Module 1 – Development of practical skills in chemistry Flashcards
Explain the method of PAG5- Preparation of a pure organic liquid:
Purifying an organic liquid
General method:
• Put the distillate of impure product into a separating funnel
• wash product by adding either: sodium hydrogencarbonate solution , shaking and
releasing the pressure from CO2 produced or sodium chloride solution
•Allow the layers to separate in the funnel, and then run and discard the aqueous layer.
•Run the organic layer into a clean, dry conical flask and add three spatula loads of drying agent (e.g. anhydrous sodium sulphate,calcium chloride) to dry the organic liquid. When dry the organic liquid should appear clear.
• Carefully decant the liquid into the distillation flask
•Distill to collect pure product
Key points:
• Sodium hydrogencarbonate will neutralise any remaining reactant acid.
• Sodium chloride will help separate the organic layer from the aqueous layer
• The layer with lower density will be the upper layer. This is usually the organic layer
• The drying agent should be insoluble in the organic liquid and not react with the organic liquid
• Decant means carefully pour off organic liquid leaving the drying agent in the conical flask
Explain the method of distillation
In general used as separation technique to separate an organic product from its reacting mixture. Need to collect the distillate of the approximate boiling point range of the desired liquid.
Note the bulb of the thermometer should be at the T junction connecting to the condenser to measure the correct boiling point
Note the water goes in the bottom of the condenser to go against gravity. This allows more efficient cooling and prevents back flow of water.
Electric heaters are often used to heat organic chemicals. This is because organic chemicals are normally highly flammable and could set on fire with a naked flame.
Explain the method for reflux
Reflux is used when heating organic reaction mixtures for long periods. The condenser prevents organic vapours from escaping by condensing them back to liquids. The reactant vapours of volatile compound are condensed and turned to the reaction mixture.
Never seal the end of the condenser as the build up of gas pressure could cause the apparatus to explode. This is true of any apparatus where volatile liquids are heated including the distillation set up
Anti-bumping granules are added to the flask in both distillation and reflux to prevent vigorous, uneven boiling by making small bubble form instead of large bubbles
Explain the general method for titration
General Method
•rinse equipment (burette with acid, pipette with alkali, conical flask with distilled water)
•pipette 25 cm3 of alkali into conical flask
•touch surface of alkali with pipette ( to ensure correct amount is added)
•adds acid solution from burette (make sure it is rinsed with the substance inside the burette)
•make sure the jet space in the burette is filled with acid and air bubbles removed
•add a few drops of indicator (as they are weak acids and could affect results) and refer to colour change at end point
•phenolphthalein [pink (alkali) to colourless (acid): end point pink colour just disappears] [used when using strong alkalis e.g when NaOH is used]
•methyl orange [yellow (alkali) to red (acid): end point orange] [use when using string acids e.g when HCl is used]
•use a white tile underneath the flask to help observe the colour change
•add acid to alkali whilst swirling the mixture and add acid drop wise at end point. Distilled water can be added to the sides of the conical flask during titration so that all acid is washed into the reaction mixture to react with the alkali.
•note burette reading before and after addition of acid. Measurement to the nearest 0.05cm^3.
•repeats titration until at least 2 concordant results are obtained- two readings within 0.1 of each other
Key ideas:
Wear PPE (acids and alkalis are corrosive and irritants )
Explain the method for measuring mass loss in thermal decomposition
Decomposition reaction. The water of crystallisation in calcium sulphate crystals can be removed as water vapour by heating as shown in the following equation:
CaSO, xH,0(s) → CaSO,(s) + *H,O(g)
Method:
•Weigh an empty clean dry crucible and lid
• Add 2g of hydrated calcium sulphate to the crucible and weigh again
• Heat strongly with a Bunsen for a couple of minutes
• Allow to cool
•Weigh the crucible and contents again
• Heat crucible again and reweigh until you reach a constant mass (do this to ensure reaction is complete).
Key ideas:
The lid improves the accuracy of the experiment as it prevents loss of solid from the crucible but should be loose fitting to allow gas to escape/enter.
Large amounts of hydrated calcium sulphate, such as 50g, should not be used in this experiment as the decomposition is likely to be incomplete.
The crucible needs to be dry otherwise a wet crucible would give an inaccurate result. It would cause mass loss to be too large as the water would be lost when heating.
Small amounts of the solid , such as 0.100 g, should not be used in thisexperiment as the percentage uncertainties in weighing will be too high.
How would you measure gas volume with a gas syringe (using an example)
1) Extract 0.20 cm3 of propanone into a hypodermic syringe and then measure the mass of this syringe
using hand protection, remove a gas syringe from the oven and note the volume of air already in the
barrel – about 5 cm3.
2) . inject the propanone through the self-seal cap into the barrel. The plunger will move straight away.
Put the gas syringe back into the oven.
3) Measure the mass of the empty hypodermic syringe immediately.
4) After a few minutes measure the volume of the gas in the gas syringe, record the temperature of the oven shelf and the pressure of the room.
Potential errors in using a gas syringe:
•gas escapes before bung inserted
•syringe sticks
• some gases like carbon dioxide or sulphur dioxide are soluble in water so the true amount of gas is not measured.
Explain the method to create a simple standard solution
Method:
• Weigh the sample bottle containing the required mass of solid on a 2 dp balance
• Transfer to beaker
• Reweigh empty sample bottle
• Record the difference in mass
• Add 100cm3 of distilled water to the beaker. Use a glass rod to stir to help dissolve the solid.
•Sometimes the substance may not dissolve well in cold water so the beaker and its contents could be heated gently until all the solid had dissolved.
• Pour solution into a 250cm3 graduated flask via a funnel.
• Rinse beaker and funnel and add washings from the beaker and glass rod to the volumetric flask.
• make up to the mark with distilled water using a dropping pipette for last few drops.
• Invert flask several times to ensure uniform solution.
Key points:
Alternatively the known mass of solid in the weighing bottle could be transferred to beaker, washed and washings added to the beaker.
Remember to fill so the bottom of the meniscus sits on the line on the neck of the flask. With dark liquids like potassium manganate it can be difficult to see the meniscus.
Shake the volumetric flask thoroughly toensure a uniform concentration
