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Chemical Reaction Orders

Chemistry Kinetics and Equilibrium


Chemical reactions can be classified based on their reaction kinetics. The general reaction form is:

aA + bB → cC + dD

Reactions are categorized as zero-order, first-order, second-order, or mixed-order (higher-order) reactions.

  1. Zero-Order Reactions

    Zero-order reactions (order = 0) have a constant rate. This rate is independent of the concentration of the reactants. The rate law is:

    rate = k, with k having the units of M/sec.

  2. First-Order Reactions

    A first order reaction (order = 1) has a rate proportional to the concentration of one of the reactants. A common example of a first-order reaction is the phenomenon of radioactive decay. The rate law is:

    rate = k[A] (or B instead of A), with k having the units of sec-1

  3. Second-Order Reactions

    A second-order reaction (order = 2) has a rate proportional to the concentration of the square of a single reactant or the product of the concentration of two reactants:

    rate = k[A]2 (or substitute B for A or k multiplied by the concentration of A times the concentration of B), with the units of the rate constant M-1sec-1

  4. Mixed-Order or Higher-Order Reactions

    Mixed-order reactions have a fractional order for their rate:

    e.g., rate = k[A]1/3

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