AS Chemistry Practical Support Document Flashcards
Determine the formula of a hydrated compound by weighing and heating;
- Weigh an evaporating basin
- Add some hydrated solid and weigh the evaporating basin and solid
- Heat to constant mass - heat for a few minutes, cool then weigh and repeat this procedure until the mass does not change
Use the deflection of a stream of liquid from a burette to indicate polarity or lack of polarity within a molecule;
- Place the liquid in a burette and place a beaker underneath the jet
- Open the tap and allow a small stream of liquid to run from the burette
- Hold a charged polythene rod close to the stream of liquid and note any deflection
- To compare the polarity of two liquids use two burettes simultaneously with the stream of liquid at same rate and the distance between the charged rod and liquid constant
Carry out tests of electrical conductivity on solids, liquids and aqueous solutions of ionic and covalent substances;
- Set up a circuit using a dc power pack and bulb
- Add graphite electrodes into the circuit and dip them into a beaker of liquid/aqueous solution, ensuring they do not touch
- Switch on the power pack and record if the bulb lights
- To test electrical conductivity of a solid, place the solid between crocodile clips
Determine the solubility of chlorine and iodine in aqueous and non-aqueous solvents;
- Add 1 cm^3 of each of the aqueous halogen solutions (chlorine water, iodine solution) into separate test-tubes
- Add equal volumes of a saturated hydrocarbon solvent e.g. hexane to each tube, stopper the tube and, shake the mixture by inverting the test-tube a few times
- Allow the two layers to settle; observe and record the colour
Quantitative determination of chlorine and iodine in aqueous solution is not required at AS. An estimate of the relative solubility of iodine could be made by comparing the colour of an iodine solution with the colour of solutions of known concentration.
Produce a reactivity order of the halogens using the displacement reactions of the halogens with other halide ions in solution;
- Add 1 cm^3 of each of the aqueous halogen solutions (chlorine water, bromine water, iodine solution) into separate test-tubes.
- Add 1 cm^3 of potassium chloride solution to each and record any colour change
- Repeat the experiment with potassium bromide solution and then with potassium iodide solution
- A saturated hydrocarbon e.g. hexane could be added to distinguish between bromine and iodine or starch solution could be added to detect iodine
Carry out the reactions of the halides with concentrated sulfuric and phosphoric acids and perform chemical tests for the products (excluding hydrogen sulfide);
- Place 1 spatula of solid potassium chloride in a test tube
- Add a few drops of concentrated sulfuric acid and record observations
- Test the gas evolved
- Repeat the experiment using solid potassium bromide
- Repeat the experiment using solid potassium iodide
- Repeat the experiment using concentrated phosphoric acid instead of concentrated sulfuric acid
Carry out an acid-base titration to determine the concentration of acid/base, the degree of hydration in hydrated metal carbonate and the percentage of ethanoic acid in vinegar
Steps to ensure accuracy
• Rinse the apparatus with the appropriate solution
• Add the solution from the burette dropwise just before the endpoint
• Swirl the flask
• Read the burette at the bottom of the meniscus
Carrying out the titration
• Rinse the pipette with the solution to be transferred to the conical flask
• Using a pipette and pipette filler place (25.0 cm^3) of the solution in the conical flask
• Rinse the burette with the solution to be placed in it
• Fill the burette with this solution and record the reading on the burette to one or two decimal places; the second decimal place should be a zero or a five
• Add 3-5 drops of a suitable indicator to the conical flask
• Add the solution from the burette, with swirling, until the indicator just changes colour
• Record the reading to one or two decimal places reading to the bottom of the meniscus (the second decimal place should be a zero or a five)
• Repeat the titration to achieve two concordant results (within +- 0.2 cm^3 of each other)
Prepare solutions of known concentration
- Calculate the required mass of the solid/liquid required to make the solution (or the required volume of liquid)
- Weigh the solid/liquid in a beaker on a top pan balance (or measure the volume of the liquid using a graduated pipette/burette/measuring cylinder, as appropriate)
- Dissolve the solid in a small volume (50-100 cm^3) of deionised water
- Transfer the solution to the appropriately sized volumetric flask
- Rinse the beaker and glass rod with deionised water and add washings to the volumetric flask
- Make up to the mark by adding deionised water until the bottom of the meniscus is on the mark
- Stopper the flask and invert to mix
Use chemical tests listed in ‘Qualitative tests’ to identify unknown substances
Flame tests
- Dip the nichrome wire into concentrated hydrochloric acid
- Dip the wire into the test solid
- Hold the wire in a blue Bunsen burner flame
- Note the colour of the flame
Use chemical tests listed in ‘Qualitative tests’ to identify unknown substances
The test for ammonium ions
- Warm the solid with a solution of sodium hydroxide
- Test any gas produced with a glass rod dipped in concentrated hydrochloric acid solution or the stopper of a concentrated hydrochloric acid bottle
Use chemical tests listed in ‘Qualitative tests’ to identify unknown substances
The test for sulfate ions
- If supplied with a solid, first make a solution of the salt e.g. a spatula measure in a test tube half-filled with deionised water
- Add (1 cm^3) of barium chloride or barium nitrate solution
Use chemical tests listed in ‘Qualitative tests’ to identify unknown substances
The test for halide ions
- If supplied with a solid, first make a solution of the halide salt in dilute nitric acid
- Add (1 cm^3) silver nitrate solution
- Add (1 cm^3) dilute ammonia solution
- Add excess (5 cm^3) dilute ammonia solution
- If the precipitate remains repeat the process using concentrated ammonia solution
Use chemical tests listed in ‘Qualitative tests’ to identify unknown substances
The test for carbonate ions
• To the solid add some dilute acid. If there is effervescence, bubble any gas produced into limewater
Use chemical tests listed in ‘Qualitative tests’ to identify unknown substances
Test for unsaturation
• Add a few cm^3 of bromine water and shake
Preparation of a halogenoalkane using the techniques of refluxing, separating with a funnel, removing acidity, drying and distillation
Method for the preparation
• Place (10g) of sodium bromide in a pear shaped/round bottomed flask
• Add (7.5 cm^3) of butan-1-ol and (10 cm^3) of water
• Lower into a beaker of cold water
• Add (10 cm^3) of concentrated sulfuric acid slowly and with swirling
• Add some anti-bumping granules
• Gently reflux the mixture (for 30-45 minutes)
• Distil off the impure bromobutane
• Remove unreacted butan-1-ol by shaking with concentrated hydrochloric acid in a separating funnel
• Remove the acid impurities from the impure product as described on page 4
• Dry the impure product as described on page 4
• Purify the product using simple distillation as described on page 4
Refluxing - a labelled diagram should include
• The condenser in the upright position
• A flask
• A heat source
• The water flowing correctly through the condenser
• Use of anti-bumping granules
• Any gaps at the joints or a closed apparatus will be penalised
Distillation - a labelled diagram should include:
• The condenser in the sideward position
• A flask
• A side-arm connection/still head
• A thermometer (the bulb is opposite the exit to the condenser)
• A heat source
• The water flowing correctly through the condenser
• An appropriate collecting vessel
• Use of anti-bumping granules
• Any gaps at the joints or a closed apparatus will be penalised
Most organic compounds are flammable; instead of a Bunsen burner a water bath, sand bath or electric heating mantle can be used.
For the first distillation to separate the crude product from reaction mixture, collect the crude product distillate over a range.
For a final distillation, if the product is pure it should distil over a narrow range at the expected boiling point.
Removal of acidic impurities using a separating funnel
• Place the impure distillate in the separating funnel
• Add a portion of sodium carbonate solution/sodium hydrogencarbonate solution
• Stopper and shake
• Release the pressure at regular intervals by inverting and opening the tap
• Allow to stand until the layers settle and separate, then run off the lower layer into a beaker
• Decide which is the aqueous layer by referring to density or adding some water and observing which layer increases in size; discard the aqueous layer
Drying
• Place the impure liquid product in a beaker or conical flask
• Add a spatula of a drying agent, for example anhydrous magnesium sulfate/anhydrous sodium sulfate/anhydrous calcium chloride and swirl
• Add more of the drying agent until the liquid is clear/no longer cloudy
• Decant/filter off the liquid
