Chemistry & reflux

Written by stan aberdeen
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Chemistry & reflux
An example of reflux reaction set-up. (petroleum distilation image by Heng kong Chen from Fotolia.com)

Reflux facilitates chemical reactions that take a long time. Evaporation, condensation and solvent recycling form the basis of reflux set-up. Reactions done through reflux can be done for an extended time (hours, days) since solvent is recycled. Reflux also allows temperature control via solvent boiling point. The cooling element that condenses evaporated solvent needs to remain functional, otherwise solvent simply evaporates away.

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Laboratory Setup

The solvent heats until it evaporates and travels up to the reflux condenser. Solvent vapour does not escape. It encounters a cooled surface on which to condense. The liquid trickles back to the original boiling solution.

Benefits

With a constant heat source, the condensate recycles into a solution at a constant temperature. Temperature control at solvent boiling point is a major reason for reflux set-up. The evaporation-condensation cycle means that, as long as the heat source is working properly, reflux reactions can go on for a long time--hours, even days.

Safety

Reflux heating is done in an open container. Gases do not build up to dangerous pressures. Upon finishing a reflux reaction, slowly cooling the flask is important. Swift cooling may cause the hot glass flask to shatter. Make sure to keep cold surfaces and liquids away from the hot flask for some time after reflux.

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