The method and the results are on the VSEPR theory lab. Do the analysis yourself first, then check the answers below.
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. These answers go with the Chemistry 11-12 lab on VSEPR theory: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.
The model answers treat deflection above 0.5 cm as evidence of polarity. Remember that this is a qualitative test, so link the results to shape and dipoles, and comment on the uncertainty.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB S2.2
Mean = (1.4 + 1.5 + 1.6) ÷ 3 = 1.5 cm. Range = 1.6 − 1.4 = 0.2 cm, so the result is 1.5 ± 0.1 cm.
Question 2 Classifying the liquids
Water (1.5), propanone (1.60) and ethanol (1.10) are above 0.5 cm: polar. Hexane (0.03) and cyclohexane (0.03) are below: non-polar.
Question 3 Why a polar stream is deflected
Each polar molecule has a δ+ end and a δ− end. A charged rod attracts the end of opposite charge more strongly than it repels the near end, because that end is closer. So the molecules turn until the opposite charge faces the rod. As a result, the rod pulls the stream toward it.
Question 4 Shape of water
Oxygen has two bonding pairs and two lone pairs, so the molecule is bent (about 104.5°). The O–H bonds are polar (3.4 against 2.2). Because of the bend, their dipoles do not cancel, so there is a net dipole toward oxygen.
Question 5 Hexane
The C–H bonds have a very small difference in electronegativity (0.4). Also, no atom in the molecule is much more electronegative. Any small dipoles spread over the chain and nearly cancel. Therefore the net dipole is about zero and the stream is not deflected.
Question 6 Ethanol compared with water
Ethanol has a non-polar ethyl group that contributes little to the dipole. Its stream also differs from water in viscosity and surface tension. With only three trials and a ±0.1 cm spread, you can justify only a rough ranking (polar above non-polar), not a precise one.
Question 7 Controlling variables
Keep these the same: the burette tap opening (flow rate), the distance of the rod from the stream, the height below the tap and the charging of the rod (same number of rubs). Also keep the temperature the same.
Question 8 Improvement
Fix the rod in a clamp at a measured distance. Then photograph the stream against a ruler, so you read the deflection from the image instead of by eye.
