QUESTION 3 [14 marks]

Question

QUESTION 3.1
The boiling points of compounds A to D are given in Table 1 below.
Table 1: Boiling points of compounds A to D.
Question 3.1.1 [2 marks]
Define the term boiling point.
Question 3.1.2 [1 mark]
Calculate the molecular mass of compound D.
Question 3.1.3 [1 mark]
In what phase is compound D at 100 °C?
Question 3.1.4 [1 mark]
Name the main type of intermolecular force present in compound A.
12
Compound Molecular formula Boiling point
A C2H4
-104 °C
B C3H6
-47 °C
C C6H12 63 °C
D C8H16 121 °C
Question 3.1.5 [4 marks]
Explain why the boiling point increases from compound A to D.
Question 3.1.6 [1 mark]
How does the vapour pressure of compound B compare to the vapour pressure
of compound C? Write down only HIGHER THAN, LOWER THAN, or EQUAL TO.
QUESTION 3.2
Consider the boiling points of the compounds, given in Table 2 below.
Table 2: Boiling point of compounds A to D
Question 3.2.1 [1 mark]
Which ONE of the substances in the table above has the strongest forces
between its molecules?
Question 3.2.2 [1 mark]
Name the type of intermolecular force found between H2 molecules.
13
Compound Molecular formula Boiling point
A H2
-252,9 °C
B HCN 25.6°C
C F2
-188 °C
D Cℓ2
-34.04 °C

Answer 1

QUESTION 3 [14 marks]

QUESTION 3.1

Question 3.1.1 [2 marks]
Define the term boiling point.
Answer: The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure surrounding the liquid, allowing it to transition from the liquid phase to the gas phase.

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Question 3.1.2 [1 mark]
Calculate the molecular mass of compound D (C8H16).
Answer:

• Molecular mass of Carbon (C) = 12.01 g/mol
• Molecular mass of Hydrogen (H) = 1.008 g/mol

Molecular mass of C8H16 = (8 x 12.01) + (16 x 1.008) = 96.08 + 16.128 = 112.208 g/mol
Therefore, the molecular mass of compound D is approximately 112.21 g/mol.

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Question 3.1.3 [1 mark]
In what phase is compound D at 100 °C?
Answer: At 100 °C, compound D (C8H16) is in the liquid phase since its boiling point is 121 °C.

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Question 3.1.4 [1 mark]
Name the main type of intermolecular force present in compound A.
Answer: The main type of intermolecular force in compound A (C2H4) is dispersion forces (London dispersion forces), which are typical for nonpolar molecules.

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Question 3.1.5 [4 marks]
Explain why the boiling point increases from compound A to D.
Answer: The boiling point increases from compound A to D due to the following reasons:

1. Molecular Size: As the number of carbon and hydrogen atoms increases from A to D, the molecular size and mass increase, leading to greater van der Waals forces (dispersion forces).
2. Surface Area: Larger molecules have a greater surface area, which allows for more significant interactions between molecules, resulting in higher energy input (heat) required to break these interactions.
3. Molecular Weight: Heavier molecules generally have higher boiling points because they require more energy to overcome the intermolecular forces holding them together.
4. Chain Length: Longer carbon chains create stronger London dispersion forces due to greater surface contact between molecules, further increasing the boiling point.

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Question 3.1.6 [1 mark]
How does the vapor pressure of compound B compare to the vapor pressure of compound C?
Answer: The vapor pressure of compound B (C3H6) is likely HIGHER THAN that of compound C (C6H12) because compound B has a lower molecular weight and weaker intermolecular forces compared to compound C, leading to a higher vapor pressure.

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QUESTION 3.2

Question 3.2.1 [1 mark]
Which ONE of the substances in the table above has the strongest forces between its molecules?
Answer: B (HCN) has the strongest forces between its molecules due to the presence of hydrogen bonding.

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Question 3.2.2 [1 mark]
Name the type of intermolecular force found between H2 molecules.
Answer: The type of intermolecular force found between H2 molecules is London dispersion forces (van der Waals forces).