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2.5.2: The Rate of a Chemical Reaction

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    The rate of a chemical reaction is the change in concentration over the change in time.


    The rate of a chemical reaction is the change in concentration over the change in time and is a metric of the "speed" at which a chemical reactions occurs and can be defined in terms of two observables:

    1. The Rate of Disappearance of Reactants \[-\dfrac{\Delta[Reactants]}{\Delta{t}}\] Note this is negative because it measures the rate of disappearance of the reactants.
    2. The Rate of Formation of Products \[\dfrac{\Delta{[Products]}}{\Delta{t}}\] This is the rate at which the products are formed.

    They both are linked via the balanced chemical reactions and can both be used to measure the reaction rate.

    rate, obtained by taking the change in concentration over a time period, for example: -0.3 M / 15 minutes. This is an approximation of the reaction rate in the interval; it does not necessarily mean that the reaction has this specific rate throughout the time interval or even at any instant during that time. The instantaneous rate of reaction, on the other hand, depicts a more accurate value. The instantaneous rate of reaction is defined as the change in concentration of an infinitely small time interval, expressed as the limit or derivative expression above. Instantaneous rate can be obtained from the experimental data by first graphing the concentration of a system as function of time, and then finding the slope of the tangent line at a specific point which corresponds to a time of interest. Alternatively, experimenters can measure the change in concentration over a very small time period two or more times to get an average rate close to that of the instantaneous rate. The reaction rate for that time is determined from the slope of the tangent lines.

    Outside links

    • website lets you play around with reaction rates and will help your understanding)
    • YouTube video of a very fast exothermic reaction.


    1. Petrucci et al. General Chemistry: Principles & Modern Applications, 9th Edition. New Jersey: Prentice-Hall Inc., 2007.
    2. Connors, Kenneth. Chemical Kinetics: The Study of Reaction Rates in Solution. New York City: VCH Publishers, Inc., 1990.


    1. Consider the reaction \(2A + B \longrightarrow C\). The concentration of [A] is 0.54321M and the rate of reaction is \(3.45 \times 10^{-6} M/s\). What Concentration will [A] be 3 minutes later?
    2. Consider the reaction \(A + B \longrightarrow C\). The rate of reaction is 1.23*10-4. [A] will go from a 0.4321 M to a 0.4444 M concentration in what length of time?
    3. Write the rate of the chemical reaction with respect to the variables for the given equation. \[2A+3B \rightarrow C+2D\]
    4. True or False: The Average Rate and Instantaneous Rate are equal to each other.
    5. How is the rate of formation of a product related to the rates of the disappearance of reactants.


    • Albert Law, Victoria Blanchard, Donald Le

    2.5.2: The Rate of a Chemical Reaction is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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