Edexcel IGCSE Chemistry · Spec 2.5-2.8C
Group 7: The Halogens & Displacement
The properties and trends of the Group 7 halogens, and displacement reactions.
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Group 7: The Halogens & Displacement
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Explained
The halogens, and which displaces which
Group 7 are the halogens: coloured, toxic non metals that exist as diatomic molecules, meaning two atoms joined by a covalent bond. Each has seven electrons in its outer shell, one short of full, and gaining one to form a 1 minus ion is the whole of their chemistry.
Appearance at room temperature
- Fluorine: a pale yellow gas.
- Chlorine: a pale green gas.
- Bromine: a brown liquid, sometimes described as orange brown.
- Iodine: a dark grey solid, which gives a purple vapour when warmed.
Two trends run down the group at once: the colour gets darker, and the state changes from gas to liquid to solid, because the melting and boiling points rise as the molecules get larger and the intermolecular forces between them get stronger.
Reactivity decreases down the group
This is the opposite of Group 1, and for a matching reason.
A halogen reacts by gaining an electron. Further down the group the outer shell is further from the nucleus and is shielded by more inner shells, so an incoming electron is attracted less strongly and is gained less easily. Fluorine is therefore the most reactive and iodine the least.
Displacement
A more reactive halogen displaces a less reactive one from a solution of its salt.
Chlorine added to sodium bromide solution displaces bromine, and the colourless solution turns orange. Bromine added to sodium chloride solution does nothing at all, because bromine is below chlorine.
The colour change is the evidence, so a question asking what you would see wants the colour, not just the word displacement.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme for a displacement question contains the sharpest wording ruling in this topic. Explaining why no reaction occurs, it credits that bromine is less reactive than chlorine, or that chlorine is more reactive than bromine. It then says: do not allow bromide is less reactive than chlorine, or chloride is more reactive than bromine.
Bromine is the element; bromide is its ion. Comparing an element with an ion compares two different kinds of thing, so the sentence does not say what it appears to say. Compare element with element.
The same ruling appears again on the redox part. It credits that astatide ions are oxidised because they lose electrons and that bromine molecules are reduced because they gain electrons, then rules out astatine is oxidised and bromide is reduced. The ion is oxidised; the element is reduced. Swapping them describes the reaction backwards.
On the equation, it awards one mark for all the formulae being correct and a second, dependent on the first, for the balancing. It ignores state symbols on that question even if they are wrong, which is unusually relaxed and worth noticing, because most chemistry mark schemes mark them.
On the colours, it accepts light green for chlorine but a companion note warns against describing bromine as red alone.
Displacement is a redox reaction
Describing it as swapping places is the observation. The explanation is electron transfer.
Chlorine molecules gain electrons and become chloride ions, so chlorine is reduced. Bromide ions lose electrons and become bromine molecules, so bromide is oxidised. The more reactive halogen is the one that takes the electrons, which is why it ends up as the ion and the other ends up as the element.
Saying which species gained and which lost electrons, using the ion names correctly, is what turns a description into a full mark answer.
Spec 2.5-2.8C
What you need to know
- Describe the halogens and what they look like
- Describe the trend in reactivity down the group
- Explain displacement reactions of the halogens
Active recall
Quick check
Answer each question before opening the answer.
How does reactivity change down Group 7, and why?
It decreases down the group because the atoms get larger, so it is harder to gain an electron.
What happens in a halogen displacement reaction?
A more reactive halogen displaces a less reactive halogen from a solution of its salt.
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