Scottish Highers Chemistry Controlling the Rate: Sodium thiosulfate, Na₂S₂O₃, can

Question

Sodium thiosulfate, Na₂S₂O₃, can be used to investigate the effect of reaction conditions on the rate of reaction.

(a) Sodium thiosulfate solution reacts with hydrochloric acid to form a precipitate of solid sulfur. By placing the reaction mixture in a conical flask over a cross drawn on paper, add 5.0 cm³ hydrochloric acid and start timing. Stop timing when the cross disappears.

(i) The equation for the reaction is

a) Na₂S₂O₃(aq) + 2HCl(aq) → S(s) + SO₂(g) + 2NaCl(aq) + H₂O(l)

Balance the equation.

(ii) In one set of experiments, the effect of varying the concentration of sodium thiosulfate was investigated.

| Experiment | Volume of 0.15 mol l⁻¹ Na₂S₂O₃ (cm³) | Volume of water (cm³) | Rate (s⁻¹) |
|------------|---------------------------------------|-----------------------|-------------|
| A          | 50                                    | 0                     | 0.0454      |
| B          | 40                                    | 0                     | 0.0370      |
| C          | 30                                    | 0                     | 0.0285      |
| D          | 20                                    | 0                     | 0.0169      |
| E          | 10                                    | 40                    | 0.0063      |

(A) Complete the table to show the volumes of water that would have been used to vary the concentration of sodium thiosulfate.

(B) Calculate the time, in seconds, for the cross to disappear in experiment C.

(iii) The reaction can also be used to investigate the effect of changing temperature on the rate of reaction. The results from an investigation are shown in the graph below.

Use the graph to determine the temperature rise, in °C, required to double the rate of the reaction.

(b) Collision theory states that for particles to react they must first collide with each other.

State two conditions necessary for the collisions to result in the formation of products.

(c) Sodium thiosulfate also reacts with iron(III) nitrate. The potential energy diagram below shows the change in potential energy during the reaction carried out without a catalyst.

(i) Draw an X on the potential energy diagram above to show where the activated complex is formed.

(ii) Cu²⁺ ions catalyse the reaction.

Add a dotted line to the diagram to show the change in potential energy with the catalyst.

Sodium thiosulfate, Na₂S₂O₃, can be used to investigate the effect of reaction conditions on the rate of reaction - Scottish Highers Chemistry - Question 1 - 2019

Worked Solution & Example Answer:Sodium thiosulfate, Na₂S₂O₃, can be used to investigate the effect of reaction conditions on the rate of reaction - Scottish Highers Chemistry - Question 1 - 2019

The equation for the reaction is

Complete the table to show the volumes of water that would have been used to vary the concentration of sodium thiosulfate.

Calculate the time, in seconds, for the cross to disappear in experiment C.

Use the graph to determine the temperature rise, in °C, required to double the rate of the reaction.

State two conditions necessary for the collisions to result in the formation of products.

Draw an X on the potential energy diagram to show where the activated complex is formed.

Add a dotted line to the diagram to show the change in potential energy with the catalyst.

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