A Level Chemistry 2017 Paper 1 Question 4 Solution

In this video we want to discuss 2017 A Level H2 Chemistry Paper 1 Question 4.

Let's take a look at this question.

2017 A Level H2 Chemistry Paper 1 Qn4 Question

Topic tested is on Intermolecular Forces between Simple Covalent Molecules.

Let's go through each statement to see if it can explain why boiling point of water is much higher.

Statement 1


2017 A Level H2 Chemistry Paper 1 Qn4 Statement 1

Water is a polar molecule with O-H bonds so can form stronger Hydrogen Bonds between molecules, while methane CH4 is non-polar with much weaker instantaneous dipole-induced dipole attractions or dispersion forces or Van Der Waals forces between molecules.

Therefore much more energy is required to overcome the intermolecular forces of attraction between water molecules, which result in a much higher boiling point for water than methane.

Hence Statement 1 is a valid explanation.

Statement 2

2017 A Level H2 Chemistry Paper 1 Qn4 Statement 2


The strength of covalent bonds in water and methane is irrelevant as boiling is a physical process so does not involving breaking of the O-H or C-H bonds.

Hence Statement 2 is not a valid explanation.

Statement 3

2017 A Level H2 Chemistry Paper 1 Qn4 Statement 3

The number of electrons will affect the electron cloud size and polarisability of the molecule which might have a consequence on instantaneous dipole-induced dipole attraction (or Van der Waals forces), so it'll be good to compare the number of electrons between water and methane.

However both molecules contain the same number of electrons which means that the id-id attractions will be similar.

Hence Statement 3 will not be able to explain the difference in boiling point and is not valid.

Finally we can conclude that only Statement 1 is valid hence answer to this question is C.

Check out this video for the question and the suggested solution!

Topic: Intermolecular Forces, Physical Chemistry, A Level Chemistry, Singapore

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