NCERT Chemical Kinetics

Introduction to Chemical Kinetics

  • Definition: Chemistry is focused on change; chemical reactions transform substances with distinct properties into others.

  • Key Concepts:

    • Feasibility of Reaction: Determined using thermodynamics, where a reaction is feasible if ΔG < 0 at constant temperature and pressure.

    • Extent of Reaction: Defined from chemical equilibrium.

    • Speed of Reaction: Time taken for a reaction to reach equilibrium; studied under chemical kinetics.

Importance of Chemical Kinetics

  • Addresses crucial questions like:

    • How fast does food spoil?

    • How do we develop quick-setting dental materials?

    • What controls fuel burn rates in engines?

  • Significance: Provides insights on how reactions proceed, including reaction rates and mechanisms.

Distinction Between Thermodynamics and Kinetics

  • Thermodynamics: Indicates feasibility (e.g., diamond to graphite is thermodynamically possible but occurs slowly).

  • Kinetics: Focuses on the rate and factors influencing how fast reactions occur.

Learning Objectives

  • Define average and instantaneous rates of reaction.

  • Express rates based on concentration changes over time.

  • Differentiate between elementary and complex reactions.

  • Distinguish molecularity from reaction order.

  • Define the rate constant and identify factors affecting reaction rates like concentration, temperature, and catalysts.

  • Derive integrated rate equations for zero and first-order reactions.

  • Calculate rate constants for these reactions.

  • Explain collision theory.

Chemical Reaction Rates

  • Rate of Reaction: Defined as change in concentration of reactants/products over time.

  • Can be analyzed as:

    • Rate of decrease in concentration of reactants.

    • Rate of increase in concentration of products.

Example of Reaction Rate Calculation

  • Consider the reaction: R → P.

  • If [R]1, [P]1 at time t1, and [R]2, [P]2 at time t2:

    • Change in concentration: Δt = t2 - t1

    • Rate expressions:

      • Rate of disappearance of R:[ Rate_{R} = -\frac{Δ[R]}{Δt} ]

      • Rate of appearance of P: [ Rate_{P} = \frac{Δ[P]}{Δt} ]

  • Sign Convention: Decrease in reactant concentration (Δ[R]<0) is converted to positive rate by multiplying by -1.

Average vs. Instantaneous Rate

  • Average Rate: Calculated over a given time interval; depends on change in concentration and time taken.

  • Instantaneous Rate: Rate at a specific moment; can be determined by drawing a tangent on concentration vs. time plot.