How to Classify Chemical Reaction Orders Using Kinetics

Chemical reactions can be classified based on their reaction kinetics, the study of reaction rates.

Kinetic theory states that minute particles of all matter are in constant motion and that the temperature of a substance is dependent on the velocity of this motion. Increased motion is accompanied by increased temperature.

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.

Zero-Order Reactions

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Chemical Kinetics Definition in Chemistry

By Anne Marie Helmenstine, Ph.D.

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

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

First-Order Reactions

A first-order reaction (where order = 1) has a rate proportional to the concentration of one of the reactants. The rate of a first-order reaction is proportional to the concentration of one reactant. A common example of a first-order reaction is radioactive decay, the spontaneous process through which an unstable atomic nucleus breaks into smaller, more stable fragments. The rate law is:

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

Second-Order Reactions

A second-order reaction (where 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. The formula is:

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

Mixed-Order or Higher-Order Reactions

Mixed order reactions have a fractional order for their rate, such as:

rate = k[A]^1/3

Factors Affecting Reaction Rate

Chemical kinetics predicts that the rate of a chemical reaction will be increased by factors that increase the kinetic energy of the reactants (up to a point), leading to the increased likelihood that the reactants will interact with each other. Similarly, factors that decrease the chance of reactants colliding with each other may be expected to lower the reaction rate. The main factors that affect reaction rate are:

• The concentration of reactants: A higher concentration of reactants leads to more collisions per unit time, which leads to an increased reaction rate (except for zero-order reactions.)
• Temperature: Usually, an increase in temperature is accompanied by an increase in the reaction rate.
• The presence of catalysts: Catalysts (such as enzymes) lower the activation energy of a chemical reaction and increase the rate of a chemical reaction without being consumed in the process. 
• The physical state of reactants: Reactants in the same phase may come into contact via thermal action, but surface area and agitation affect reactions between reactants in different phases.
• Pressure: For reactions involving gases, raising pressure increases the collisions between reactants, increasing the reaction rate.

While chemical kinetics can predict the rate of a chemical reaction, it does not determine the extent to which the reaction occurs.