Rates of reaction part 2: measure change in colour or turbidity

Investigate how changes in concentration affects the rates of reactions by measuring change in colour or turbidity.

In this required practical, students use a change in colour or turbidity to see how quickly Hydrochloric Acid reacts with Sodium Thiosulfate to give a cloudy solution.

This follows the rates of reaction by measuring the volume of gas produced practical, which tests two different concentrations of acid to see how quickly and how much gas is produced when it is reacted with a set length of Magnesium Ribbon.

Equipment (per participant)

• Safety goggles
• 100ml conical flask
• 25ml measuring cylinder x 2
• Stopwatch
• Paper square with a cross
• Washing up bowl (for use as a stop bath containing 0.5M Sodium Carbonate and Phenolphthalein solution)

• Sodium Thiosulfate (40g/L)
• Hydrochloric Acid (1M)
• CLEAPSS Hazcards: 32, 47A, 59A, 95A

Method

Students should collect the necessary equipment and place a 100ml conical flask onto a square of white paper onto which a large cross has been drawn.
They should measure 10mls of Sodium Thiosulfate solution and put this into the conical flask. They then need to add 40mls of water to dilute the Sodium Thiosulfate solution a little and ensure the conical flask is put back onto the cross.
Using a clean measuring cylinder, students should measure 10mls of Hydrochloric acid. This is added to the flask and the flask is swirled to mix whilst starting the stopwatch. The flask must be placed back onto the cross after the solution is mixed.
Students must now observe the solution from above (at a distance), taking care not to inhale any sulfur fumes given off. They should watch for when the cross disappears. As soon as the cross disappears, the students must stop the stopwatch. They then record this time in a suitable table.
The waste solution from the reaction should be emptied into the stop bath provided containing 0.5M Sodium Carbonate solution, to keep the sulfur fumes in the laboratory to a minimum.
The students then repeat steps 2-5 of the process using 20mls, 30mls, 40mls and 50mls of Sodium Thiosulfate (& 30mls, 20mls, 10mls and 0mls of water to dilute the Sodium Thiosulfate). The total amount of solution in the flask each time should total 50mls.
They must then carry out a complete repeat two further times to produce results from which a mean average can be taken, and their results can be plotted on a graph.

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Technician tips

• A washing up bowl should be provided as a stop bath. A Sodium Carbonate solution (0.5M) is added to the washing up bowl, and students can empty the waste from their experiments into this so that it stops the reaction between the acid and the Thiosulfate. This will also help to reduce the sulfur fumes in the laboratory.
• In order to make the Sodium Carbonate solution for the stop bath, 53g of Sodium Carbonate anhydrous is dissolved in 1L of water. Phenolphthalein should be added, and the stop baths should be checked periodically and be refreshed as needed to ensure they are still reacting to neutralize the waste products.
• 40g/L Sodium Thiosulfate solution is made up wearing eye protection by dissolving 10g of the solid in 250mls of water.