Question

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.

Experimental Record
Flask Volume of
HCl Volume of
Sodium Thiosulfate Volume of
Water Time
1 10 mL 10 mL 40 mL 14 seconds
2 10 mL 20 mL 30 mL
3 10 mL 30 mL 20 mL
4 10 mL 40 mL 10 mL

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.

Answer 1

Based on the given experimental record and our knowledge of factors that affect the rates of chemical reactions, we can predict that the time taken for the cross to become invisible in flask 2 will be shorter than that in flask 1, the time taken in flask 3 will be shorter than that in flask 2, and the time taken in flask 4 will be shorter than that in flask 3.

This is because the volume of sodium thiosulfate is increasing while the volume of water is decreasing in each successive flask. Sodium thiosulfate is a reactant in the chemical reaction that produces the yellow precipitate, and the greater the volume of sodium thiosulfate, the greater the concentration of the reactants and the faster the reaction will occur. This means that as the volume of sodium thiosulfate increases, the time taken for the cross to become invisible will decrease, indicating a faster reaction rate.

Conversely, as the volume of water decreases, the total volume of the solution in each flask is decreasing, which means that the rate of collisions between the reactant molecules is decreasing. This can slow down the reaction rate and cause the time taken for the cross to become invisible to increase. However, the effect of decreasing volume of water is likely to be smaller than the effect of increasing volume of sodium thiosulfate, leading to an overall decrease in the time taken for the cross to become invisible in each successive flask. Therefore, we can predict that the trend in the last column of the experimental record will be a decrease in the time taken for the cross to become invisible as the volume of sodium thiosulfate increases and the volume of water decreases.