Quick answer: Molecular shape follows VSEPR theory: electron domains around the central atom repel and spread as far apart as possible, with lone pairs repelling more than bonding pairs. This Chemistry 11-12 lab on VSEPR theory gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Can you tell whether a liquid is polar without any instrument but a plastic rod? In this lab you bend thin streams of five liquids with a charged rod and link the size of the deflection to molecular shape and bond polarity.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB S2.2
Aim
To compare the polarity of five liquids by measuring how far a thin stream of each is bent by a charged rod.
Equipment
- Burette (50 cm³) with a clamp and stand
- Beaker to catch the liquid
- Plastic rod and a dry cloth
- Ruler
- Water, ethanol, propanone, hexane and cyclohexane, 15 cm³ of each
- Safety goggles
Safety
Propanone, hexane, cyclohexane and ethanol are highly flammable and their vapours are harmful: no flames or sparks, work in a ventilated area, wear goggles and keep the containers closed. Do not pour liquids down the sink.
Method
- Clamp the burette above the beaker and fill it with the first liquid.
- Rub the plastic rod hard with the cloth to charge it.
- Open the tap to give a thin, steady stream and hold the rod beside the stream about 3 cm below the tap.
- Measure with the ruler how far the stream moves sideways, d, in cm. Repeat three times.
- Rinse the burette with a little of the next liquid and repeat for each liquid.
Results
Kofi in Accra recorded the sideways movement d of the stream (cm).
| Liquid | Trial 1 | Trial 2 | Trial 3 | Mean d / cm |
|---|---|---|---|---|
| Water | 1.4 | 1.5 | 1.6 | 1.50 |
| Propanone | 1.5 | 1.7 | 1.6 | 1.60 |
| Ethanol | 1.0 | 1.2 | 1.1 | 1.10 |
| Hexane | 0.0 | 0.0 | 0.1 | 0.03 |
| Cyclohexane | 0.0 | 0.1 | 0.0 | 0.03 |
Analysis questions
Question 1 Mean for water
Calculate the mean deflection of water and the range of its three results.Question 2 Classifying the liquids
Use 0.5 cm as the dividing line. Classify each liquid as polar or non-polar from the means.Question 3 Why a polar stream is deflected
Explain, using partial charges, why a stream of polar molecules is attracted toward a charged rod.Question 4 Shape of water
Draw (describe) the shape of the H₂O molecule and explain why it is polar.Question 5 Hexane
Hexane has C–H bonds that are slightly polar. Explain why hexane is not deflected.Question 6 Ethanol compared with water
Ethanol is deflected less than water. Suggest two reasons, and say how far the data justify a ranking.Question 7 Controlling variables
State three variables that must be kept constant for a fair comparison.Question 8 Improvement
Suggest one improvement that would make the measurement of d more reliable.
