Collecting gas from a reaction
Use gas-volume readings to calculate how quickly a reaction produces collected gas.
Choose the time interval
Mean rate = change in quantity ÷ time taken. Subtract the two volume readings and the two time readings for the interval asked about. With volume in cm³ and time in seconds, rate is in cm³/s.
A mean rate describes the whole interval. It is not necessarily the rate at the start or end. A steeper section of a gas-volume graph shows a faster collection rate.
Explain the curve
The curve often becomes less steep as reactants are used up. A flat section means no further increase is being measured. The gas already collected has not disappeared.
Smaller pieces of the same mass of solid expose more surface area and can react faster. With the other reactant in excess, the final gas amount may stay the same. A faster rate does not automatically mean more product.
Check the method
Gas can escape before the apparatus is connected or through a leak. Some may dissolve in liquid. Keep collection conditions the same when comparing runs, including temperature and pressure.
Use suitable supervised gas-collection apparatus. Never seal a gas-producing reaction in an unsuitable container. Repeat readings and explain how a method problem would affect the volume measured.
Worked example
Volume rises from 40 cm³ at 40 s to 56 cm³ at 80 s. Mean rate = (56 − 40) ÷ (80 − 40) = 0.40 cm³/s.
Read the detailed explanation
A gas syringe measures the volume collected as a reaction proceeds. The change in volume over a stated time interval gives a mean rate for that interval.
Choose the interval
Mean rate = change in quantity ÷ time interval. For gas volume measured in cm³ and time in seconds, the rate unit is cm³/s.
For an interval that starts after zero, subtract both the starting volume and the starting time. Dividing the final reading by the final time would answer a different question.
Read what the graph shows
A collected-volume graph is usually steeper when gas is being produced more quickly. As the limiting reactant is used up, the graph becomes less steep and may level off.
A mean rate between two readings describes the whole interval. It does not give the instantaneous rate at either reading. To estimate the rate at one instant, draw a tangent and calculate its gradient.
Separate rate from final amount
With the same mass of calcium carbonate and acid remaining in excess, smaller pieces expose more surface area. This can increase the initial rate without increasing the final amount of carbon dioxide.
The conclusion depends on the conditions. Changing the amount of limiting reactant can change the final gas amount. Leaks, dissolving gas and changing temperature can also affect measured volume.
Another worked example
Collected volume rises from 40 cm³ at 40 s to 56 cm³ at 80 s. Mean rate = (56 − 40) ÷ (80 − 40) = 0.40 cm³/s.
Practical reasoning
- Select a gas-collection method suitable for the reaction and gas.
- Record volume at stated times and use the same temperature and pressure conditions for comparisons.
- Connect the apparatus before starting gas production where the supervised method permits this.
- Describe how a limitation affects the measurement. For example, gas escaping before connection makes the collected volume too low.
Check your reasoning
- A larger final gas volume alone does not establish a faster initial rate.
- A flat collected-volume graph means no further gas is being collected at a measurable rate; it does not mean the collected gas has vanished.
- Never seal a gas-producing reaction in unsuitable apparatus. Carry out practical work only with the appropriate supervised method.