Beginner Topic 421 of 1102 Kinetics & Equilibrium
Reaction Rate
The change in concentration of a reactant or product per unit time.
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In-depth explanation of Reaction Rate
The rate of a reaction is the change in concentration of a reactant or product per unit time, and it is reported as a positive quantity. Reactant concentrations fall, so their rate is defined with a minus sign, while product concentrations rise. For a general reaction aA + bB -> cC + dD the rate is written as minus one over a times d[A]/dt, or one over c times d[C]/dt, so that dividing each species change by its stoichiometric coefficient gives one common value. The usual SI unit is mol per cubic metre per second, though mol L^-1 s^-1 is common in laboratory work. Rates are highest at the start of a reaction and decrease as reactants are used up.
Going deeper: Reaction Rate
Key ideas: Convert between species by dividing by the coefficient: for aA -> bB, rate of reaction = -(1/a)d[A]/dt = (1/b)d[B]/dt. In a gas at constant volume you can follow pressure instead of concentration. Rate has units of concentration per time, such as mol per L per s, so check units when comparing runs reported in minutes or hours.
Common mistakes and connections: A common mistake is to report the rate of disappearance of A and the rate of appearance of B as equal when their coefficients differ. Another is giving a negative rate: the minus sign in the definition makes the reported value positive. This links directly to rate laws, since the rate law is written for this single reaction rate.
Visualization of Reaction Rate
For 2N2O5 to 4NO2 + O2, NO2 grows four times as fast as O2 and twice as fast as N2O5 disappears; dividing by coefficients gives one common rate. The interactive lesson shows this as a live animation or chart.
Worked example and use case
For 2N2O5 -> 4NO2 + O2 the rate of O2 formation is one quarter of the rate of NO2 formation.
Frequently asked questions about Reaction Rate
What is Reaction Rate?
The change in concentration of a reactant or product per unit time.
Is Reaction Rate a beginner, intermediate or advanced topic?
Reaction Rate is a beginner topic in Kinetics & Equilibrium. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before Reaction Rate?
Study these earlier topics in Kinetics & Equilibrium first: Chemical Kinetics.
What comes after Reaction Rate?
The next topics in Kinetics & Equilibrium are Average and Instantaneous Rate and Rate Law.
Can I practise Reaction Rate with quizzes?
Yes. The interactive lesson includes a self-check question on Reaction Rate, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Reaction Rate fits in Kinetics & Equilibrium
Reaction Rate is topic 2 of 91 in Kinetics & Equilibrium.