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Question 23M.2.HL.TZ2.5

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Date May 2023 Marks available [Maximum mark: 6] Reference code 23M.2.HL.TZ2.5
Level HL Paper 2 Time zone TZ2
Command term Deduce, Explain Question number 5 Adapted from N/A
5.
[Maximum mark: 6]
23M.2.HL.TZ2.5

The table shows thermal properties of water and methane.

Property Freezing
point / °C		 Boiling
point / °C		 Heat capacity / J g-1 °C-1 Heat of
vaporization / J g-1
Water (H2O) 0 100 4.2 3357
Methane (CH4) -182 -160 2.2 760
(a)

Water molecules are polar and methane molecules are non-polar. Explain how this difference affects the thermal properties of these substances.

[2]

Markscheme
  1. water forms hydrogen bonds but methane does not/hydrogen bonds form between water molecules, but are absent in methane;
  2. energy needed to break hydrogen bonds/intermolecular attractions;
  3. hydrogen bonds raise the freezing point/boiling point/heat capacity/heat of vaporization;

Mpa: A clear difference between the 2 substances is expected.

Mpc: Do not accept “water has a high boiling point”, etc. if no reference is made to hydrogen bonds.

Examiners report

SL:

This question proved to be a good discriminator. Surprisingly few could describe Hydrogen bonds between the water molecules (and they should also have stated that they do not exist between methane molecules).

HL:

There were many answers where candidates failed to indicate the contrast between water and methane and just explained about water molecules. Hydrogen bonds were not always mentioned.

(b)

Using the data in the table, deduce the reasons for methane being a gas on Earth.

[2]

Markscheme
  1. boiling point of methane is −160 °C
    OR 
    methane is in gaseous state when temperatures are above/higher than −160 °C;
  2. temperatures on Earth are always above −160 °C;

Mpb: accept reference to Earth average temperature being warmer / higher than methane boiling point.

Examiners report

SL:

Many gained both marks in (b), for stating that nowhere on Earth is colder than −160ºC, so methane is always going to be above its boiling point.

HL:

Some answers were rather vague, with limited reference to Earth's mean temperatures being above −160ºC, however, many candidates were able to indicate that methane would be in a gaseous state whenever temperatures are above −160ºC (methane's boiling point).

(c)

Water is used as a coolant in sweat. Using the data in the table, explain the reasons for methane not being as suitable as water for use as a coolant.

[2]

Markscheme
  1. heat of vaporization is low/heat of vaporization is only 760 J g−1
    OR
    methane has a lower heat of vaporization compared to water;
  2. no hydrogen bonds that need to be broken;
  3. not enough heat removed when methane evaporates;
  4. methane boils at −160 °C so would already be a gas (in/on the human body);

Mpa: the second statement aims at the idea of a comparison

Mpa: accept vice versa.

If methane is not referred to directly in the answer, then award [1 max]

Examiners report

SL:

A discussion in terms of heat of vaporization was looked for in part (c).

HL:

There were some answers obtaining 2 marking points. In many cases, though, it was observed that candidates tended to confuse heat capacity with heat of vaporization, and not relating sweating with vaporization. In many cases no reference to methane was made or to the lack of hydrogen bonds.