Required Practical 5: Investigating Effect of Concentration on Rate of Reaction
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
- Set up the apparatus as shown in the diagram
- Measure 20 cm3 of 1 mol / dm3 hydrochloric acid using the 25 cm3 measuring cylinder
- Transfer the acid into the 250 cm3 conical flask
- 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
- Add a 3 cm strip of magnesium ribbon to the flask, connect the bung and delivery tube to the flask and start the stopwatch
- Record the volume of hydrogen gas given off every ten seconds
- Continue timing until no more gas appears to be given off
- 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
| Time (s) | Volume of hydrogen gas produced (cm3) | |
| 1 mol / dm3 acid | 0.5 mol / dm3 acid | |
| 10 | ||
| 20 | ||
| 30 | ||
| 40 | ||
| 50 | ||
| 60 * | ||
* 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.
Worked example
A student studied the effect of concentration on the rate of reaction between magnesium and hydrochloric acid. They obtained the following results.
| Time (s) | Volume of hydrogen gas produced (cm3) | |
| 1 mol / dm3 acid | 0.5 mol / dm3 acid | |
| 10 | 20 | 10 |
| 20 | 34 | 19 |
| 30 | 44 | 26 |
| 40 | 48 | 33 |
| 50 | 50 | 38 |
| 60 | 50 | 43 |
| 70 | 47 | |
| 80 | 49 | |
| 90 | 50 | |
| 100 | 50 | |
a)
Plot a graph of time against the volume of hydrogen gas produced.
b)
State what conclusion can be drawn about how concentration affects the rate of this reaction. Explain your conclusion using particle theory.
Answers:
a)
The graph of the results is:
b)
Statement:
- As the concentration increases, the overall volume of hydrogen gas produced remains the same
AND
But it is produced faster with a higher concentration of acid
Explanation:
- This is because increasing the concentration means that there are more particles in the same volume / amount of space
- This leads to more frequent, successful collisions and a faster rate of reaction
Exam Tip
- The two specified practicals for rates of reaction are:
- Investigation of the factors affecting the rate of reaction between dilute hydrochloric acid and sodium thiosulfate.
- Investigation of the factors that affect the rate of reaction using a gas collection method - typically using magnesium and hydrochloric acid.
- In exams, you will often find that the experiments are switched so that you are asked about:
- The effects of concentration relating to the dilute hydrochloric acid and sodium thiosulfate experiment.
- The effects of temperature (or another factor, e.g. surface area) relating to the magnesium and hydrochloric acid experiment.
