Science 7–10 · Year 10
Gas volume against time: reading a rate from the gradient
Reactions
School laboratory, not for home
In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
The idea
Rate is the gradient of a product-against-time graph, steepest at the start when reactant concentration is highest and zero when the magnesium has all reacted.
Safety card
Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
Hazards
- hydrogen is extremely flammable
- hydrochloric acid 1 mol/L is an irritant
Controls
- no ignition sources in the room
- eye protection
- hydrogen vented outside or in a fume cupboard
Note
NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson.
What you need
- two pieces of magnesium ribbon, each weighed to 0.04 g and cleaned
- hydrochloric acid 1 mol/L, 50 mL; a second run with 0.5 mol/L
- 100 mL conical flask with bung and delivery tube, trough, 100 mL measuring cylinder inverted over water or a 100 mL gas syringe, stop clock
How to do it
- Put 50 mL of 1 mol/L acid in the flask and set up the gas collection.
- Add the magnesium, bung the flask at once and start the clock; record the gas volume every 10 seconds until it stops rising.
- Repeat with 0.5 mol/L acid and the second 0.04 g piece.
- Plot volume against time for both runs. Draw tangents at 0 s, 30 s and 60 s and calculate the gradients in mL/s.
What you should see
Both runs level off near 42 mL (the hydrogen from 0.04 g of magnesium at 25 °C, saturated with water vapour) because all the magnesium has reacted while acid is left over: even 50 mL of the 0.5 mol/L acid could react with about 0.30 g of magnesium, more than seven times the 0.04 g used. The 1 mol/L run reaches the plateau sooner. The tangent gradient falls from its highest value at the start to zero at the plateau. The learner knows it worked when the two plateaus agree within 5 mL and the initial gradient of the 1 mol/L run is clearly the steeper; the ratio of the two initial gradients is the class's own measure of how rate depends on concentration.
What changes
- What you change
- acid concentration
- What you measure
- volume of hydrogen against time
- What you keep the same
- mass of magnesium
- acid volume
- temperature
- apparatus
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- The stronger acid makes more gas.
- The rate is constant until the reaction suddenly stops.
- The plateau means the acid has run out.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC5-RXN-02SC5-WS-06
- Australian Curriculum v9AC9S10U07AC9S10I05
Sources
The pages the author read to write this activity.
- curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
- edu.rsc.org/experiments/the-rate-of-reaction-of-magnesium-with-hydrochloric-acid/1916.article
- edu.rsc.org/lesson-plans/interpreting-rate-of-reaction-graphs-14-16-years/95.article
- education.nsw.gov.au/teaching-and-learning/curriculum/science/science-curriculum-resources-k-12/science-7-10-curriculum-resources/s4-s5-chemical-world