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Chemical Reaction Orders

Chemistry Kinetics and Equilibrium

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Chemical reaction orders are classified using kinetics.

Chemical reaction orders are classified using kinetics

Adrianna Williams, Getty Images

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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