Chemical Reaction Rate Study Guide

The reaction rate in chemistry is the speed of a chemical reaction.

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What is the reaction rate? It is the measure of how fast a chemical reaction proceeds. Consider a combustion reaction like the burning of wood, which takes place in a few hours as compared to rusting which can take several days.

Fire-combustion-fuel Source

  • In a chemical reaction, where A & B are the reactants and AB is the product or A B → AB.
  • The reaction rate in chemistry is determined by how fast the concentration of A & B increases or decreases.

Rate-of-a-Reaction Source


It is defined as the estimate of the conversion in the concentration of the reactants through a certain time interval. It may be expressed as

           Rate = Δ concentration / Δ time

According to the above reactants A & B, the reaction rate may be written as:

            Rate = - Δ (A) / Δ t

            Rate = - Δ (B) / Δ t

            Rate =   Δ (AB) / Δ t

Here Δ(A) is the difference in the concentration of reactant A through time (t₂ - t₁). The bracket indicates the concentration in Molarity of the substance, and the negative signs show the decrease in the concentration of only the reactants.

Therefore, Δ(A) = (A₂) – (A₁) where A₂ is the final concentration and A1 is the initial concentration of reactant A. It will always be negative since the concentration of the reactant decreases with time during the reaction.


A reaction rate is the conversion of the concentration of reactants with time. A 2.00M solution of A reacts with B to form AB from the above equation. It takes place in 20 seconds, and the concentration of A drops from 2.00M to 0.72M. Rate can be calculated as Δ(A) = (A₂) – (A₁) / Δ t


      Rate = 0.72M – 2.00 M / 20 s  

            = -(-1.28) M / 20 s  

            = 0.064 M/s

The reaction rate unit is measured in molarity per second or M/s. So for the above example, A's molarity decreases at a rate of 0.064 seconds.


  • The reaction rate in chemistry is the difference in the concentration of a reactant or product divided by the time taken.
  • The rate of a reaction is measured as molarity per second.


1. How do you calculate the rate of a reaction per second?

It is calculated as rate or Δ (A) = (A₂) – (A₁) / Δ t

2. How do you calculate the rate of reaction from molarity?

If the initial Molarity of a reactant was 1.00M and the final Molarity changes to 0.64M over 15 seconds, then the rate of the reaction is calculated as

Rate = 0.64M – 1.00 M / 15 s

= -(- 0.36) M / 15 s

= 0.024 M/s

The rate of the reaction is expressed as 0.024M/s.

3. What is the rate of reaction formula?

The rate may be calculated as:

rate = - Δ (A) / Δ t

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  1. Reaction Rate. https://www.ck12.org/c/chemistry/reaction-rate/lesson/Chemical-Reaction-Rate-CHEM. Accessed 17 February 2022.
  2. Reaction Rate. https://www.britannica.com/science/reaction-rate. Accessed 18 February 2022.