Equilibria Involving Carbon Dioxide in Aqueous Solution

Experiment & Report Higher Chemistry

EEXPERIMENT && CCHEMISTRHEMISTR RREPORT HigherHigher (HC)(HC) YY

EQUILIBRIA INVOLVING CARBON DIOXIDE IN AQUEOUS SOLUTION

INTRODUCTION Demonstration and class practical

Soda water is placed in a syringe and the plunger pulled out to reduce the pressure above it. Bubbles of carbon dioxide are seen forming (‘out-gassing’) as its solubility decreases. Methyl red indicator added to the soda water turns from red to yellow, showing that the solution has become less acidic as the equilibria in solution adjust.

Methyl red indicator is red below pH 4.2 and yellow above pH 6.3.

METHOD

Soda water or carbonated mineral water - A fresh, unopened bottle is best. Flavoured fizzy drinks, such as lemonade or cola, are not suitable here because they contain added acids, such as citric acid. 1 A smaller syringe will do but the changes are less easily visible. Modify the syringe as follows: pull out the plunger so that the volume of air in the syringe is 50 cm3 (see diagram). Warm the nail in a Bunsen

of 2 24-五月-18 Experiment & Report Higher Chemistry flame and push it through the stem of the plunger as shown in the diagram. When the nail is in place, the plunger can be ‘locked’ at the 50 cm3 mark.

Procedure

1)Pour 10 – 20 cm3 of soda water into the beaker and add a few drops of methyl red indicator to give a red solution. 2)Remove the nail from the syringe and insert the plunger completely. Draw about 5 cm3 of the soda water and indicator solution into the syringe. Place a syringe cap over the end of the syringe (or use a finger), pull the plunger out to the 50 cm3 mark and lock it with the nail. Bubbles of carbon dioxide will be seen out- gassing from the water and the indicator will begin to turn orange. Shake the syringe to speed up the out-gassing. 3)Hold the syringe vertically with the nozzle pointing upwards, remove the syringe cap and the nail, and push in the plunger to expel the gas but not the solution. Seal the syringe again and repeat the out-gassing cycle in b. More bubbles will be seen and the indicator will turn further towards a yellow colour. Several more such cycles can be repeated until the indicator becomes completely yellow.

Hazard Wear eye protection Methyl Red indicator - Hazardous in case of ingestion. Slightly hazardous in case of skin contact (irritant), of eye contact (irritant), of inhalation.

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