Chemical Kinetics
Class 12 · Chemistry
4 ideas in Class 12 Chemistry, explained in plain words, with games and quizzes to practise.
Reaction Rate and Rate Law
Part of: Rate Laws and Integrated Forms
Rate measures concentration change per second, averaged over intervals or read from tangents. The rate law multiplies concentrations raised to experimental powers, whose sum is the order, while molecularity counts particles in an elementary step and stays whole.
Why it matters. Factories, medicines and pollution models all run on rate laws. Reading exponents off balanced equations invents wrong orders.
Formulas
- Average rate = change / time
- Rate = k [A]^x [B]^y
- Order = x + y from experiments
Key words
- Reaction rate
- : Change in concentration per unit time, in mol per litre per second.
- Rate law
- : Experimental link between rate and reactant concentrations.
- Order
- : Sum of concentration powers in the rate law.
- Molecularity
- : Particle count colliding in one elementary step.
Try it in two minutes
- Divide concentration change by time for average rate.
- Fit powers to rate data, never to the equation.
- Add exponents for order and derive k units.
Try at home. Compare rusting rates of oiled versus bare nails.
Integrated Laws and Half-Life
Part of: Rate Laws and Integrated Forms
Zero-order concentration falls linearly with slope minus k, and its half-life shrinks with remaining amount. First-order concentration decays exponentially, its log falls linearly, and half-life stays fixed at 0.693 over k whatever the start.
Why it matters. Medicine timing, hospitals and shelf life all use these clocks. Treating zero-order half-life as constant misjudges every finish time.
Formulas
- Zero order: [R] = [R]0 - k t
- First order: ln[R] = ln[R]0 - k t
- First half-life: t = 0.693 / k
Key words
- Integrated rate law
- : Concentration as a function of time for a given order.
- Half-life
- : Time for concentration to fall to half.
Try it in two minutes
- Plot concentration or its log against time to identify order.
- Divide 0.693 by k for first-order half-life.
- Halve repeatedly to track remaining amounts.
Try at home. Track how many half-lives leave 1% of a medicine.
Temperature and Arrhenius Equation
Part of: Temperature, Catalyst and Collisions
Rates roughly double per 10 K because more molecules top the activation barrier. The Arrhenius equation k = A e^(-Ea/RT) captures this, with slope minus Ea over 2.303R on a log k versus 1/T plot, while catalysts open lower-barrier paths without touching equilibrium.
Why it matters. Cooking, engines and industrial reactors all manage heat and catalysts. Expecting catalysts to shift equilibrium misreads their role.
Formulas
- Arrhenius: k = A e^(-Ea/RT)
- Log form: log k = log A - Ea/(2.303 R T)
- Catalyst lowers Ea, keeps equilibrium same
Key words
- Activation energy
- : Minimum extra energy for reactants to reach the transition state.
- Arrhenius equation
- : Rate constant as A times e to minus Ea over RT.
- Catalyst
- : Substance speeding reaction via a lower-energy path, unchanged itself.
Try it in two minutes
- Compare molecular energies with the barrier.
- Plot log k against 1/T for slope.
- Add a catalyst and redraw a lower hump.
Try at home. Explain why fridges slow food spoilage.
Collision Theory
Part of: Temperature, Catalyst and Collisions
Molecules must collide with threshold energy and correct orientation to react through a high-energy activated complex. Real rates trail collision counts because most hits are too weak or misaligned, captured by a steric factor below one.
Why it matters. Explosions, enzymes and inhibitors all hinge on effective hits. Counting every collision as reactive overstates rates astronomically.
Formulas
- Effective hits need energy plus orientation
- Threshold = Ea + average molecular energy
- Rate far below total collision count
Key words
- Threshold energy
- : Minimum collision energy for reaction, Ea plus average energy.
- Activated complex
- : Unstable peak arrangement between reactants and products.
- Steric factor
- : Fraction of collisions with suitable orientation.
Try it in two minutes
- Count collisions per second.
- Keep only those above threshold energy.
- Keep only well-oriented ones.
Try at home. Explain why petrol needs a spark despite oxygen around.
What your child will practise
- Collision Theory: Candy Quest (Flip and Match)
- Collision Theory: Jungle Quick Fire (Quick Fire)
- Integrated Laws and Half-Life: Candy Puzzle (Clue Puzzle)
- Reaction Rate and Rate Law: Desert Quick Fire (Quick Fire)
- Temperature and Arrhenius Equation: Space Puzzle (Clue Puzzle)
- Quiz: Collision Theory (8 questions)
- Quiz: Integrated Laws and Half-Life (8 questions)
- Quiz: Reaction Rate and Rate Law (8 questions)
- Quiz: Temperature and Arrhenius Equation (8 questions)