Topic 20 · page 14 of 15

Choosing a procedure and controlling risk

Match the technique to a property of the substances, and match each control measure to a hazard. A good answer names both the procedure and the property that makes it suitable.

14 min read20.3(iii)20.3(iv)

Read first: Recrystallisation, melting and boiling temperatures

Choosing a practical procedure 20.3(iii)

Pages 11 to 13 gave you the techniques. A question may now describe a reaction and ask which procedure to use. The answer is always the same shape: name the technique and link it to a property of the substances: volatile, flammable, reacts with water, solid or liquid.

Situation Procedure Reason
slow reaction with a volatile reactant or solvent heat under reflux vapour condenses and returns, so nothing is lost
making an aldehyde from a primary alcohol warm and distil the aldehyde off as it forms it leaves before it can be oxidised to the acid
making a carboxylic acid from a primary alcohol excess oxidising agent, heat under reflux the aldehyde stays in the flask until it is oxidised
water-sensitive reagent (Grignard reagent, LiAlH₄, PCl₅, acyl chloride) dry apparatus and solvent, with a drying tube the reagent reacts with water
flammable liquid or solvent (ether, ethanol) water bath or electric heater, no naked flame the vapour could ignite
toxic or irritant vapour (HCN, bromine, ethanoic anhydride) work in a fume cupboard the vapour is not breathed in
product is a solid filter under reduced pressure, then recrystallise removes the impurities and gives pure crystals
liquid product with acid impurities wash with Na₂CO₃(aq), separate, dry, distil removes acid, then water, then other liquids
product dissolved in water solvent extraction the product is more soluble in the solvent
volatile, water-insoluble liquid that decomposes when heated to its boiling temperature steam distillation distils below 100 °C
mixture of liquids with different boiling temperatures distillation the lowest boiling temperature comes over first
exothermic reaction, or adding concentrated acid add slowly, cool in ice prevents the mixture boiling over
Two columns showing how to purify a product. A solid product: filter from the mixture, recrystallise from the minimum volume of hot solvent, filter under reduced pressure and wash with ice-cold solvent, dry, then check the melting temperature. A liquid product: separate in a separating funnel, wash with sodium carbonate solution and then water, dry with an anhydrous salt, distil and collect the fraction at the boiling temperature, then check the boiling temperature.
Purifying a solid and a liquid, step by step. Each step removes a different impurity, and each ends with a purity check.

Worked exampleChoosing the sequence for an ester

Ethyl ethanoate is made from ethanol, ethanoic acid and a little concentrated sulfuric acid. Ethyl ethanoate boils at 77 °C. Suggest the procedure to make it and purify it.

  1. Heat under reflux. The reaction is slow and the reactants are volatile and flammable, so use a heating mantle or water bath and a vertical condenser.
  2. Distil the mixture and collect the distillate that comes over near 77 °C. This separates the ester from the sulfuric acid and other less volatile material.
  3. Wash the distillate with sodium carbonate solution in a separating funnel to remove acids. Then wash with water.
  4. Dry the ester with anhydrous calcium chloride or magnesium sulfate until it is clear and the solid no longer clumps.
  5. Distil again, collecting the fraction over a narrow range around 77 °C, and check that the boiling temperature matches the data book.

Hazards and control measures 20.3(iv)

A hazard is a property of a substance that can cause harm, such as being flammable, corrosive or toxic. Risk is the chance that the harm happens and how serious it would be. A control measure cannot change a hazard, but it can reduce the risk by lowering the exposure.

Substance Hazard Control measure
ethoxyethane (ether) highly flammable; vapour causes drowsiness no naked flames: heat with a water bath or electric heater; fume cupboard
ethanol flammable no naked flames; water bath
ethanoic anhydride flammable; corrosive; harmful vapour fume cupboard; gloves and eye protection
concentrated H₃PO₄ or H₂SO₄ corrosive gloves and goggles; add a few drops at a time; wash any splashes with plenty of water
bromoethane harmful; volatile fume cupboard; keep the container stoppered
potassium cyanide very toxic; releases HCN gas with acid fume cupboard; gloves; keep it away from acids
LiAlH₄ reacts violently with water; flammable dry apparatus and solvent; no water nearby
acidified potassium dichromate toxic; carcinogenic; oxidising gloves; avoid skin contact; wash spills at once
Hazard Typical control
flammable no naked flame; water bath, electric heater or mantle; keep away from ignition sources
toxic or harmful vapour fume cupboard; use small amounts
corrosive gloves and eye protection; add slowly; dilute where possible
reacts with water dry apparatus; store and use away from water
oxidising keep away from flammable material

Worked exampleControls for making a Grignard reagent

Hazard data given. Ethoxyethane is highly flammable and its vapour is heavier than air. Bromoethane is harmful and volatile. Magnesium turnings are a flammable solid. The Grignard reagent reacts with water.

  1. Flammable ether: use a water bath or electric heater and no naked flames. Reflux with a condenser so ether vapour is condensed and not released.
  2. Volatile, harmful bromoethane: work in a fume cupboard with a stoppered container.
  3. Reaction with water: use dry apparatus and ether, with a drying tube, so the reagent is not destroyed.
  4. Contact with skin or eyes: wear gloves and eye protection.

Check yourself

  1. Name a suitable procedure for each, with a reason: (a) heating ethanol and ethanoic acid for an hour, (b) separating propanal from a hot mixture of propan-1-ol and acid, (c) drying a crude liquid.
  2. Bromine is toxic and corrosive, and its vapour is harmful. Give two control measures and match each to a hazard.
  3. State the difference between a hazard and a risk.
  4. Explain why ethoxyethane should be heated in a water bath and not with a Bunsen burner.
  5. A student plans to make propanal by heating propan-1-ol under reflux with acidified dichromate. Explain why this will not work well and what to do instead.
  6. A solid product is mixed with a small amount of a soluble impurity. Give the sequence of procedures to obtain a pure sample and show how you would check its purity.

Answers cover these until you have tried the questions

  1. (a) Reflux, because the reactants and the ester are volatile and flammable, so the vapour must be condensed and returned. (b) Distillation: propanal has the lowest boiling temperature and is removed before it can be oxidised. (c) Add an anhydrous salt, such as magnesium sulfate, which takes up the water; stop when the solid no longer clumps.
  2. Fume cupboard, because the vapour is toxic and harmful. Gloves and eye protection, because the liquid is corrosive and would burn the skin and eyes.
  3. A hazard is something that can cause harm, such as being toxic. Risk is the chance of that harm happening, and how serious it would be. Control measures reduce risk, not the hazard.
  4. Ether is highly flammable, and its vapour can ignite from a naked flame, even from a distance. A water bath cannot get hot enough to ignite it.
  5. Under reflux the aldehyde stays in the hot oxidising mixture and is oxidised to propanoic acid. Instead, warm gently and distil the propanal off as it forms, so that it leaves the oxidising agent.
  6. Recrystallise from the minimum volume of hot solvent, filter hot if needed, cool, filter under reduced pressure, wash with a little ice-cold solvent, and dry. To check purity, measure the melting temperature: a sharp range that matches the data book shows the solid is pure.