Simple molecular structures
5 marks · HardThe table shows data for three simple molecular substances:
| Substance | Relative molecular mass (Mr) | Melting point (°C) | Boiling point (°C) | Electrical conductivity |
|---|---|---|---|---|
| A | 32 | -210 | -183 | does not conduct |
| B | 44 | -56 | -1 | does not conduct |
| C | 160 | -7 | 59 | does not conduct |
Describe the trend shown by the melting points and boiling points as the relative molecular mass increases.
Substance C is liquid at room temperature (25°C). Explain why substance C is a liquid at this temperature but substance A is a gas.
Explain why none of these substances conduct electricity in any state.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) As relative molecular mass increases, both the melting point and boiling point increase.
(b) At 25°C, substance C is between its melting and boiling points so it is liquid. Substance A’s boiling point is much lower (–183°C), so at 25°C it is already a gas. This is because C has stronger intermolecular forces due to higher molecular mass.
(c) Simple molecular substances do not have delocalised electrons or ions that can move to conduct electricity.
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · States or clearly describes that melting (or boiling) points increase as relative molecular mass increases.
A2 (a) · 1 mark · Correctly links both melting and boiling points to increasing molecular mass or refers to both properties.
B1 (b) · 1 mark · States substance C’s melting and boiling points straddle 25°C (liquid at this temperature), substance A is gas because boiling point is below 25°C.
B2 (b) · 1 mark · Explains that substance C’s higher Mr leads to stronger intermolecular forces, so higher boiling/melting points than substance A.
C1 (c) · 1 mark · States that simple molecular substances cannot conduct electricity due to lack of mobile charged particles (no delocalised electrons or ions).
Worked solution
(a) As relative molecular mass increases from A to C, both the melting points and boiling points increase. For example, A has lowest melting/boiling points, C has highest.
(b) Substance C is a liquid at 25°C because its melting point is –7°C and its boiling point is 59°C; 25°C lies between them. Substance A boils at –183°C, so at 25°C it has already boiled and is a gas. Substance C, with greater molecular mass, has stronger intermolecular forces so its melting/boiling points are higher.
(c) None of these simple molecular substances conduct electricity because their molecules do not supply delocalised electrons or ions that can move to carry electric current.