In a kinetics experiment, a conical flask of sodium thiosulphate solution stands on white paper marked with a cross. Dilute hydrochloric acid is added, a stopwatch is started, and the time is noted when the cross can no longer be seen from above. Repeating with a more dilute thiosulphate solution takes longer. What hides the cross, and what does the result show?
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Na2S2O3 + 2HCl → 2NaCl + H2O + SO2 + S; the finely divided sulphur makes the solution turbid and hides the cross. Since the time taken is longer at lower thiosulphate concentration, the rate decreases as concentration decreases.
- A.
Sulphur dioxide stays dissolved and does not cloud the solution; a longer time means a slower rate.
- B.
Sodium chloride is soluble and does not precipitate; the change in time shows the rate depends on concentration.
- C.
A longer time for the cross to vanish means a slower reaction, so the rate falls with concentration.
NCERT: Class 12 Chemistry Laboratory Manual