Question

For the reaction \[A \to B\], it was found that the concentration of B increased by \[0.3\;mol\;{L^{ - 1}}\]in 2 hours. What is the average rate of reaction?

Answer 1

Hint: Rate of reaction is change in concentration of reactants or products per unit time, i.e., change in concentration divided by the time taken for that change.
Rate of reaction or Reaction rate, is defined as the speed at which a chemical reaction proceeds. It is often expressed in terms of either the concentration (amount per unit volume) of a product that is formed in a unit of time or the concentration of a reactant that is consumed in a unit of time. Alternatively, it may be defined in terms of the amounts of the reactants consumed or products formed in a unit of time.

Complete step by step answer:
The average rate of reaction is calculated by considering this change over a period of time while the instantaneous rate of reaction is calculated by taking the slope of tangent to the concentration versus time curve. In the present case, however, the average rate is required to be calculated. This is shown below in a stepwise manner.
Change in concentration = \[0.3\;mol\;{L^{ - 1}}\] (given)
Time taken for the above change in concentration =
Average rate of reaction
\[=\dfrac{{Change\;in\;concentration}}{{Time\;taken}} = \dfrac{{0.3\;mol\;{L^{ - 1}}}}{{2\;hours}} = 0.15 \approx 0.2mol{L^{ - 1}}{h^{ - 1}}\] (Answer)

Additional Information: In case of reactants, there is decrease in their concentration because they get consumed as the reaction proceeds, hence, their change in concentration should be taken with a negative sign. Further, the coefficients of reactants and products also determine how fast the change in concentration takes place. For example, in the reaction \[2C \to D\], the reactant C gets consumed twice as fast as D is formed because the coefficient of C is 2 and that of D is 1.

Note: In this reaction, B is a product and so change in concentration meant the increase in its concentration because as the reaction proceeds the concentration of products increase over time.