4B.9-4B.11 - Halogen Trends And Displacement

Reactivity From Displacement Results

A chemist adds chlorine water to aqueous potassium bromide, shakes the mixture, then adds a small volume of an organic solvent. The organic layer becomes orange. The same chlorine water added to potassium iodide gives a purple organic layer, but iodine water added to potassium chloride gives no displacement.

Those observations are not separate facts to memorise. They are evidence for a reactivity order:

$\ce{F2}$ > $\ce{Cl2}$ > $\ce{Br2}$ > $\ce{I2}$

In this lesson, "more reactive" means "more able to gain electrons from halide ions". A reactive halogen is a stronger oxidising agent because it removes electrons from another species and is itself reduced.

For solutions known to contain exactly one of chloride, bromide or iodide, a displacement matrix can identify the unknown halide. If neither chlorine nor bromine displaces a new halogen, chloride is supported; if chlorine produces bromine but bromine gives no new product, bromide is supported; if both produce iodine, iodide is supported. Use the colour of the molecular halogen and distinguish newly formed product from unchanged added reagent. A no-reaction result still supplies useful evidence about relative oxidising strength.

The displacement row in the specification names $\ce{Cl2}$, $\ce{Br2}$ and $\ce{I2}$ reacting with halide ions in aqueous solution. Fluorine belongs in the general Group 7 trend, but reacts with water and cannot be compared using this aqueous procedure.

Group 7 Physical Trends

Group 7 elements exist as diatomic molecules: $\ce{F2}$, $\ce{Cl2}$, $\ce{Br2}$ and $\ce{I2}$. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid.

The melting and boiling temperatures increase down the group:

$\ce{F2}$ lowest < $\ce{Cl2}$ < $\ce{Br2}$ < $\ce{I2}$ highest

The reason is intermolecular, not covalent. Each molecule is simple molecular, so melting or boiling separates molecules from each other; it does not break the covalent bond inside $\ce{X2}$.

Down the group:

• the molecules contain more electrons
• the electron clouds are larger and more easily distorted
• instantaneous dipole-induced dipole attractions, also called London forces, become stronger
• more energy is needed to separate the molecules
• melting and boiling temperatures therefore increase

[DIAGRAM: asset_name: Edexcel A Level Chemistry L021 Group 7 Trend Ladder; asset_slug: edexcel_a_level_chemistry_l021_group_7_trend_ladder; recommended_method: deterministic_code; description: Halogen molecules grow down Group 7: London forces increase while oxidising strength decreases. Pale 1600 by 900 diagram rendered with Pillow; technical geometry and Unicode chemical labels inspected.]

Worked example: explaining why bromine is a liquid but chlorine is a gas at room temperature

Chlorine and bromine are both simple molecular substances with diatomic molecules. Bromine molecules are larger and contain more electrons than chlorine molecules. The electron cloud in $\ce{Br2}$ is more polarisable, so London forces between $\ce{Br2}$ molecules are stronger. More energy is needed to overcome these intermolecular forces, so bromine has a higher boiling temperature and is liquid at room temperature, while chlorine is gaseous.

The key chain is:

larger molecule / more electrons → stronger London forces → more energy needed to separate molecules → higher boiling temperature

Common-error contrast

Do not write that bromine has a higher boiling temperature because the covalent bond in $\ce{Br2}$ is stronger. Boiling does not break the $\ce{Br-Br}$ bond; it overcomes attractions between neighbouring $\ce{Br2}$ molecules.

Electronegativity And Reactivity

Electronegativity is the ability of an atom to attract a shared pair of electrons in a covalent bond. The Data Booklet gives this order for the halogens:

F 4.0 > $\ce{Cl}$ 3.0 > $\ce{Br}$ 2.8 > $\ce{I}$ 2.5

Electronegativity decreases down Group 7. The atoms become larger and have more occupied electron shells. Shielding increases, so the nucleus attracts an outer shared pair less strongly, even though the nuclear charge has increased.

The same particle model explains the reactivity trend. A halogen atom reacts by gaining one electron to form a halide ion:

$\ce{X2 + 2e- -> 2X-}$

Down the group, the outer shell is further from the nucleus and more shielded by inner shells. The attraction for an incoming electron is weaker, so the halogen molecule is less readily reduced. Reactivity as an oxidising agent decreases:

$\ce{Cl2}$ is a stronger oxidising agent than $\ce{Br2}$

$\ce{Br2}$ is a stronger oxidising agent than $\ce{I2}$

Displacement And Redox

The more reactive halogen gains electrons and forms its own halide ion. The halide ion loses electrons and forms the less reactive halogen.

Worked example: chlorine and bromide ions

Observation after adding organic solvent: an orange organic layer shows that bromine has formed.

Half-equations:

$\ce{Cl2 + 2e- -> 2Cl-}$ — chlorine is reduced

$\ce{2Br- -> Br2 + 2e-}$ — bromide ions are oxidised

Overall ionic equation:

Oxidation number check:

$\ce{Cl}$ in $\ce{Cl2}$: 0 to -1 reduction

$\ce{Br}$ in $\ce{Br-}$: -1 to 0 oxidation

This proves that chlorine is a stronger oxidising agent than bromine because chlorine removes electrons from bromide ions.

Displacement outcomes for $\ce{Cl2}$, $\ce{Br2}$ and $\ce{I2}$

For an "explain" answer, include the direction of electron transfer. For example, "chlorine displaces iodine from iodide" is a start, but it is not the full reasoning. The full explanation is that chlorine is more readily reduced than iodine, so $\ce{Cl2}$ oxidises $\ce{I-}$ to $\ce{I2}$ while $\ce{Cl2}$ is reduced to $\ce{Cl-}$.

Organic Solvent Evidence

The organic solvent step makes the halogen product easier to identify. Molecular halogens dissolve better in the organic layer than in the aqueous layer, so after shaking and allowing the layers to separate, the colour of the organic layer gives evidence for which halogen molecule is present.

[DIAGRAM: asset_name: Edexcel A Level Chemistry L021 Halogen Solvent Extraction; asset_slug: edexcel_a_level_chemistry_l021_halogen_solvent_extraction; recommended_method: deterministic_code; description: Separate chlorine, bromine and iodine references in an upper organic layer; the lower layer is aqueous. Pale 1600 by 900 diagram rendered with Pillow; technical geometry and Unicode chemical labels inspected.]

Typical observations are:

In many school demonstrations the organic solvent is less dense than water, so the organic layer is the upper layer. The important evidence is the colour in the organic layer, not the layer position; a different organic solvent could change which layer is on top.

Practical reasoning

Use a fresh separate portion for each halogen–halide combination, with the same measured volumes and concentrations to make comparisons fair. Use small volumes, stopper the tube before shaking, and allow the layers to settle before recording the colour. Halogens are harmful and volatile, and many organic solvents are flammable, so the method needs eye protection, good ventilation or a fume cupboard when required, and careful waste handling. Do not use the solvent-layer colour as a silver nitrate halide test; that is a different reaction family and belongs to the next lesson.

Check

Recap

Group 7 physical trends come from increasingly strong London forces between larger molecules. Group 7 reactivity decreases down the group because larger, more shielded atoms attract an incoming electron less strongly. Displacement reactions turn that model into evidence: a more reactive halogen oxidises a less reactive halide ion, and the organic solvent layer helps identify the molecular halogen produced.