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What is the order of a reaction?

Class 12 ChemistryNCERT chapter: Chemical Kineticsइसे हिंदी में पढ़ें

Exam-ready answer

The order of a reaction is the sum of the powers of the concentration terms of reactants in the experimentally determined rate law. If rate = k[A]ˣ[B]ʸ, the overall order is x + y. Order can be zero, a whole number or a fraction, and it must be found by experiment, not from the balanced equation.

Explanation

The rate of a reaction depends on the concentrations of the reactants. This dependence is written as a rate law, rate = k[A]ˣ[B]ʸ, where k is the rate constant. The exponents x and y are found experimentally by changing one concentration at a time and measuring how the rate changes. The sum x + y is the overall order, while x alone is the order with respect to A.

Order cannot be predicted from the stoichiometric coefficients. For 2NO + O₂ → 2NO₂, the experimental rate law is rate = k[NO]²[O₂], so the reaction is second order in NO, first order in O₂ and third order overall. For CHCl₃ + Cl₂ → CCl₄ + HCl, rate = k[CHCl₃][Cl₂]^½, giving an order of 1.5. Order applies to both elementary and complex reactions.

The units of the rate constant depend on the order: (mol L⁻¹)¹⁻ⁿ s⁻¹ for an nth-order reaction. So k has units of mol L⁻¹ s⁻¹ for zero order, s⁻¹ for first order and L mol⁻¹ s⁻¹ for second order. Order is different from molecularity, which is the number of reacting species colliding together in an elementary step and is always a whole number.

Points to remember

  1. Order of a reaction: the sum of the powers of the concentration terms in the experimentally determined rate law.
  2. For rate = k[A]ˣ[B]ʸ, overall order = x + y; x and y are the orders with respect to A and B.
  3. Order can be 0, 1, 2, 3 or a fraction; it is determined only by experiment.
  4. Units of k = (mol L⁻¹)¹⁻ⁿ s⁻¹: zero order mol L⁻¹ s⁻¹, first order s⁻¹, second order L mol⁻¹ s⁻¹.
  5. Molecularity is theoretical, applies only to elementary reactions and is never zero or fractional; order is experimental.

Exam tip

Write the general rate law rate = k[A]ˣ[B]ʸ, state order = x + y, and add one example with a third or fractional order to show that order is found by experiment.

Questions students also ask

Can the order of a reaction be zero?
Yes. In a zero-order reaction the rate does not depend on reactant concentration, so rate = k. An NCERT example is the decomposition of gaseous ammonia on a hot platinum surface at high pressure, where the surface is fully covered and the rate stays constant.
What is the difference between order and molecularity?
Order is the sum of the powers in the experimental rate law and can be zero or fractional. Molecularity is the number of atoms, ions or molecules that must collide at the same time in an elementary step; it is always a whole number such as 1, 2 or 3.
Why can't order be found from the balanced equation?
Most reactions take place through several elementary steps. The slowest step controls the overall rate, so the rate law reflects that step rather than the overall stoichiometry. That is why the exponents in the rate law often differ from the coefficients in the balanced equation.

Chapter labels help you find the topic in your textbook. Check your own edition: chapter names and order can change. These are Know.it explanations, not official NCERT solutions.