Learning focus
Explain chemical changes using particles, collisions, equilibrium and electron transfer; interpret graphs and industrial conditions; and apply the ideas to unfamiliar reactions.
Apparatus and method
A conical flask is connected by airtight tubing to a gas syringe. One reactant is added, the bung is fitted immediately and the timer starts. Gas volume is read at regular intervals until no further change occurs.
Advantages
Gas volume is measured directly and can be recorded repeatedly without opening the apparatus. A complete curve gives both initial rate and final gas amount. A gas syringe is usually more accurate than collecting gas over water when the gas is water-soluble.
Limitations
Leaks cause volumes to be too low. Gas can be produced before the bung is fitted. The plunger may stick, and the syringe has a maximum capacity. Hot gas may cool and contract, affecting volume.
Improving method
Test for leaks, use a consistent mixing and starting procedure, choose a suitable syringe size and clamp it horizontally. Repeat conditions and calculate means. Use the same apparatus for all comparisons.
Practical or data skill
Plan a marble-chip and hydrochloric-acid investigation using a gas syringe. Include a suitable concentration range, repeats and safety precautions.
Examination tip
When evaluating gas-syringe data, distinguish rate evidence from final-yield evidence.
Review questions and suggested answers
Question 1
What does the initial graph gradient show?
Suggested answer
The initial gas-production rate.
Question 2
How does a gas leak affect results?
Suggested answer
Measured gas volumes are lower than the actual amount formed.
Question 3
Why must the bung be fitted quickly?
Suggested answer
To prevent early gas escaping before measurement.
