Experiment 10: Rate of Reaction — Magnesium and Acid
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Topic: Rates of Reaction
Level: Ordinary & Higher Level
Exam Focus: Gas Collection + Rate Graphs
1. Experiment Snapshot
Feature
Details
Experiment
To investigate the rate of reaction between magnesium ribbon and dilute hydrochloric acid.
Main observation
Effervescence occurs as hydrogen gas is produced.
Main measurement
Volume of hydrogen gas produced over time.
Common method
Gas syringe or inverted measuring cylinder over water.
Difficulty
⭐⭐⭐ Medium
Exam importance
⭐⭐⭐⭐⭐ Very high for rate graphs, gas tests and variables.
2. Aim
To investigate how changing a factor such as acid concentration affects the rate of reaction between magnesium and hydrochloric acid by measuring the volume of hydrogen gas produced over time.
3. Quick Theory
Magnesium reacts with dilute hydrochloric acid to form magnesium chloride and hydrogen gas.
The rate of reaction can be followed by measuring the volume of hydrogen gas produced at regular time intervals.
Rate = change in volume of gas ÷ time taken
Examiner Tip: The gas produced is hydrogen. Test it with a lighted splint; hydrogen gives a squeaky pop.
4. Apparatus and Chemicals
Apparatus
Chemicals
Conical flask, rubber bung, delivery tube, gas syringe or inverted measuring cylinder, stopwatch, measuring cylinder, balance or ruler, clamp stand.
Magnesium ribbon, dilute hydrochloric acid, water if using water displacement.
Safety: Wear goggles. Hydrochloric acid is irritant/corrosive. Hydrogen is flammable, so keep flames away except for a controlled gas test.
5. Labelled Experiment Diagram
Gas Syringe Method
6. Variables
Variable Type
In this Experiment
Independent variable
Acid concentration, length/mass of magnesium, temperature, or surface area depending on the investigation.
Dependent variable
Volume of hydrogen gas produced per unit time / reaction rate.
Controlled variables
Volume of acid, mass/length of magnesium, temperature, same apparatus, same time intervals.
Examiner Trap: If investigating acid concentration, keep the same length/mass of magnesium and same volume of acid.
7. Procedure
Measure a fixed volume of dilute hydrochloric acid into a conical flask.
Set up the flask with a rubber bung, delivery tube and gas syringe.
Measure a fixed length or mass of magnesium ribbon.
Add the magnesium to the acid and quickly replace the bung.
Start the stopwatch immediately.
Record the volume of hydrogen gas in the gas syringe every 10 or 20 seconds.
Continue until no more gas is produced or the volume becomes constant.
Repeat using different acid concentrations if required.
Plot a graph of volume of hydrogen gas against time.
Compare the steepness of the curves to compare rates.
Examiner Tip: Fit the bung quickly after adding magnesium to avoid losing hydrogen gas at the start.
8. Results Table Template
Time / s
Gas volume with dilute acid / cm³
Gas volume with more concentrated acid / cm³
0
0
0
20
12
24
40
21
39
60
28
48
80
32
52
100
34
53
120
34
53
Conclusion: The more concentrated acid produces hydrogen faster because the graph is steeper at the start.
9. Graph Skills
The graph usually shows volume of hydrogen gas on the y-axis and time on the x-axis.
Graph Feature
Meaning
Steep slope at start
Fast reaction rate.
Curve becomes less steep
Reaction slows as reactants are used up.
Flat line / plateau
Reaction has finished; no more gas is produced.
Higher final volume
More gas produced overall, often due to more limiting reactant.
Typical Rate Graph
Examiner Tip: The initial rate can be compared by comparing the initial slope of the graph.
10. Collision Theory Explanation
Increasing acid concentration increases the number of acid particles per unit volume. This causes more frequent collisions with magnesium particles. More successful collisions per second means the rate increases.
Exam answer sentence:
A higher acid concentration gives more H⁺ ions per unit volume, so there are more frequent successful collisions with magnesium, producing hydrogen faster.
11. Gas Test for Hydrogen
Hydrogen gas is tested using a lighted splint.
Hydrogen + lighted splint → squeaky pop
Safety: Only test a small sample of gas. Hydrogen is flammable.
12. Sources of Error and Improvements
Source of Error
Effect
Improvement
Gas escapes before bung is fitted
Measured gas volume too low.
Fit bung quickly or use a side-arm flask.
Leak in apparatus
Gas volume too low.
Check all connections are airtight.
Magnesium oxide layer on ribbon
Slower start to reaction.
Clean magnesium with sandpaper before use.
Temperature changes
Rate changes unpredictably.
Use a water bath or keep temperature constant.
Inaccurate timing
Unreliable rate data.
Use regular intervals and repeat experiment.
Gas syringe plunger sticks
Incorrect gas volume.
Check gas syringe moves freely before starting.
13. Student Common Mistakes
Forgetting to start the stopwatch immediately.
Not fitting the bung quickly enough.
Using different lengths of magnesium between trials.
Forgetting to keep acid volume constant.
Confusing oxygen test with hydrogen test.
Saying the graph becomes flat because the gas syringe is full instead of because the reaction has finished.
Forgetting that steeper slope means faster reaction.
Drawing time on the y-axis instead of the x-axis.
14. Examiner Tips
Tip 1: Hydrogen is identified by a squeaky pop with a lighted splint.
Tip 2: Use the phrase “volume of hydrogen gas produced per unit time” for rate.
Tip 3: On graphs, the steeper line shows the faster reaction.
Tip 4: The curve levels off when the limiting reactant is used up.
15. Examiner Traps
Trap: The gas is hydrogen, not carbon dioxide.
Trap: Higher acid concentration gives faster rate, but not necessarily more final gas if the same amount of magnesium is used.
Trap: A flat graph means the reaction is finished, not that the reaction is fastest.
Trap: If magnesium is the limiting reactant, changing acid concentration affects speed but may not change total gas volume.
16. 🎮 Interactive Simulator: Hydrogen Gas Rate Lab
Choose Conditions
Gas Collection
0 cm³ H₂
Choose conditions, then run the reaction.
Practice Score: 0/0
17. Past Paper Style Questions
State the aim of this experiment. [2 marks]
Write the word equation for the reaction. [2 marks]
What gas is produced and how is it tested? [3 marks]
Name two variables that must be kept constant when investigating acid concentration. [2 marks]
Explain why increasing acid concentration increases the rate. [3 marks]
What does the flat part of a gas-volume graph show? [2 marks]
Give one source of error and one improvement. [2 marks]
Why might the first gas reading be lower than expected? [2 marks]
18. Mark Scheme Answers
Q1. To investigate the effect of a factor such as acid concentration [1] on the rate of reaction between magnesium and hydrochloric acid [1].
Q3. Hydrogen [1]; test with a lighted splint [1]; squeaky pop [1].
Q4. Length/mass of magnesium / volume of acid / temperature / same apparatus / same time intervals. Any two [2].
Q5. Higher concentration has more acid particles/H⁺ ions per unit volume [1]; collisions with magnesium are more frequent [1]; more successful collisions per second increases rate [1].
Q6. No more gas is being produced [1] because the reaction has finished/limiting reactant used up [1].
Q7. Gas escapes before bung fitted [1]; fit bung quickly/check airtight apparatus [1]. Other valid answers accepted.
Q8. Hydrogen may escape before the bung is sealed [1], so not all gas is collected/measured [1].
19. Quick Revision Card
Key Point
Memory Hook
Reaction
Mg + acid → salt + hydrogen
Gas test
Hydrogen gives squeaky pop.
Rate measurement
Volume of H₂ per unit time.
Higher concentration
Faster reaction due to more frequent collisions.
Graph slope
Steeper = faster.
Plateau
Reaction finished.
Main error
Gas lost before bung fitted / leaks.
20. Mastery Checklist
I can state the aim of the magnesium and acid rate experiment.
I can draw and label the gas collection setup.
I can write the word and symbol equation.
I can test for hydrogen gas.
I can identify variables in the experiment.
I can explain the effect of acid concentration using collision theory.