Measuring Rates of Reaction

Measuring rates of reaction

How the average rate of a reaction is measured will depend on what the reaction is, what the reactants are, and what product forms. Look back at the reactions that have been discussed so far. In each case, how was the average rate of the reaction measured? The following examples will give you some ideas about other ways to measure the average rate of a reaction:

Measuring the volume of gas produced per unit time

The volume of gas produced in a reaction may be measured by collecting the gas in a gas syringe (see the figure below).

The gas syringe method.

Optional Video: Rates of Reaction - Size Comparison

As more gas is produced, the plunger is pushed out and the volume of the gas in the syringe can be recorded.

By measuring the volume at set time intervals, we can graph the data (see figure below) and hence determine the rate of the reaction.

A plot of the volume of gas collected at set time intervals.

Examples of reactions that produce gas are listed below:

Reactions that produce hydrogen gas:

When a metal reacts with an acid, hydrogen gas is produced. The hydrogen can be collected in a test tube. A lit splint can be used to test for hydrogen. The 'pop' sound shows that hydrogen is present.

For example, magnesium reacts with sulfuric acid to produce magnesium sulfate and hydrogen.

$\text{Mg}(\text{s}) + \text{H}_{2}\text{SO}_{4}(\text{aq})$ $\to$ $\text{MgSO}_{4}(\text{aq}) + \text{H}_{2}(\text{g})$
Reactions that produce carbon dioxide:

When a carbonate reacts with an acid, carbon dioxide gas is produced. When carbon dioxide is passed through limewater, it turns the limewater milky. A burning splint will also stop burning (be extinguished) in the presence of $\text{CO}_{2}$ gas. These are a simple tests for the presence of carbon dioxide.

For example, calcium carbonate reacts with hydrochloric acid to produce calcium chloride, water and carbon dioxide.

$\text{CaCO}_{3}(\text{s}) + 2\text{HCl}(\text{aq})$ $\to$ $\text{CaCl}_{2}(\text{aq}) + \text{H}_{2}\text{O}(\text{l}) + \text{CO}_{2}(\text{g})$
Reactions that produce oxygen:

Hydrogen peroxide decomposes in the presence of a manganese(IV) oxide catalyst to produce oxygen and water.

In the zinc and hydrochloric acid experiment the learners collect gas in a balloon. Do not light the gas in the balloon or allow it to be near flame. There will be an explosion.

Optional Experiment: Measuring reaction rates

Aim

To measure the effect of concentration on the average rate of a reaction.

Apparatus

Solid zinc granules, $\text{1}$ $\text{mol.dm$^{-3}$}$ hydrochloric acid $(\text{HCl})$
Two conical flasks, two beakers, two balloons, bunsen burner, splint of wood

Method

Warning:

Do not get hydrochloric acid ($\text{HCl}$) on your hands. We suggest you use gloves and safety glasses whenever handling acids and handle with care.

Label a conical flask A. Weigh $\text{5}$ $\text{g}$ zinc granules into it.

Repeat with the second conical flask but label it B.
Label a beaker $\text{1}$. Pour $\text{10}$ $\text{cm$^{3}$}$ $\text{HCl}$ into it.

Label the other beaker $\text{2}$. Pour $\text{5}$ $\text{cm$^{3}$}$ $\text{H}_{2}\text{O}$ into it. Add $\text{5}$ $\text{cm$^{3}$}$ $\text{HCl}$ to this second beaker.
Quickly: Pour the liquid in beaker $\text{1}$ into conical flask A and pour the liquid in beaker $\text{2}$ into conical flask B. Attach one balloon firmly to each conical flask.
Note which balloon filled more quickly.
Fill a test tube with the gas formed. Light only the gas in the test tube. Keep open flames away from the balloons.

The equation for this reaction is:

$\text{Zn}(\text{s}) + 2\text{HCl}(\text{aq})$ $\to$ $\text{ZnCl}_{2}(\text{aq}) + \text{H}_{2}(\text{g})$

Results

Which beaker contained the more concentrated solution of $\text{HCl}$?
Which balloon filled more quickly?
What happened when you lit the gas in the test tube?

Conclusions

The more concentrated solution led to a faster reaction rate (i.e. the balloon filling with $\text{H}_{2}$ gas more quickly). The test for hydrogen gas would have resulted in a loud pop when the lit splint was placed near the mouth of a test tube.

Measuring Rates of Reaction