The rate of the reaction at any time is the slope of a tangent to this curve at that time. Three tangents are shown by blue lines on the graph. From these it can be seen that the slope of the tangents (and therefore the rate of reaction) decreases as the reaction proceeds. This is due to a decrease in the concentrations of reactants and an increase in the concentrations of products.
The initial rate of the reaction is the rate during the first few percent of reaction. This is the slope of the steepest tangent near time equals zero.
Experiments where the initial rate of reaction is measured for different concentrations of reactants show how the initial rate depends on the concentration of each reactant. These experiments can be used to deduce the order of the reaction with respect to each reactant, and then the differential rate law (rate equation).
To find the order of the reaction with respect to a reactant:
Choose two experiments where the concentration of only one reactant is different.
The order with respect to that reactant is
- zero if doubling its concentration does not change the rate
- one if doubling its concentration doubles the rate
- two if doubling its concentration quadruples the rate
To calculate the rate constant:
Substitute the rate and reactant concentrations from one experiment into the experimental rate law as determined above.
To calculate the rate of reaction using the rate constant and the concentration of each reactant:
Substitute into the differential rate law (rate equation).