Required practical 5: compare a defined disappearing-cross endpoint
הוצג| English | משמעות |
|---|---|
| turbidity/tɜːˈbɪdɪti/ | עכירות |
| reciprocal-time proxy/rɪˈsɪprəkl taɪm ˈprɒksi/ | מדד הפוך-זמן |
What would explain this observation?
- Sulfur particles make an acid–thiosulfate mixture cloudy until a black cross is no longer visible. The endpoint is an observation threshold, not the instant all reactants have been used up.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- The acquired RP5 turbidity 浑浊度 activity dilutes 40 g/dm³ sodium thiosulfate stock with water to a constant 50 cm³ before adding the same 10 cm³ dilute hydrochloric acid. Use stock/water volumes 10/40, 20/30, 30/20, 40/10 and 50/0 cm³, giving pre-acid concentrations 8,16,24,32 and 40 g/dm³. Put the same flask on a printed black cross, add acid, swirl consistently and time until the cross is no longer visible.
- turbidity: Cloudiness caused by suspended particles affecting how clearly a target is seen; reciprocal-time proxy 时间倒数指标: One divided by time to a comparable fixed endpoint, used as a relative rate measure.
Why keep the total diluted thiosulfate volume constant before adding acid?
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Keeping the total volume constant with water keeps the depth and path length identical, so only concentration changes between trials. Without it, the cross would disappear at different depths for reasons unrelated to rate.
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
- The concentration labels refer to the 50 cm³ pre-acid dilution; adding acid further dilutes the mixture consistently. Maintain total volumes, flask, viewing depth, lighting, temperature and endpoint judgement. Repeat and calculate mean endpoint time. For a comparable fixed turbidity threshold, 1/mean time is a relative rate proxy: a shorter time means a larger proxy. It does not measure a known mass of sulfur $\dfrac{1}{\text{s}}$ or the exact completion time of the entire reaction.
- Carry out only the school-approved method with ventilation and eye protection. Sulfur dioxide is released and can aggravate breathing difficulties; do not lean over the vessel or deliberately inhale fumes. The teacher selects controls and an appropriate alternative observation arrangement for affected students. Record raw times and reasons for any justified anomalous-result exclusion; do not discard values merely because they spoil a trend. Compare repeats within a method before claiming reproducibility across groups.
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 repeat endpoint times are 48,50 and 52 s. Mean time=(48+50+52)/3=50 s; relative proxy=1/50=0.020 $\dfrac{1}{\text{s}}$. A second concentration gives mean 25 s and proxy 0.040 $\dfrac{1}{\text{s}}$, twice as large under the same observation threshold. This does not mean sulfur production was exactly constant within either trial.
A comparable fixed endpoint takes 40 s. Find the reciprocal-time proxy. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A comparable fixed endpoint takes 40 s. Find the reciprocal-time proxy.
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The result is 0.025 per s. Known: endpoint at 40 s. Use the reciprocal-time proxy = 1/time. Proxy = 1/40 = 0.025 per second. The proxy doubles when the endpoint time halves, under the same observation threshold.
Check the conclusion and its limits
- A disappearing cross is not a direct gas-volume reading, and one method alone does not fulfil both RP5 approaches. Do not dilute stock by reducing solution volume without replacing water, because depth and total volume would also change. A subjective endpoint needs consistent judgement; repeat averaging cannot correct a systematic viewing difference.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
The cross disappearing proves the whole reaction has finished. This claim is false: The cross disappears when enough sulfur has formed to obscure it - an arbitrary visibility threshold reached LONG before the reaction finishes. It measures time to a fixed turbidity, not completion.
Required practical 5: compare a defined disappearing-cross endpoint: The concentration labels refer to the 50 cm³ pre-acid dilution; adding acid further dilutes the mixture consistently. Maintain total volumes, flask, viewing depth, lighting, temperature and endpoint judgement. Repeat and calculate mean endpoint time. For a comparable fixed turbidity threshold, 1/mean time is a relative rate proxy: a shorter time means a larger proxy. It does not measure a known mass of sulfur $\dfrac{1}{\text{s}}$ or the exact completion time of the entire reaction.
The cross disappearing proves the whole reaction has finished.
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The cross disappears when enough sulfur has formed to obscure it - an arbitrary visibility threshold reached LONG before the reaction finishes. It measures time to a fixed turbidity, not completion.
Cloudiness caused by suspended particles affecting how clearly a target is seen: write the technical term.
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turbidity means Cloudiness caused by suspended particles affecting how clearly a target is seen.