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Concentration affecting Reaction Rate

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A series of free IGCSE Chemistry Activities and Experiments (Cambridge IGCSE Chemistry).

This experiment investigates the effect of concentration on the rate of the reaction between sodium thiosulfate and dilute hydrochloric acid. The rate of reaction is measured by timing the disappearance of an × marked under the flask. The disappearance is caused by the sulfur precipitate produced in the reaction.

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

1. Using a pen, draw a × on some white paper.
2. Place a conical flask over the × so that you can see the × from above.
3. Measure 50 cm3 of the sodium thiosulfate solution tnto the flask.
4. Add 5 cm3 of the hydrochloric acid and at the same time start the stopclock.
5. Viewing from directly above the flask, note the time taken for the cross marked on the paper to 'disappear' i.e. be obscured by the sulfur precipitating.
6. Repeat the experiment using different concentrations of sodium thiosulfate.
7. Record your results in a table.

(In the following video, the concentration of Na2S2O3 is calculated using the formula
c1v1 = c2v2, where c1 is concentration and v1 is the volume before diluting and c2 is concentration and v2 is the volume after diluting.
For example, in the second trial she used 40.0 mL of 0.15 mol/L stock solution and 10.0 mL of water to form 50.0 mL of diluted solution.
(0.15 mol/L)(0.0400 L) = (0.0500 L)(c2). Solve for c2 to get 0.12 mol/L)

1. What happened when the acid was added to the sodium thiosulfate solution?

2. How can this be used to tell you how quick the reaction is?

3. Therefore the shorter the time taken for the × to disappear, the _____ the reaction.

4. State the effect of increasing the concentration of sodium thiosulfate on the reaction time.

5. Using the collision theory, explain how increasing the concentration of sodium thiosulfate can increase the rate of reaction.

  • Show Solutions
    1. Sulfur precipitate forms.
    2. The time taken for the precipitate to form will indicate the speed of the reaction.
    3. faster
    4. Increasing the concentration of sodium thiosulfate will reduce the reaction time.
    5. As the concentration of sodium thiosulfate increases, the particles become closer and there is an increase chance of them colliding with the hydrochloric acid particles and reacting. Therefore the effect of increasing the concentration is to increase the rate of reaction

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