Chemistry 9701/52 — May/June 2011
Cambridge A-Level · Planning, Analysis and Evaluation · worked solutions for every part, with the mark scheme
Topics Analysis, Conclusions and Evaluation · Planning
Reactions involving two aqueous solutions are dependent on collisions occurring between the particles of the two reagents.
As the temperature of the system is raised, the average kinetic energy of the particles increases.
You are to plan an experiment to investigate how the rate of the reaction between hydrochloric acid and sodium thiosulfate, , depends on the temperature of the reaction. When these two reagents react, after a short period they slowly produce a white or yellow precipitate of sulfur. As more sulfur is produced, the reaction mixture becomes more cloudy until it cannot be seen through (i.e. it is opaque). The time taken for the mixture to become opaque can be dependent on the relative concentrations of the reagents or the temperature of the reaction mixture.
Predict how the rate of reaction will change if the temperature of the reagents is increased. Using the idea of how the kinetic energy of the particles changes as the temperature of the reagents increases, explain your prediction in terms of particle collisions.
prediction
explanation
Display your prediction in the form of a sketch graph, clearly labelling the axes.
In the experiment you are about to plan, identify the following.
the independent variable
the dependent variable
Draw a diagram of the apparatus and experimental set up you would use to carry out this experiment. Your apparatus should use only standard items found in a school or college laboratory and show clearly the following.
(i) the apparatus used as the reaction vessel and how the thermometer will be positioned in order to measure the temperature of the solution as accurately as possible
(ii) how the solution will be heated
Label each piece of apparatus used, indicating its size or capacity and the temperature range that the thermometer should cover.
Using the apparatus shown in (c) design a laboratory experiment to test your prediction in (a).
In addition to the standard apparatus present in a laboratory you are provided with the following materials,
aqueous sodium thiosulfate,
hydrochloric acid,
A stop-watch/clock with second hand.
Give a step-by-step description of how you would carry out the experiment by stating
(i) the number of experiments you would do, and their temperature range (minimum and maximum temperatures),
(ii) what you would keep constant in all the experiments,
(iii) what temperature measurements you would make,
(iv) how you would use the cloudiness (opacity) of the reaction mixture to measure the time taken for each reaction.
State a hazard that must be considered when planning the experiment and describe precautions that should be taken to keep risks to a minimum.
Draw a table with appropriate headings to show the data you would record when carrying out your experiments and the values you would calculate in order to construct a graph to support or reject your prediction in section (a). The headings must include the appropriate units.
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