A reflux setup Figure 1. A reflux setup is analogous to a distillation, with the main difference being the vertical placement of the condenser. The liquid remains at the boiling point of the solvent or solution during active reflux.
A reflux apparatus allows for facile heating of a solution, but without the loss of solvent that would result from heating in an open vessel. In a reflux setup, solvent vapors are trapped by the condenser, and the concentration of reactants remains constant throughout the process.
The main purpose of refluxing a solution is to heat a solution in a controlled manner at a constant temperature. But often the boiling point of the solvent is chosen as the reaction temperature because of its practicality.
Figure 1. Do not turn off the water flowing through the condenser until the solution is only warm to the touch. After a few minutes of air cooling, the round bottomed flask can be immersed in a tap water bath to accelerate the cooling process Figure 1. Use an extension clamp on the round bottomed flask to connect to the ring stand or latticework. Attach the condenser, and connect the hoses so that water travels against gravity cooling water comes into the bottom and drains out the top.
Be sure there is a secure connection between the round bottomed flask and condenser, as vapors escaping this joint have the potential to catch on fire. Circulate water through the condenser, then begin heating the flask by using a heating mantle, sand, water, or oil bath.
Use an adjustable platform so the heat can be lowered and removed at the end of the reflux, or if something unexpected occurs. Heat so that the "reflux ring" is seen in the lower third of the condenser.
Turn down the heat if the refluxing vapors reach higher than halfway up the condenser. At the end of the reflux period, lower the heat source from the flask or raise the apparatus. Keep circulating water in the condenser until the flask is just warm to the touch.
After air cooling somewhat, the flask can be quickly cooled by immersing in a container of tap water. Likewise, in a reaction which calls for reflux, you bring the temperature to a high value first, to get the solvent boiling. Then you lower the temperature to the minimum value that will sustain the boiling process.
An indication that you are about to form a reflux ring is the formation of liquid droplets above the boiling liquid that look like sweat. The point at which the rate of evaporation equals the rate of condensation determines is the ideal rate of heating. To know that boiling point has been reached, bubbles of vapour are produced inside the liquid. If the heating rate is turned up, the temperature of the reactants does not change, but the rate at which the boiling liquid changes into vapour form increases.
This increase is caused by the increased supply of energy that facilitates more liquid molecules into overcoming their intermolecular interactions to enter the gas phase.
When a mixture of two or more volatile compounds is heated, the total vapour pressure PT of the mixture equals the sum of the vapour pressures of compound 1 and 2 P1 and P2 in the mixture. The magnitude of the vapour pressure exerted by each compound is determined by the vapour pressure of that compound P0 and the mole fractions of both compounds 1 and 2 present in the mixture X1 and X2.
Depending on the mixture, the temperature boils variably. Homogenous mixtures boil at a temperature between the boiling points of the pure compounds, but the exact value depends on the amount mass or volume of each compounds. For example, a liquid mixture when boiled will yield a vapour that will contain a larger percent of the more volatile compound. When it comes to separating compounds a common method that is used in organic chemistry is distillation which separates compounds based on differences in boiling points.
In more sophisticated experiments, reflux and distillation can be carried out at the same time. For example, while a reaction is refluxing, a microscale distillation can be carried out using specialized equipment. Microscale distillation is to shorten the distillation path to reduce the chance for material loss in the process. Sidenote: The vapours of the heated liquid rise upward and cool to condense on either the inside walls of the Hickman head or on the walls of the condenser.
The conical vial should be securely attached to the Hickman distillation head and the air condenser using the compression cap and the metal clamp. All pieces should have a ground-glass joint and fit together well such that no major leaks occur. The spin vane should be placed in the conical vial and be pointed downward.
The flat septum and the small compression cap are used to close the side port of the Hickman head. The entire setup is placed in the appropriate hole in the Al-block or sand bath and centered on the hotplate before starting to stir otherwise the spin vane will flip over and rotate improperly. The Hickman head and the air condenser have to be cooled with a wet paper towel. The heat source should be set so that the compound of interest distils over slowly.
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