Chemistry 11–12 · Year 11

Magnesium in hydrochloric acid: gas volume against time, surface area and concentration

Module 3: Reactive Chemistry

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

The gradient of a volume-time graph is the reaction rate; it is steepest at the start, falls as reactant is used up, and rises with surface area and acid concentration.

Safety card

Medium riskLearners carry it out, with a teacher supervising

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

hydrogen is flammable
1 mol/L hydrochloric acid irritant
flask warms

Controls

no flames on the bench
bung fitted quickly but not forced
eye protection
acid spills rinsed at once

Note

NSW Department of Education Chemical Safety in Schools (CSIS) package, 2021 Technical Update: Section 1.7 (risk assessment) and Volume 2 Appendix D (generic assessment advice and DoE chemical categories); record a RiskAssess (riskassess.com.au) risk assessment before the lesson and check the school's hazardous chemical register (CSIS Section 1.9) for local restrictions.

What you need

Magnesium ribbon 3 cm lengths (about 0.040 g) cleaned with emery paper, and magnesium turnings of the same mass
Hydrochloric acid 1.0 mol/L and 0.5 mol/L, 50 mL per run
100 mL conical flask with one-hole bung and delivery tube, or a 100 mL gas syringe
Trough of water and an inverted 100 mL measuring cylinder if collecting over water
Stopwatch, or a gas-pressure sensor with data logger for continuous readings
Thermometer

How to do it

Pour 50 mL of 1.0 mol/L acid into the flask; record its temperature.
Drop in the ribbon, fit the bung at once and start the clock; record the gas volume every 10 s until it stops changing (about two to three minutes).
Repeat with turnings of the same mass in fresh 1.0 mol/L acid.
Repeat with ribbon in 0.5 mol/L acid.
Plot volume against time for all three runs on one graph; draw a tangent at t = 0 and at t = 30 s for the ribbon run and calculate the gradients in mL/s.
Fit each run in the simulation by adjusting the rate constant; compare the fitted constants across the three conditions.

What you should see

Each run levels off near 41 mL of hydrogen for 0.040 g of magnesium (Mg + 2 HCl gives MgCl2 + H2; 1.65 mmol of gas at 25 degrees Celsius and 100 kPa), and a volume collected over water reads about 3 percent high rather than low, because the collected gas is saturated with water vapour (about 3.17 kPa at 25 degrees Celsius), which outweighs the small amount of hydrogen that dissolves in the displaced water. The RSC practical notes that 3 cm of ribbon is typically 0.04 g and yields about 40 mL. The gradient is steepest at the start and falls towards zero as the metal is used up; turnings give a steeper start and finish sooner; 0.5 mol/L acid gives a gentler start and a longer run with the same final volume. With the model's default rate constant (0.020 per second) the initial gradient is 0.82 mL/s; the learner fits the constant to each of their own runs. The flask warms by a few degrees.

What changes

What you change: surface area of the magnesium, or acid concentration
What you measure: volume of hydrogen collected at each time (mL) and the initial rate (mL/s)
What you keep the same: mass of magnesium
acid volume
starting temperature
acid concentration (in the surface-area comparison)

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The reaction rate is constant until the magnesium runs out (rate falls continuously as acid concentration at the surface and the metal area shrink).
Turnings make more gas than ribbon (same mass, same final volume; only the rate differs).
The total gas volume tells you the rate (volume gives the amount; the gradient gives the rate).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH11-10 CH11/12-3 CH11/12-4
Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document); a Working Scientifically outcome not among those the syllabus targets in Module 3, which it allows in any moduleCH11/12-5
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-11-03 CH-11WS-04 CH-11WS-05
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities