Rate of Reaction Using Gas Collection (WJEC GCSE Chemistry)

Specified Practical: Rate of Reaction Using Gas Collection

Introduction

• Magnesium and dilute hydrochloric acid react to produce hydrogen

magnesium + hydrochloric acid → magnesium chloride + hydrogen

Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g)

• The production of hydrogen gas can be used to determine the rate of reaction

• This experiment will study the effect of changing the concentration of the hydrochloric acid on the rate of the reaction

Apparatus

• Magnesium ribbon cut into 3 cm lengths

• 1 mol/dm³  hydrochloric acid

• 250 cm³ conical flask

• Single-holed rubber bung with delivery tube to fit the conical flask

• Trough / plastic washing-up bowl

• 100 cm³ measuring cylinder

• 250 cm³ measuring cylinder

• Clamp stand, boss and clamp

• Stopwatch

Diagram

Collecting gas with an inverted measuring cylinder

Method

1. Set up the apparatus as shown in the diagram

2. Measure 20 cm³ of 1 mol / dm³ hydrochloric acid using the 25 cm³ measuring cylinder

3. Transfer the acid into the 250 cm³ conical flask

4. Fill the 100 cm³ measuring cylinder with water

   • Ensure it stays filled with water when you turn it upside down and clamp it above the trough

5. Add a 3 cm strip of magnesium ribbon to the flask, connect the bung and delivery tube to the flask and start the stopwatch

6. Record the volume of hydrogen gas given off every ten seconds

7. Continue timing until no more gas appears to be given off

8. Repeat steps 2 - 7 using 10 cm³ of 1 mol / dm³ hydrochloric acid and 10 cm³ of water

   • 10 cm³ of hydrochloric acid and 10 cm³ of water gives a total volume of 20 cm³ with a concentration of 0.5 mol / dm³.

• An alternative method involves collecting the gas using a gas syringe rather than an inverted measuring cylinder:

Collecting gas with a gas syringe

Practical Tip

• Magnesium and sulfuric acid will react immediately so the flask must be connected to the gas syringe straight after adding the magnesium to avoid gas escaping

Analysis of results

An example results table

* It may be necessary to extend the table past 60 seconds.

Evaluation

• Plot a line graph of the results for both concentrations of hydrochloric acid and label the lines appropriately, using appropriate scales on each axis

  • The independent variable, time, is on the x-axis

  • The dependent variable, the volume of hydrogen gas, is on the y-axis

Conclusion

• Describe what the graph shows:

  • e.g. when the concentration is higher, the same volume of hydrogen gas is produced but in a faster time.

• If it is appropriate, give a more detailed description.

• Use particle theory to explain the description.