CP7.1: Gas Collection and Temperature-Dependent Turbidity

A suitable acid–carbonate reaction produces carbon dioxide that can be collected with an appropriate apparatus. Record gas volume against time using a consistent start, mixing and sampling rule. Check for leaks, delayed connection and a gas syringe that moves freely. Keep reactant amounts, temperature and relevant solid dimensions comparable unless one is the intended variable.

The average volume rate over an interval is change in volume divided by elapsed time. A progress curve can become less steep as reactants are depleted. Straight lines joining recorded points give interval averages, not automatically an instantaneous tangent. Compare equivalent intervals and retain the actual units.

In a suitable thiosulfate–acid comparison, formation of a sulfur suspension can obscure a mark viewed through the mixture. Record the time to a defined visual endpoint under the approved laboratory method. Use comparable liquid depth, viewing position, mixing, lighting and the same visibility rule.

For the same small change and comparable starting conditions, reciprocal time can be a relative rate proxy. It is not a directly calibrated mass of sulfur per second. Subjective judgement and timing delays can affect the result. Follow the centre’s arrangements for the reaction and any released gases rather than using it as a home experiment.

A faster reaction need not produce more final product. If a fixed carbonate amount limits the reaction and acid is sufficiently in excess in both runs, different rates can lead to the same eventual gas amount under comparable gas conditions. Changing a reactant amount can instead change both rate and final yield, so state which design was used.

Use repeats, suitable summaries and a graph matched to the measured quantity. A graph of endpoint time against temperature is not the same graph as reciprocal time against temperature. Explain how concentration, temperature or surface area affects collisions only when the corresponding factor and other controls are clear.

Pearson CP7.1 measures gas from hydrochloric acid reacting with marble chips, then investigates a thiosulfate–acid turbidity endpoint at different temperatures by warming the thiosulfate solution. Record the actual temperature and keep concentrations, total depth, mixing and visibility criteria comparable in that second part.

Plot the specified time-to-endpoint against temperature and distinguish it from a reciprocal-time rate-proxy graph. A lower endpoint time indicates a faster comparable threshold process, but it is not an automatically calibrated sulfur-mass rate. Use suitable repeats and explain the limitations of the visual endpoint.

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.