The questions are on the intermolecular forces test. Take it under real conditions before reading the mark scheme.
Quick answer: Intermolecular forces are the attractions between molecules: London forces act between all particles and grow with the number of electrons, permanent dipole–dipole forces act between polar molecules, and hydrogen bonds act when H bonded to N, O or F meets a lone pair. This mark scheme gives the model answers and the marks for the 8 questions of the Chemistry 11-12 test on intermolecular forces, from boiling bromine to pentane and butan-1-ol, with the equivalent answers to accept.
Marks are given for naming the correct force and for linking it to the boiling point. Answers that say covalent bonds break on boiling lose the relevant mark. Accept equivalent wording.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB S2.2
Question 1 Boiling bromine (2 marks)
London forces are broken. The Br–Br covalent bond is within the molecule and is far stronger, so the molecules remain intact as Br₂.Marking: 1 for London forces; 1 for covalent bonds not broken.
Question 2 Krypton and argon (3 marks)
Krypton has more electrons (36 against 18), so its electron cloud is larger and more easily distorted. Instantaneous dipoles are larger, so the London forces are stronger and more energy is needed to separate the atoms.Marking: 1 for more electrons; 1 for larger instantaneous dipoles; 1 for stronger forces needing more energy.
Question 3 Alkanes and alcohols (4 marks)
a. Alcohols boil higher than alkanes with the same number of carbons.b. Alcohols have O–H groups and form hydrogen bonds, which are stronger than the London forces between alkane molecules.
c. As the chain lengthens the London forces become a larger part of the total attraction, so the contribution of hydrogen bonding becomes relatively smaller.
Marking: (a) 1; (b) 1 for hydrogen bonding, 1 for stronger than London; (c) 1.
Question 4 Propanol and methoxyethane (2 marks)
Propan-1-ol has an O–H group and forms hydrogen bonds. Methoxyethane has no H bonded to oxygen and only dipole–dipole and London forces, which are weaker.Marking: 1 for hydrogen bonds in propan-1-ol; 1 for none in methoxyethane.
Question 5 Hydrogen bond in water (2 marks)
The hydrogen atom of one molecule is δ+ because oxygen is highly electronegative. It is attracted to a lone pair on the oxygen of a neighbouring molecule, with the three atoms in a nearly straight line.Marking: 1 for δ+ H bonded to O; 1 for attraction to a lone pair on a neighbour.
Question 6 Energy to vaporise water (2 marks)
n = 36.0 ÷ 18.0 = 2.00 mol. Energy = 2.00 × 40.7 = 81.4 kJ.Marking: 1 for n; 1 for the energy with unit.
Question 7 Why ice floats (3 marks)
In ice each water molecule forms four hydrogen bonds in a regular open lattice. The hydrogen bonds hold the molecules at fixed distances and leave empty spaces. In the liquid the molecules are closer together on average, so ice is less dense.Marking: 1 for four hydrogen bonds; 1 for open lattice or spaces; 1 for molecules further apart than in the liquid.
Question 8 Pentane and butan-1-ol (2 marks)
Pentane has only London forces. Butan-1-ol also forms hydrogen bonds, which are much stronger, so more energy is needed to separate its molecules.Marking: 1 for London only in pentane; 1 for hydrogen bonds in butan-1-ol.
