Chemistry - Required Practicals

What is the recrystallisation method?

Dissolve the impure compound in a minimum volume of hot solvent so that the desired compound is saturated in solution. Filter the solution hot so any insoluble impurities can be removed and crystals cannot be reformed. Cool the filter solution slowly in ice to increase formation of crystals but soluble impurities still remain. Suction filter with a Buchner flask to separate the crystals, the water pump reduces pressure so speeds up filtration. Wash the crystals with distilled water to remove soluble impurities. Dry the crystals to remove water than affects the percentage yield.

Making up a volumetric solution and carrying out an acid base titration

Weigh by difference for the solid. Add distilled water and stir until all of the solid has dissolved. Transfer to a volumetric flask with washings. Make upt o 250cm3 and shake the flask. Pour 100cm3 of this solution into the burette. Pour 100cm3 of the unknown solution into the beaker under the burette. Add phenolpthalein drops and note initial colour. Record initial burette reading. Titrate contents until a permanent colour change is seen. Record final burette reading and find titre. Repeat until you get at elast 2 concordant results.

How rate of reaction changes with temperature

Add a small amount of sodium thiosulphate to a test tube. Place this test tube in a container making sure the cross is visible through the hole with a thermometre. Add the same amount of HCl to another test tube. Record start temp and then add 1cm3 of the acid and start timing. Record until cross disappears from view. Record final temp. Add water no higher than 55 degrees to the plastic container, leave the tube to warm up and repeat the experiment for different temps.

Measuring rate of reaction

Add hydrogen peroxide to a beaker. Add sulphuric acid to a separate beaker. To this, add distilled water and using a pipette add starch solution. Add some potassium iodide and finally add sodium thisulphate. Stir the mixture. Immediately add the hydrogen peroxide solution to the beaker and start timing. Stop the timer when the mixture turns blue black. Record the time. Repeat but changing concentration of potassium iodide. Plot a graph of inital rate against concentration to get order.

Purifying an organic liquid

Put distillate into a separating funnel. Wash product with sodium hydrogen carbonate solution, invert and release pressure. it also removes acidic impurities by neutralisation. Allow layers to separate funnel and then run and discard the aqueous layer. Run the organic liquid into a conical flask. Add anhydrous sodium sulphate as a drying agent. Decant liquid into a distillation flask and distill to collect pure product.

Distillation of a reaction product

Dissolve potassium dichromate in sulphuric acid and measure out 12cm³ of this solution into a test tube. Cool the boiling tube in cold water. Set up distillation apparatus with the boiling tube being suspened and another tube being suspended in a boiling tube in cold water.