification of order, half-life, activation energy, and the role of catalysts using numerical data, short reasoning, assertion-reason, and graph-based interpretation.
Difficulty
Medium
Study time
70-90 min
Plan by time
Pick the window that matches what you have right now.
If you have 15 min
Last-pass revision
Skim the Quick Revision table — definitions, formulas, and the traps board examiners reuse.
Open Quick RevisionIf you have 45 min
Targeted practice
Read the high-priority concepts, then take the chapter MCQ quiz to find weak spots.
Start MCQ QuizIf you have 70 min
First full pass
Walk every concept in chapter order, then revise and quiz. Best for the first time you study this chapter.
Open Key ConceptsChapter Learning Map
Start with one of the buckets below, then open the full map when you want the complete concept roadmap.
Key Concepts
Work through the concepts in order. Each one has an explanation, a worked example, a self-check and an exam answer.
All Concepts
All 5 concepts of the chapter, in the order the NCERT chapter teaches them.
Rate of Reaction
Rate of reaction is the change in concentration of a reactant or product per unit time. For a reactant it is written with a negative sign because its concentration decreases, while for a product it is written with a positive sign because its concentration increases.
Rate Law, Order and Molecularity
Rate law expresses reaction rate in terms of molar concentrations of reactants raised to experimentally determined powers. Order is the sum of these powers. Molecularity is the number of reacting species taking part in a single elementary step.
Zero, First and Second Order Reactions
Zero, first, and second order reactions are classified by how rate depends on reactant concentration. Their integrated rate equations relate concentration and time, allowing calculation of rate constant, concentration remaining, or half-life.
Arrhenius Equation
The Arrhenius equation relates the rate constant of a reaction to temperature and activation energy: k = Ae−Ea/RT, where A is the frequency factor, Ea is activation energy, R is the gas constant, and T is absolute temperature.
Effect of Catalyst
A catalyst increases reaction rate by providing an alternate pathway with lower activation energy. It does not change the overall enthalpy change of the reaction and is regenerated by the end of the reaction.
Exam Intelligence
The traps that cost marks in this chapter, and how its main concepts turn up in the paper.
Common Traps
- Ignoring stoichiometric coefficients while writing rate expressions.
- Using balanced equation coefficients as rate law powers for overall reactions.
- Applying first order half-life formula to zero or second order reactions.
- Mixing seconds and minutes in k calculations.
- Using Celsius instead of kelvin in Arrhenius equation.
- Forgetting the negative sign in the Arrhenius plot slope.
- Claiming that a catalyst changes Delta H or equilibrium constant.
How CBSE Asks This Chapter
- Rate of Reaction
Questions ask students to calculate average rate from concentration data, compare rates of disappearance and formation, or identify the correct expression using a balanced chemical equation.
- Rate Law, Order and Molecularity
CBSE questions commonly give initial rate data and ask for order, rate law, or unit of k. Assertion-reason questions often contrast order and molecularity.
- Zero, First and Second Order Reactions
This concept appears as numerical problems on k, t1/2, concentration left, and graph-based identification of order. Long-answer questions may ask for derivation or comparison of equations and half-life relations.
- Arrhenius Equation
Questions ask for activation energy from slope, comparison of rate constants at two temperatures, explanation of temperature effect, and interpretation of energy distribution or activation barrier diagrams.
Quick Revision
Concept, formula or equation to remember, and the trap that loses marks — in one scannable view.
Remember
Average rate for reactant = -Delta[R]/Delta t; average rate for product = +Delta[P]/Delta t.Trap to avoid
Students often forget to divide by stoichiometric coefficients, especially for species such as H2 or NH3 in balanced equations.
Rate Law, Order and Molecularity
Remember
Rate law: rate = k[A]a[B]b.Trap to avoid
Students often take powers directly from the balanced equation for every reaction, but rate law powers are experimental for overall reactions.
Zero, First and Second Order Reactions
Remember
Zero order: [A] = [A0] - kt, t1/2 = [A0]/2k, k unit = mol L−1 s−1.Trap to avoid
Students often use t1/2 = 0.693/k for every order, but that formula applies only to first order reactions.
Remember
Arrhenius equation: k = Ae−Ea/RT.Trap to avoid
Students sometimes use Celsius temperature in Arrhenius calculations.
Remember
Catalyst effect in Arrhenius terms: lowering Ea increases k in k = Ae−Ea/RT.Trap to avoid
Students often write that a catalyst increases product yield at equilibrium.
| Concept | Remember | Trap to avoid |
|---|---|---|
| Rate of Reaction | Average rate for reactant = -Delta[R]/Delta t; average rate for product = +Delta[P]/Delta t. | Students often forget to divide by stoichiometric coefficients, especially for species such as H2 or NH3 in balanced equations. |
| Rate Law, Order and Molecularity | Rate law: rate = k[A]a[B]b. | Students often take powers directly from the balanced equation for every reaction, but rate law powers are experimental for overall reactions. |
| Zero, First and Second Order Reactions | Zero order: [A] = [A0] - kt, t1/2 = [A0]/2k, k unit = mol L−1 s−1. | Students often use t1/2 = 0.693/k for every order, but that formula applies only to first order reactions. |
| Arrhenius Equation | Arrhenius equation: k = Ae−Ea/RT. | Students sometimes use Celsius temperature in Arrhenius calculations. |
| Effect of Catalyst | Catalyst effect in Arrhenius terms: lowering Ea increases k in k = Ae−Ea/RT. | Students often write that a catalyst increases product yield at equilibrium. |
Practice
Use short concept checks first, then move into the full chapter test.
Free Chapter MCQ Quiz
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