Group VII properties

Cambridge IGCSE Chemistry 0620 · Core and Extended

Syllabus points

• Describe the Group VII halogens, chlorine, bromine and iodine, as diatomic non-metals.
• State the appearance of chlorine, bromine and iodine at room temperature and pressure.
• Describe the general trends down the group of increasing density and decreasing reactivity.
• Predict the properties of other elements in Group VII, given information about the elements.
• Describe and explain the displacement reactions of halogens with other halide ions.

Every objective in 8.3 is Core. There is no Supplement content in this subtopic, so a Core candidate needs all of it.

Appearance, which is pure recall

Three facts sit inside that table and each is asked separately.

The formulae are all diatomic: two atoms joined by a single covalent bond, because each atom has seven outer electrons and needs one more.

The states run gas, liquid, solid down the group, because the melting and boiling points increase as the molecules get larger. Iodine is the only one that is solid at room temperature, and it sublimes, turning straight from a grey-black solid to a purple vapour without melting.

The density increases down the group, in step with the increasing mass of the molecules.

Reactivity decreases down the group

This is the opposite direction to Group I, and the explanation is the mirror image.

A halogen atom reacts by gaining one electron to complete its outer shell. Going down the group each atom has one more occupied shell, so the outer shell is further from the nucleus and shielded by more inner shells. The nucleus therefore attracts an incoming electron less strongly, the electron is gained less easily, and the element is less reactive.

Chlorine is the most reactive of the three and iodine the least. Fluorine, above chlorine, is more reactive than all of them.

Colours in solution

These matter because they are how displacement is observed.

• Chlorine water is very pale, almost colourless with a faint green tint.
• Bromine water is orange.
• Iodine solution is brown in water, and purple in an organic solvent such as hexane.

Displacement reactions

A more reactive halogen displaces a less reactive halogen from a solution of its salt. Since reactivity falls down the group, chlorine displaces bromine and iodine, bromine displaces iodine only, and iodine displaces neither.

The equations:

Cl₂ + 2KBr → 2KCl + Br₂

Cl₂ + 2KI → 2KCl + I₂

Br₂ + 2KI → 2KBr + I₂

Written ionically, with the potassium as a spectator ion:

Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂

Displacement is redox. Chlorine gains electrons, so it is reduced and is acting as the oxidising agent. Bromide ions lose electrons, so they are oxidised. Chlorine is a better oxidising agent than bromine, which is another way of saying it is more reactive, and the oxidising power falls down the group in step with the reactivity.

The colour change is the evidence, so quote it. Adding chlorine water to colourless potassium bromide solution turns it orange because bromine has been set free. Adding it to potassium iodide turns the solution brown because iodine has been set free, and a grey-black solid may settle out if enough forms.

Worked example. Excess chlorine is bubbled through a solution containing 2.38 g of potassium bromide. What mass of bromine is displaced? The relative formula mass of KBr is 119 and of Br₂ is 160.

moles of potassium bromide = 2.38 / 119 = 0.0200 mol

The equation shows 2 mol of KBr giving 1 mol of Br₂, so

moles of bromine = 0.0200 / 2 = 0.0100 mol

mass of bromine = 0.0100 × 160 = 1.60 g

Compounds of the halogens

Each halogen gains one electron to form a 1− ion: chloride Cl⁻, bromide Br⁻, iodide I⁻. With a metal that gives an ionic compound such as NaCl or MgBr₂. With a non-metal it shares instead, giving covalent HCl.

The halide ions are identified with acidified silver nitrate solution, and the three precipitate colours are recall marks:

• Chloride gives a white precipitate of silver chloride.
• Bromide gives a cream precipitate of silver bromide.
• Iodide gives a yellow precipitate of silver iodide.

Chlorine itself is used to kill bacteria in drinking water and in swimming pools, and in the manufacture of bleach.

Predicting the next element

Astatine, below iodine, follows every trend:

• A solid at room temperature, since the states run gas to liquid to solid down the group.
• Darker than iodine, so black.
• Higher density than iodine.
• Less reactive than iodine, so it displaces no other halogen and is displaced by all of them.
• Forms the astatide ion At⁻ and the compound NaAt.

Fluorine, above chlorine, runs the other way: a pale yellow gas, less dense, and the most reactive halogen of all, displacing every other halide from solution.

Common mistakes

• Writing the halogens as single atoms, Cl rather than Cl₂.
• Saying reactivity increases down Group VII, by carrying the Group I trend across.
• Explaining decreasing reactivity by "fewer electrons" instead of by the outer shell being further from the nucleus and shielded.
• Giving iodine as purple in the solid state, when it is grey-black and only its vapour is purple.
• Saying iodine displaces bromine, which is the reaction that does not happen.
• Confusing the colours of bromine water, which is orange, with an iodine solution, which is brown.
• Forgetting that a displacement reaction is redox, and that the displacing halogen is the oxidising agent.