Practical

Investigate the effects of changing the conditions of a reaction on the rates of chemical reactions by measuring the production of a gas.

It is important in this practical to use appropriate apparatus to make and record a range of measurements accurately, including mass, time and volume. This includes the safe use of apparatus, and monitoring chemical changes.

This outlines one way to carry out the practical. It is also possible to collect the gas in an inverted measuring cylinder in place of the gas syringe. Eye protection must be worn.

Aims

To investigate the effect of changing the conditions on the rate of a reaction.

Magnesium reacts with dilute hydrochloric acid to produce hydrogen:

magnesium + hydrochloric acid → magnesium chloride + hydrogen

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

The volume of hydrogen gas produced can be measured using a gas syringe.

Changing the concentration of acid

Method

A sealed conical flask connected to a gas syringe. The flask contains a reaction mixture that is giving off bubbles of gas.
  1. Support a gas syringe with a stand, boss and clamp.
  2. Using a measuring cylinder, add 50 cm3 of dilute hydrochloric acid to a conical flask.
  3. Add a 3 cm piece of magnesium ribbon to the flask. Immediately connect the gas syringe and start a stop clock.
  4. Record the volume of gas produced every 5 seconds. Continue timing until no more gas is given off.
  5. When the reaction is complete, clean the apparatus as directed by your teacher.
  6. Repeat steps 1 to 5 with different concentrations of hydrochloric acid.

Results

Record the results in a table.

Time (s)Volume
00 cm3
5
10
15

Analysis

  1. For each concentration of hydrochloric acid, plot a graph on the same set of axes to show:
    • volume of gas (cm3) on the vertical axis
    • time (s) on the horizontal axis
    • a curve of best fit
  2. For each concentration of acid, calculate the mean rate of reaction: mean~rate~of~reaction~(cm^{3}/s = \frac{total~volume~of~gas~produced~(cm^{3})}{time~to~collect~this~volume~(s)}
  3. Describe the effect of increasing the concentration of acid on the mean rate of reaction. Use your graphs and calculations in step 2 to help you.

Evaluation

Question

Describe how you can tell that the reaction is complete.

No more gas is produced, and the line on the graph becomes horizontal.

Question

Suggest why the rate is not measured by finding the mass of hydrogen given off as the reaction takes place.

Hydrogen has a very low density, so the mass of gas that is lost is too small to be measured accurately.

Extending the experiment

It is also possible to use this experiment to investigate the effect of temperature on the rate of the reaction. The experiment is carried out in the same way, but:

  • keep the concentration of acid the same - it is best to use a more dilute solution, or else the reaction will be too fast to measure at higher temperatures
  • warm the acid to different temperatures using a hot water bath, or a Bunsen burner, tripod and gauze
  • measure and record the temperature of the acid
  • describe the effect of increasing the temperature on the mean rate of reaction

Hazards, risks and precautions

Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:

HazardHarmPrecaution
Hot hydrochloric acidCauses skin and eye irritation; burns to the skin Wear gloves and eye protection; do not heat above 60°C
Fizzing in the reaction mixtureSpray or foam escaping, which may damage skin and eyesUse a large conical flask so there is plenty of space inside; do not look over the top when adding the magnesium ribbon