(07.05 HC)

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. (5 points)

The time taken for the cross to disappear decreases as we move down the table

Explanation:

We have to take note of three important things we can see in the table.

1) the volume of HCl was held constant

2) the volume of the thiosulphate solution was increased steadily down the table

3) the volume of water was decreased steadily down the table.

We know that the rate of reaction increases with increase in the the concentration of reactants. The more the volume of reactants compared to the volume of the volume of water present, the higher the concentration of reactants and the faster the rate of reaction. This implies that it takes a shorter time for the reaction to get to completion.

As we increase the volume of thiosulphate and decrease the volume of water, we are manipulating the reactant concentration. In this case, concentration is being increased. Hence the reaction proceeds faster and it takes a shorter time for the cross to disappear. Hence the time taken for the cross to disappear will decreased steadily down the table.

answer; natural resources;

it would be a gas formation