Reactions of Halogens

Section 1 of 8Halogens with Group 1 metals

What forms

• What forms A halogen and an alkali metal combine to make an ionic metal halide, which is a salt.
• Each halogen atom gains one electron and each metal atom loses one, so the compound contains M⁺ and X^- ions.

Example equations

• Example equations sodium + chlorine → sodium chloride: 2Na + Cl₂ → 2NaCl
• lithium + bromine → lithium bromide: 2Li + Br₂ → 2LiBr
• potassium + iodine → potassium iodide: 2K + I₂ → 2KI

Flame colours

• Flame colours Lithium compounds burn with a crimson flame, sodium with an orange flame and potassium with a lilac flame, whichever halogen is used.

Safety and trend

• Safety and trend The halogens are toxic, so these reactions must be done in a fume cupboard.
• Reactivity decreases down Group 7, so the reactions become less vigorous from chlorine to bromine to iodine.

Section 2 of 8Halogens with iron

Halogen	What is seen with heated iron wool	Product
Fluorine	Even cold iron wool burns	White iron(III) fluoride
Chlorine	Hot iron wool burns vigorously	Orange-brown iron(III) chloride
Bromine	Hot iron wool burns vigorously	Orange-brown iron(III) bromide
Iodine	Hot iron wool reacts slowly with the vapour	Grey iron(III) iodide

Section 3 of 8Equation and trend for iron

1

Iron reacts with a halogen to make an iron(III) halide, so the formula needs three halogen atoms for each iron atom.

2

Word equation for chlorine: iron + chlorine → iron(III) chloride.

3

Balanced symbol equation: 2Fe + 3Cl₂ → 2FeCl₃.

4

The reaction is done in a fume cupboard and becomes less vigorous down the group, matching the fall in reactivity.

Section 4 of 8Displacement reactions (HT only)

1Mix

• Mix A halogen solution is added to a colourless solution of a halide salt, such as potassium bromide.

2Compete

• Compete The more reactive halogen takes the place of the less reactive halogen in the compound.

3Observe

• Observe The freed halogen colours the solution: orange for bromine, brown for iodine.

4Conclude

• Conclude A reaction means the added halogen is more reactive; no change means it is less reactive.

Section 5 of 8Displacement results and equations (HT only)

Reaction	Observation	Equation
Chlorine + potassium bromide	Solution turns orange	Cl2 + 2KBr → 2KCl + Br2
Chlorine + potassium iodide	Solution turns brown	Cl2 + 2KI → 2KCl + I2
Bromine + potassium iodide	Solution turns brown	Br2 + 2KI → 2KBr + I2
Bromine + potassium chloride	No reaction	Bromine is less reactive than chlorine
Iodine + potassium bromide or chloride	No reaction	Iodine is the least reactive of the three

Section 6 of 8Why displacement is better evidence (HT only) and uses

Predictions

• Predictions Fluorine should displace every other halogen from its compounds, and astatine should be displaced by all of them.
• Astatine is so rare that these predictions cannot be tested.

Stronger evidence

• Stronger evidence The halogens are different states at room temperature, so heating them with iron is not a fair comparison.
• In displacement reactions the halogens compete directly in the same solution, which gives better evidence for the trend.

Chlorine and iodine

• Chlorine and iodine Both are diatomic and toxic,
  • but chlorine is a green-yellow gas while iodine is a dark grey solid that gives a purple vapour when warmed.
• Chlorine sterilises drinking water and swimming pools; iodine is used as an antiseptic. Levels must be monitored because both are toxic.

Section 7 of 8Key terms

Term	Meaning
Halogen	An element in Group 7 of the periodic table; they exist as diatomic molecules.
Metal halide	An ionic salt made from a metal and a halogen, such as sodium chloride.
Displacement reaction	A reaction in which a more reactive element takes the place of a less reactive one in a compound.
Diatomic	Made of molecules containing two atoms joined together, such as Cl2.
Fume cupboard	A ventilated enclosure that removes toxic gases and vapours during an experiment.

Section 8 of 8Exam technique

1

Writing chlorine as Cl instead of Cl₂ in equations - halogens are diatomic, so the equation will not balance otherwise.

2

Saying reactivity increases down Group 7 - it decreases, so fluorine is the most reactive.

3

Predicting that iodine displaces bromine - a halogen can only displace one below it in the group.

4

Forgetting that iron forms iron(III) halides with halogens, giving FeCl₃ rather than FeCl₂.