Topic 10 · page 18 of 27
Heating under reflux and distillation
Reflux keeps a reaction mixture hot without losing anything, because the vapour condenses and runs back. Distillation does the opposite: it boils a liquid off and collects it elsewhere. The apparatus and its set-up decide which you get.
Heating under reflux 10.1910.19(i)
Many organic reactions are slow and need heating for some time. The liquids are often volatile and flammable. If you heat them in an open flask, they boil away. Heating under reflux solves that.
Points to include when you describe or draw reflux apparatus:
- A round-bottomed flask with anti-bumping granules (to give smooth boiling).
- A vertical condenser directly above the flask, open at the top. It must never be sealed: a closed system that is heated can burst.
- Water enters at the bottom of the condenser jacket and leaves at the top, so the jacket is always full and the cooling is most effective.
- A heating mantle or a water bath, not a naked flame, for flammable liquids.
The liquid boils, the vapour rises into the condenser, condenses on the cold walls and falls back. The temperature stays at the boiling temperature of the mixture. That is why reflux is used for the reactions on pages 6, 7, 17 and 22.
Distillation 10.1910.19(iii)
Distillation separates a liquid from a mixture by boiling it and collecting the condensed vapour.
- The thermometer bulb is level with the side arm of the still head, so that it reads the temperature of the vapour that is leaving.
- The condenser slopes downwards towards the receiver, and water enters at the bottom.
- The receiving flask is not sealed. For a volatile product it may stand in a beaker of ice.
- The vapour that distils at a steady temperature is the pure liquid. Collect only the fraction that boils in the expected range.
| Reflux | Distillation | |
|---|---|---|
| Condenser | vertical, above the flask | sloping downwards, away from the flask |
| Liquid | returns to the flask | is collected in a receiver |
| Thermometer | not needed | bulb at the side arm |
| Use | to carry out a reaction | to separate a product, or to purify it |
Check yourself
- State what is meant by heating under reflux, and give one reason for using it.
- Describe three features of reflux apparatus that are needed for safety or efficiency.
- State where the cooling water enters the condenser, and why.
- Where should the thermometer bulb be in a distillation, and what does it read?
- State two differences between reflux and distillation apparatus.
- A student seals the top of a reflux condenser with a bung. Explain the danger.
Answers cover these until you have tried the questions
- A reaction mixture is heated in a flask with a vertical condenser, so the vapour condenses and returns. It allows long heating without losing volatile liquids.
- Any three: vertical condenser, open at the top; water in at the bottom, out at the top; anti-bumping granules; heating by mantle or water bath, not a flame; round-bottomed flask.
- At the bottom, so that the jacket fills completely and the cooling is effective.
- Level with the side arm, so that it reads the temperature of the vapour that is passing into the condenser, which is the boiling temperature of the substance that is distilling.
- Reflux: vertical condenser, liquid returns, no receiver. Distillation: sloping condenser, liquid collected in a receiver, thermometer at the side arm.
- A closed system that is heated builds up pressure as the vapour expands, so the apparatus could burst or explode.