refluxing


Learn How to Use Reflux in Chemical Engineering and Chemistry

Heating accelerates the rate of a chemical reaction by increasing the average
kinetic energy of the molecules. The temperature of a reaction mixture can be controlled in several ways, the simplest and most convenient being to use a reaction solvent that has a boiling point within the desired temperature range for the reaction. The reaction is than conducted at the boiling point of the solvent, using a water-cooled condenser to return the solvent vapors to the reaction vessel and prevent their escape.


1. Position the heat source, usually a heating mantle or hot plate.

2. Select a round-bottomed flask of the proper size. The reactants should fill the flask half
way.

3. Clasp the flask to the heat source.

4. Add the reactants via a funnel to the flask. Mix the reactants by swirling or stirring.

5. Insert the reflux condenser into the flask and clamp it to secure.

6. Connect the water inlet (the lower connector) on the condenser jacket to a cold-water tap
with rubber tubing. Run another length of tubing from the water outlet (the upper connector) to a sink.

7. Turn on the water carefully so that the condenser jacket slowly fills with water from the
bottom up. Adjust the water pressure so that a narrow stream flows from the outlet. It is important to have a flow
rate that is great enough to maintain a continuos flow of water in spite of pressure changes in the water line.
Also make sure to keep the condenser at the temperature of the tap water during the reaction.

8. Turn on the heat source. Adjust to maintain a gentle boiling of the solvent.

9. Reflux has started when liquid begins to drip into the flask from the condenser.

10. The vapors passing into the condenser will then form a “reflux ring” of condensate
that should be clearly visible. Below this point, solvent will be seen flowing back into the flask; above it, the
condenser should be dry. If the reflux ring rises more than halfway up the condenser, the heating rate should be
reduced to prevent the escape of solvent vapors.

11. At the end of the reaction period, turn off the heat source and remove it from contact with
the flask. Let the apparatus cool, then turn off the condenser water, and pour the reactants into a suitable container.

 

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