A student investigates a reaction used in some reusable heat packs. Sodium thiosulfate solution and dilute hydrochloric acid form a sulfur solid, making the...
Question 1 Report
A student investigates a reaction used in some reusable heat packs. Sodium thiosulfate solution and dilute hydrochloric acid form a sulfur solid, making the mixture cloudy. Fig. 1 shows a flask positioned over a black cross. The student records the time for the cross to disappear from view. Table 1 contains the results. In every trial, the total volume of liquid is 50 cm3 and the temperature is 22 °C.
| Volume of sodium thiosulfate solution / cm3 | Volume of water / cm3 | Time for cross to disappear / s |
|---|---|---|
| 10 | 30 | 96 |
| 20 | 20 | 51 |
| 30 | 10 | 33 |
| 40 | 0 | 24 |
(a) Name the solid that causes the cloudiness. [1]
(b) State the independent variable. [1]
(c) Describe the trend shown by the results. [2]
(d) Explain why adding water changes the time taken for the cross to disappear. [3]
(e) Give two improvements to the method that would make the conclusion more reliable. [2]
(f) Calculate the rate, using 1/time, for the 40 cm3 trial. Give your answer to three significant figures. [1]
Answer Details
(a) The cloudiness is caused by solid sulfur. [1]
(b) The independent variable is the volume or concentration of sodium thiosulfate solution. [1]
(c) As sodium thiosulfate volume or concentration increases, the time for the cross to disappear decreases. [1] Therefore the reaction rate increases. [1] [2]
(d) Adding water dilutes the sodium thiosulfate solution. [1] There are fewer reacting particles in a given volume, [1] so fewer successful collisions occur each second and sulfur forms more slowly. [1] [3]
(e) Repeat every trial and calculate a mean [1], and use a colourimeter or light sensor rather than judging the cross by eye. [1] Other valid improvements include accurate pipettes, the same observer, or a water bath to keep temperature constant. [2]
(f) Rate \(=1/\text{time}\):
\[\frac{1}{24\ \mathrm{s}}=0.0417\ \mathrm{s^{-1}}\]
\(0.0417\ \mathrm{s^{-1}}\), to three significant figures. [1]
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