Required practical 5: test concentration with gas-volume curves
Đã giới thiệu| English | Ý nghĩa |
|---|---|
| gas-volume method/ɡæs ˈvɒljuːm ˈmeθəd/ | phương pháp đo thể tích khí |
| start delay/stɑːt dɪˈleɪ/ | độ trễ khởi đầu |
What would explain this observation?
- More concentrated acid can react faster with magnesium. A gas-volume curve tests that prediction, but a delayed bung or a leak can lose the early hydrogen.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- RP5 requires a concentration investigation using both gas-volume measurement and colour or turbidity change. In the acquired gas activity, equal cleaned 3 cm magnesium ribbon lengths react with 50 cm³ of teacher-prepared dilute hydrochloric acid of two concentrations. Hydrogen is collected through a delivery tube into an inverted water-filled measuring cylinder in a trough. The handbook permits a gas syringe as an alternative. Develop a hypothesis before measuring.
- gas-volume method 气体体积法: Monitoring gas product volume over time to investigate reaction rate; start delay 启动延迟: A delay between reaction beginning and measurement or collection starting.
Which change would undermine a concentration-only comparison?
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Testing concentration means changing ONLY concentration. Different magnesium masses would change the surface area and the total gas as well, so no difference could be attributed to concentration alone. Same temperature and same acid volume protect the comparison.
Match each technical term to its precise meaning.
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Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Fit the collector and check for unobstructed movement and an open gas path into the collector. Add magnesium, replace the bung promptly and start timing consistently. Record gas volume at suitable intervals, for example every 10 s, until it becomes constant. Plot both volume–time curves on the same axes and compare steepness at comparable times. Identical final volumes require equal limiting magnesium and enough acid; real collection losses or sample differences can change recorded plateaux.
- Perform the approved actual school experiment with eye protection, a stable clamped collector and no ignition sources near hydrogen. Use matched ribbon mass/length and consistent oxide removal, acid volume, initial temperature and apparatus. Never seal gas-producing apparatus with no outlet. Repeat trials where feasible and compare methods. A syringe avoids some water-collection problems but can stick or leak; a water collector needs consistent reading conditions and gas not excessively soluble in water.
Which two habits make the investigation or model in this case more defensible?
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Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: original illustrative readings give 18 cm³ after 20 s in the lower-concentration trial and 30 cm³ after 20 s in the higher one, both starting at zero. Endpoint mean rates are 0.90 and 1.50 $\dfrac{\text{cm}^3}{\text{s}}$. The latter is faster over that interval; the ratio is 1.50/0.90≈1.67. These fictional values are not the handbook’s technician data or a rule that rate exactly tracks concentration in every reaction.
A gas trial starts at zero and collects 27 cm³ in 30 s. Find mean collection rate. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A gas trial starts at zero and collects 27 cm³ in 30 s. Find mean collection rate.
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The result is 0.9 cm³ per s. Known: 27 cm^3 collected in 30 s, starting from zero. Mean rate = volume / time = 27/30 = 0.9 cm^3 per second over that interval.
Check the conclusion and its limits
- A written graph answer cannot certify practical participation. Do not ignore gas lost before fitting the bung, interpret a smaller collected plateau automatically as slower chemistry, or use unequal magnesium pieces as a concentration-only test. The molar concentration labels identify provided solutions; calculating a mol/s rate remains Higher-only.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Gas lost before the bung is fitted can be ignored when evaluating the volume curve. This claim is false: Gas escaping before the bung seals is lost product: the recorded curve then starts below the truth and every later reading is low. Evaluating the method means acknowledging that systematic loss, not ignoring it.
Required practical 5: test concentration with gas-volume curves: Fit the collector and check for unobstructed movement and an open gas path into the collector. Add magnesium, replace the bung promptly and start timing consistently. Record gas volume at suitable intervals, for example every 10 s, until it becomes constant. Plot both volume–time curves on the same axes and compare steepness at comparable times. Identical final volumes require equal limiting magnesium and enough acid; real collection losses or sample differences can change recorded plateaux.
Gas lost before the bung is fitted can be ignored when evaluating the volume curve.
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Gas escaping before the bung seals is lost product: the recorded curve then starts below the truth and every later reading is low. Evaluating the method means acknowledging that systematic loss, not ignoring it.
Monitoring gas product volume over time to investigate reaction rate: write the technical term.
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gas-volume method means Monitoring gas product volume over time to investigate reaction rate.