# How can rates of reactions be measured?

May 4, 2016

See below.

#### Explanation:

The reaction rate can be found from either the rate of consumption of a reactant or rate of production of a product.

In the following two examples, I will show you how this could be done, and I will attach corresponding videos to the experiments.

Example 1:
Consider the reaction of magnesium metal with an aqueous solution of hydrochloric acid.

The reaction is the following:

$M g \left(s\right) + 2 H C l \left(a q\right) \to M g C {l}_{2} \left(a q\right) + {H}_{2} \left(g\right)$

The easiest way to monitor the progress of this reaction is by collecting the hydrogen gas formed and calculate its amount in volume for example.

The differential rate law of this reaction could be written as:

$R = \frac{\Delta \left[{H}_{2}\right]}{\Delta t}$

since the $\left[{H}_{2}\right]$ is inversely proportional to the volume ($V$) of hydrogen produced, then the rate could be written as:

$R = \frac{\Delta {V}_{{H}_{2}}}{\Delta t}$

by measuring the volume of hydrogen produced over time, we can calculate the rate of this reaction.

More details in the following video:
Lab Experiment 19: Effect of Concentration on the Reaction Rate.

Example 2:
Consider the reaction of crystal violet ($C {V}^{+}$) with an aqueous solution of sodium hydroxide.

The reaction is the following:
$C {V}^{+} \left(a q\right) + O {H}^{-} \left(a q\right) \to C V O H \left(a q\right)$

Note that the $C {V}^{+}$ has a violet colour and that $C V O H$ is colourless, therefore, the progress if the reaction could be monitored by the consumption of $C {V}^{+}$ through the fading go its colour during the course of the reaction.

Using Beer-Lambert law, $A = \epsilon \times l \times C$, we can find the change on concentration over time and the rate law could be found by:

$R = \frac{\Delta \left[C {V}^{+}\right]}{\Delta t}$

More details in the following video:
AP Chemistry Investigation 11: Rate Law of the Fading of Crystal Violet.

For more theory related to this topic, you can watch the following video lesson, which is one of a series of 6 videos:
Chemical Kinetics | Reaction Rates & Rate Law.