Chemistry 9701/31 — October/November 2012

Method

Experiment 1

• Use the measuring cylinder labelled A to transfer $50\text{ cm}^3$ of FA 1 into a $250\text{ cm}^3$ beaker.
• Use the measuring cylinder labelled B to measure $20\text{ cm}^3$ of FA 2.
• Tip the FA 2 from measuring cylinder B into the beaker and immediately start timing.
• Stir the contents of the beaker once and place the beaker on the printed insert.
• View the print from directly above and through the solution in the beaker.
• Stop timing when the printed material on the insert is just no longer visible.
• Record the reaction time to the nearest second.
• Calculate and record the value of $\frac{1000}{\text{reaction time}}$ to 3 significant figures.
• Empty, rinse and dry the beaker.

Experiment 2

• Use the measuring cylinder labelled A to transfer $40\text{ cm}^3$ of FA 1 into the rinsed and dried beaker.
• Use the same measuring cylinder labelled A to transfer $10\text{ cm}^3$ of distilled water into the same beaker.
• Use the measuring cylinder labelled B to measure $20\text{ cm}^3$ of FA 2.
• Tip the FA 2 from measuring cylinder B into the beaker and immediately start timing.
• Stir the contents of the beaker once and place the beaker on the printed insert.
• View the print from directly above and through the solution in the beaker.
• Stop timing when the printed material on the insert is just no longer visible.
• Record the reaction time to the nearest second.
• Calculate and record the value of $\frac{1000}{\text{reaction time}}$ to 3 significant figures.
• Empty, rinse and dry the beaker.

Experiments 3–5

• Repeat the experiment using the following volumes of FA 1 and distilled water.
  • Experiment 3: $30\text{ cm}^3$ of FA 1 + $20\text{ cm}^3$ of distilled water
  • Experiment 4: $20\text{ cm}^3$ of FA 1 + $30\text{ cm}^3$ of distilled water
  • Experiment 5: $10\text{ cm}^3$ of FA 1 + $40\text{ cm}^3$ of distilled water

Experiment 6

When you have completed experiments 1–5, carry out one further experiment using a different volume of both FA 1 and distilled water.

When you have completed all of your experiments, empty and rinse the beaker.

Record the results for all six experiments in the space below.
Your table should include columns for the following.

• experiment number
• volume of FA 1
• volume of distilled water
• reaction time
• $\frac{1000}{\text{reaction time}}$

The rate of reaction can be represented by the following formula.

On the next page plot a graph of 'rate' against the volume of FA 1.

Start each of the axes at zero.

Draw the line of best fit.

A student carried out the experiments in a $100\text{ cm}^3$ beaker instead of a $250\text{ cm}^3$ beaker.
State and explain what effect this would have on the times recorded.

FA 1 is $0.150\text{ mol dm}^{-3}$ $\text{Na}_2\text{S}_2\text{O}_3$.
Calculate the initial concentration of $\text{Na}_2\text{S}_2\text{O}_3$ in the reaction mixture in Experiment 5.
Show your working.

In your experiments, the volume of FA 1 represents the initial concentration of sodium thiosulfate in the reaction mixture.
A text book states that the rate of reaction between aqueous sodium thiosulfate and hydrochloric acid is directly proportional to the concentration of sodium thiosulfate.
Use your graph to decide whether the statement in the text book is correct or not.
Explain your answer.

When viewing the insert through the solution it is difficult to judge the exact moment when the printed material just disappears.
This uncertainty is different for each experiment and is greater for longer reaction times when the printed material disappears slowly.
Complete the table below, assuming the uncertainties given.

Complete the headings in the table below to record the volume of FA 1 (aqueous sodium thiosulfate), the volume of distilled water and the volume of FA 2 (hydrochloric acid).

In the second row copy the volumes used in Experiment 3 from your table of results on page 4.

In the following two rows suggest volumes of each of the reagents that could be used in two further experiments, Experiment 7 and Experiment 8, to investigate how the rate of reaction varies with a change in the concentration of the acid.

Do not carry out these experiments.