Simple molecular structures
4 marks · NormalAnalyze the data in the table below and answer the questions.
| Halogen | Formula | Relative molecular mass (Mr) | Melting point (°C) | Boiling point (°C) |
|---|---|---|---|---|
| Fluorine | F2 | 38 | –220 | –188 |
| Chlorine | Cl2 | 71 | –101 | –34 |
| Bromine | Br2 | 160 | –7 | 59 |
| Iodine | I2 | 254 | 114 | 184 |
Describe the trend in boiling points as you go down the group from fluorine to iodine.
Explain this trend in terms of molecular structure and forces.
Which halogen is a solid at room temperature (25°C)? Explain how you decided.
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.
Show answer
(a) Boiling point increases down the group. · must show boiling point increases
(b) Relative molecular mass increases, so intermolecular forces get stronger, requiring more energy to boil. · must show increase in Mr means stronger intermolecular forces, more energy needed to separate molecules
(c) Iodine, because its melting point (114°C) is much higher than 25°C. · must show iodine is solid at 25°C because melting point > room temperature
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Trend correctly stated: boiling point increases down the group.
B2 (b) · 1 mark · Relative molecular mass increases down the group.
B3 (b) · 1 mark · Stronger intermolecular forces require more energy to boil.
B4 (c) · 1 mark · Correct identification of iodine as a solid at room temperature, explained using melting point higher than 25°C.
Worked solution
a) The boiling point increases from fluorine to iodine.
b) As you go down the group, the relative molecular mass increases, causing intermolecular forces between molecules to become stronger. This requires more energy (higher temperature) to separate the molecules and change state.
c) Iodine is a solid at 25°C because its melting point (114°C) is much higher than room temperature.