Class 12 Chemistry - ISC
Chemical Kinetics
Chemical Kinetics is a crucial physical chemistry chapter in the Class 12 ISC curriculum that explores the rates of chemical reactions and the factors affecting them. You will learn about the rate of a reaction, rate law, molecularity, order of reaction (especially zero and first-order reactions), and the Arrhenius equation which relates reaction rate to temperature. This chapter carries significant weight in the ISC Chemistry board exams, usually around 5-7 marks, featuring both conceptual reasoning questions and numerical problems based on integrated rate equations and half-life periods.
Start Learning FreeKey Concepts
Rate of a Reaction
The change in concentration of a reactant or product per unit time, expressed in units like mol L^-1 s^-1.
Rate Law and Specific Rate Constant
An expression that relates the rate of a reaction to the molar concentrations of reactants with each term raised to some power, where the proportionality constant is the rate constant (k).
Order of a Reaction
The sum of the powers of the concentration terms of the reactants in the rate law expression, which can be zero, fractional, or integer values determined experimentally.
Molecularity of a Reaction
The number of reacting species colliding simultaneously in an elementary reaction to bring about a chemical change; it is always a whole number and cannot be zero or fractional.
Integrated Rate Equations
Mathematical equations that give the relationship between concentration and time for zero-order and first-order reactions, allowing us to calculate half-life periods.
Arrhenius Equation
An equation (k = A * e^(-Ea/RT)) that describes the quantitative relationship between the rate constant and temperature, highlighting the concept of activation energy.
Important Formulas
Board Exam Info
In the ISC Class 12 Chemistry paper, Chemical Kinetics typically carries about 5 to 7 marks. The board exam frequently features direct numerical problems based on first-order half-life calculations, activation energy using the Arrhenius equation, and theoretical questions distinguishing between the order and molecularity of a reaction, or defining pseudo-first-order reactions.
Frequently Asked Questions
What is the difference between order and molecularity of a reaction?
Order is an experimental quantity representing the sum of concentration term powers in the rate law, and it can be zero or fractional. Molecularity is a theoretical concept representing the number of colliding species in an elementary step and is always a positive integer (1, 2, or 3).
Can the order of a reaction be negative or fractional?
Yes, unlike molecularity, the order of a reaction can be fractional (like the decomposition of acetaldehyde) or even zero, as it is determined purely by experimental rate laws.
What is a pseudo-first-order reaction?
A reaction that is actually of a higher order (usually second-order) but behaves as a first-order reaction because one of the reactants is present in large excess, making its concentration practically constant throughout the reaction.
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