Reactions of the halogens
3 marks · NormalThe melting and boiling points of the halogens increase down the group.
Chlorine is a gas at room temperature (25 B0C) and boils at 76 B0C. Bromine is a liquid at room temperature and boils at 59 B0C. Iodine is a solid at room temperature and sublimes at 84 B0C.
Use the information above to state the physical state (solid, liquid or gas) of each halogen at room temperature (7F25 B0C).
Explain, using the relative molecular mass and forces present, why the melting and boiling points increase from chlorine to iodine.
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) Chlorine: gas; Bromine: liquid; Iodine: solid · must show All three states correctly matched to the element
(b) Melting and boiling points increase because the relative molecular mass increases down the group, leading to stronger intermolecular forces that require more energy to overcome. · must show Reference to relative molecular mass, Reference to intermolecular forces, Explanation that more energy is needed
Common mistakes: Stating iodine is a gas at room temperature; Mixing up melting and boiling points
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correct state for chlorine (gas at room temp)
B2 (a) · 1 mark · Correct states for bromine (liquid) and iodine (solid) at room temperature
B3 (b) · 1 mark · Any valid link between increasing relative molecular mass, stronger intermolecular forces, and higher melting/boiling points (must relate both molecular mass and forces)
Worked solution
a) Chlorine: gas at room temperature (boiling point -34 B0C, so it is gaseous at 25 B0C).
Bromine: liquid at room temperature (melting point -7 B0C, boiling point 59 B0C).
Iodine: solid at room temperature (sublimation point 184 B0C, so remains solid at 25 B0C).
b) Down group 7, the relative molecular mass of the halogens increases. This results in stronger intermolecular forces (van der Waals forces) between molecules, so more energy is needed to overcome these forces, raising the melting and boiling points.