In an experiment, hydrochloric acid reacted with different volumes of sodium thiosulfate in water. A yellow precipitate was formed during the reaction. A cross drawn at the base of each flask became gradually invisible due the formation of this yellow precipitate. The time taken for the cross to become invisible was recorded. A partial record of the experiment is shown.

Based on your knowledge of factors that affect the rates of chemical reactions, predict the trend in the last column of the experimental record. Use complete sentences to explain the trend you predicted. You do not have to determine exact values for time; just describe the trend you would expect (increase or decrease) and why it occurs.

The time taken for the reaction to go to completion(time taken for the cross to disappear) decreases steadily down the column for time.

Explanation:

If we look at the table closely we would quickly observe the following;

1) the volume of HCl was held constant

2) the volume of sodium thiosulphate was increased steadily

3) the volume of water was decreased steadily

The increase in volume of thiosulphate and decrease in the volume of water meant that reactant concentration was increased steadily. From our knowledge of factors that affect the rate of reaction, we know that the rate of reaction (typically read off as time taken for a given reaction to go to completion, in this case, decrease in the time taken for the cross to disappear) increases with increase in concentration of reactants.

Hence it is expected that as the concentration of reactants increase, the rate of reaction increases. As the rate of reaction increases, the time taken for the cross to disappear decreases. Hence the answer above.

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