Quick answer: A Lewis structure shows how valence electrons are shared as bonds and lone pairs, and formal charges help choose the best one. This AP Chemistry lab on vsepr theory gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
A stream of a polar liquid bends toward a charged rod, while a nonpolar liquid does not. In this lab you test five liquids and connect the result to molecular shape and polarity.
Level: AP Chemistry · Curriculum: AP Chem Unit 2
Aim
To test whether liquids are polar by observing how a stream of each liquid responds to a charged rod, and to relate the result to molecular shape.
Equipment
- Burette (or a plastic pipette) on a stand
- Plastic rod and cloth
- Small beakers and a ruler
- Water, ethanol, propanone, hexane and cyclohexane (about 20 mL of each)
Safety
Ethanol, propanone, hexane and cyclohexane are flammable: no flames in the room. Wear goggles, work in a ventilated space and keep the quantities small. Dispose of waste liquids as instructed.
Method
- Fill the burette with the first liquid and let it run in a thin continuous stream.
- Rub the plastic rod with the cloth.
- Hold the rod 2 cm from the stream, at the height 10 cm below the tap.
- Measure the sideways deflection of the stream with the ruler.
- Repeat three times, then test the other liquids.
Results
| Liquid | Deflection (mm) | Dipole moment (D, reference) |
|---|---|---|
| Water | 14 | 1.85 |
| Propanone | 16 | 2.88 |
| Ethanol | 11 | 1.69 |
| Hexane | 0 | 0 |
| Cyclohexane | 0 | 0 |
Analysis questions
Question 1 Pattern
Describe the pattern in the table between the deflection and the dipole moment.Question 2 Why a stream bends
Explain why a charged rod attracts a stream of a polar liquid whatever the sign of the rod’s charge.Question 3 Nonpolar liquids
Explain why hexane shows no deflection, using its structure.Question 4 Shape of water
Draw the Lewis structure of water and explain why the bent shape makes the molecule polar.Question 5 Propanone
The structure of propanone is (CH₃)₂C=O. State the shape around the carbonyl carbon and explain why the molecule is polar.Question 6 Reliability
Suggest two sources of error in measuring the deflection and one way to reduce each.Question 7 Limits of the test
Ethanol gives a smaller deflection than water despite being polar. Suggest two reasons the deflection is not a perfect measure of the dipole moment.
