Polarity of molecules

Introduction

The traditional practical way to show whether the molecules of a substance are polar was to put the liquid in a burette and use a charged rod to see if it would deflect the liquid as it ran out of the burette.

A much neater way is to use an ordinary household microwave oven. The first microwave oven was invented by Percy Spencer during the Second World War. Microwave ovens work by creating oscillating electromagnetic radiation in the microwave region. Polar molecules heat up as they are constantly trying to re-arrange their polarity to line up with the oscillating field. Non-polar molecules are unaffected by the microwave radiation and so no temperature change occurs. This fact can be utilised easily in the lab and can be extended to look at the amount of polarity possessed by a substance. The experiment can be demonstrated by the teacher but is better done by students themselves.

Teacher’s notes

Any microwave oven will do but it is probably best to experiment beforehand with your particular oven to determine the time to heat about 10 cm3 of water in a beaker by about 40 oC as the power output of the microwave oven can vary. The best non-polar liquid to use is tetrachloromethane but this is not available in many schools and if you do use it then the experiment should be performed in a fume cupboard. Other non-polar liquids (or nearly non-polar liquids) that could be used are for example, cyclohexane or methylbenzene. Unlike tetrachloromethane these are flammable so care should be taken. I have done the experiment several times with cyclohexane and not experienced any problems but it is not clear that there is absolutely no risk. Be very careful with the ethanol experiment. Students probably will think that as ethanol is less polar than water it will not heat up as much. In fact it will rapidly boil as the specific heat capacity of ethanol is much lower (2.44 J K-1 g-1) than the specific heat capacity of water (4.18 J K-1 g-1). If you have some different alcohols such as propan-1-ol and butan-1ol then students can experiment to see how much the temperature increases and try to relate this to the expected polarity and specific heat changes.

smiley laugh Student worksheet

POLARITY OF MOLECULES

This experiment uses an ordinary household microwave oven to explore the polarity of several different substances. A microwave oven works by producing a rapidly oscillating field of electromagnetic radiation in the microwave region. Polar molecules continually try to align themselves with the field with the result that they are rapidly rotating. This kinetic energy results in the temperature of the substance increasing.

SAFETY:

Although microwaves are dangerous the oven only works when the door is closed. Some of the substances you use are flammable so keep them away from sparks and naked flames. Certain substances are poisonous and should only be used in a fume cupboard. Take advice from your teacher on this.

ENVIRONMENTAL CARE:

Since no reactions are carried out, substances can be returned to their original bottle at no cost to the environment. This means that you must be careful that they are not contaminated in any way before they are put back into the original bottle. If in doubt then place them in the ‘waste organic’ container.

PROCEDURE:

1. Place 10.0 cm3 of water into a 100 cm3 open beaker. Record the temperature of the water. Put the beaker in the centre of the rotating plate in the microwave oven without the thermometer. Switch the oven on for a short time (e.g. 10 seconds). Take the temperature of the water as soon as you have removed it from the microwave oven. Experiment with the time to find out how long it requires to raise the temperature by about 40 oC.

2. Place 10.0 cm3 of the non-polar liquid you have been provided with in a 100 cm3 beaker. Switch the oven on for exactly the same amount of time that you determined in 1. Record the temperature after you have removed the beaker and its contents from the oven.

3. Repeat 2. with 10.0 cm3 of ethanol. Be very careful as you may be surprised by what happens.

4. Repeat with 10.0 cm3 of other alcohols that you have been provided with. Try to predict what will happen before you do each experiment.

QUESTIONS:

1. Explain why the temperature of water rises rapidly after only a short burst of microwaves whereas there is no change in temperature when the non-polar substance undergoes the same treatment.

2. Suggest a reason why ice is slow to heat up in a microwave oven.

3. Compare the polarity of ethanol with that of water. Would you expect ethanol to heat up more, or less, or the same as water under the same conditions? Suggest a reason for the result you obtained with ethanol.

4. If you are able to repeat the experiment with other alcohols try to explain the results you achieved.

This worksheet can also be downloaded from:

  Polarity of molecules

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