Chemistry · Chemistry of the elements · Grade 8

Non-metals and halogens

Why does chlorine displace bromine, and why does iodine turn violet in petrol?

The situation

A toothpaste box says “fluoride”, a pack of salt says “iodised”, and the pharmacy sells tincture of iodine. Fluorine, chlorine, bromine and iodine are one family, the halogens: all their atoms have 7 electrons in the outer shell. Yet fluorine and chlorine are gases, bromine is a liquid and iodine is a solid crystal. Why do members of one family differ so much? Why does chlorine water displace bromine from its salt, and why is iodine violet in petrol but brown in water?

Topic

Non-metals and the halogen family. Fluoride toothpaste, iodised salt and bromine: which halogen displaces which, and why iodine is violet in petrol

Key ideas

The halogen series
Displacement
Extraction

Formulas

Cl_{2} + 2 KBr \overset{}{\to} 2 KCl + Br_{2}

A more active halogen displaces a less active halogen from the solution of its salt (lab work 5).

Br_{2} + 2 KI \overset{}{\to} 2 KBr + I_{2}

Bromine displaces iodine; the other way round — $I_{2}$ + KBr — there is no reaction.

F_{2} > Cl_{2} > Br_{2} > I_{2}

Oxidising power decreases down the group, while the atomic radius increases.

K = \frac{c_{petrol}}{c_{water}}

The distribution law: in two immiscible liquids the ratio of the concentrations of a substance is constant (for iodine, hexane–water $K$ ≈ 35).

q = \frac{V_{water}}{V_{water} + K V_{petrol}}

The fraction of iodine left in the water after one extraction; if it is repeated $n$ times — $q_{1} q_{2} \dots q_{n}$ .

Theory

Non-metals (§ 21)

Non-metals are in the upper right part of the periodic table, above the B–At diagonal; hydrogen and the noble gases are also non-metals. Their atoms have many electrons in the outer shell (usually 4–8; hydrogen and helium are exceptions), so in reactions they often accept electrons — they are oxidising agents. Their elements at room temperature are gases ( $H_{2}$ , $N_{2}$ , $O_{2}$ , $F_{2}$ , $Cl_{2}$ , the noble gases), a liquid (only bromine) or solids (carbon, phosphorus, sulfur, iodine). They conduct heat and electricity poorly (graphite is an exception), the solids are brittle, and they usually have no metallic lustre (iodine and graphite do). When one element forms several simple substances, this is allotropy: $O_{2}$ and ozone $O_{3}$ , diamond and graphite, white and red phosphorus, rhombic and monoclinic sulfur. The oxides of non-metals are usually acidic, and their hydrogen compounds ( $HCl$ , $H_{2} S$ , $NH_{3}$ , $CH_{4}$ ) are volatile substances.

The halogens: position and atomic structure (§ 22)

The halogens (“salt-formers”) are Group VIIA (Group 17): fluorine, chlorine, bromine, iodine (and radioactive astatine). The outer shell is $n s^{2} n p^{5}$ — 7 electrons, one short of an octet: the atom accepts one electron and becomes an $X^{-}$ ion (oxidation state −1). Fluorine is the most electronegative element (3.98) and has only −1; chlorine, bromine and iodine also have +1, +3, +5, +7 in oxygen compounds. Their elements are diatomic $X_{2}$ molecules. Going down the group the atomic radius increases (F 57 → I 139 pm), and the electronegativity and oxidising power decrease; the molecules get heavier and the attraction between them stronger — the melting and boiling points rise: fluorine and chlorine are gases, bromine is a liquid and iodine is a solid.

Fluorine, bromine, iodine (§ 28)

Fluorine is a pale yellow gas, the most reactive non-metal: even water burns in it ( $2 F_{2} + 2 H_{2} O \overset{}{\to} 4 HF + O_{2}$ ), and it attacks glass; it was isolated by electrolysis in 1886 by Henri Moissan. Fluorides strengthen tooth enamel — many toothpastes contain 0.10–0.15 % (1000–1500 ppm) of fluorine. Bromine is the only liquid non-metal: red-brown, it evaporates quickly, its vapour is poisonous and burns the skin. Iodine is a violet-black crystal with a metallic lustre; when heated it begins to turn into a violet vapour (sublimation). A 5 % solution of iodine in alcohol is tincture of iodine, which disinfects wounds. Iodine is essential for the thyroid gland: in Uzbekistan table salt is iodised by law (potassium iodate is added). Starch gives a blue colour with iodine — the qualitative test for iodine.

Lab work 5 and practical 1: displacement

A more active halogen displaces a less active halogen from the solution of its salt: $Cl_{2} + 2 KBr \overset{}{\to} 2 KCl + Br_{2}$ , $Cl_{2} + 2 KI \overset{}{\to} 2 KCl + I_{2}$ , $Br_{2} + 2 KI \overset{}{\to} 2 KBr + I_{2}$ . Iodine can displace neither bromine nor chlorine. To see the liberated halogen, petrol is added to the test tube and shaken: halogens dissolve better in petrol than in water and pass into the upper layer — bromine gives an orange colour, iodine violet. The result is the series of oxidising power $F_{2} > Cl_{2} > Br_{2} > I_{2}$ . Fluorine is not used in solutions — it reacts with the water itself. Practical work: unknown solutions of KCl, KBr and KI can be told apart with chlorine water and petrol: no change — KCl, orange — KBr, violet — KI (the second method is silver nitrate, in the “Chlorine and hydrochloric acid” experiment).

Lab work 6: dissolving and extraction

Iodine dissolves very little in water: ≈ 0.3 g/L at 20 °C (1 g of iodine needs ≈ 3.5 L of water), and the solution is pale yellow-brown. It dissolves well in alcohol (brown — tincture of iodine) and also well in petrol, giving a violet colour: the non-polar $I_{2}$ molecule dissolves well in a non-polar solvent (“like dissolves like”). Between two liquids that do not mix a substance is distributed in a constant ratio: $K = c_{petrol} / c_{water}$ . Extraction is moving a substance from one solvent to another in this way; the mixture is shaken in a separating funnel, and when the layers have separated the upper (petrol) layer is taken off. After one extraction a fraction $q = V_{water} / (V_{water} + K V_{petrol})$ remains in the water; it is more effective to divide the solvent and extract several times.

Safety and the model

Safety: work with chlorine water and bromine water only in a fume cupboard — their vapours burn the airways; bromine on the skin causes severe burns — gloves and safety goggles are essential. Petrol catches fire easily — do not light a flame near it. Never taste a substance; if you need to smell it, waft the vapour towards you with your hand over the vessel. Model: the ampoules in the thermostat show the state of the substance at normal pressure (101.3 kPa) (in reality fluorine is not kept in a glass vessel — it attacks glass); the colour of the vapour is schematic, following the saturated vapour pressure; displacement is instant and goes to completion; hexane is used in place of petrol, with $K$ = 35 for iodine (an approximate laboratory value); in a solution containing iodide ions the iodine binds to form $I_{3}^{-}$ and so more of it stays in the water — this is taken into account; for chlorine and bromine the distribution is shown only for the colour.

Did you know?

Iodine was discovered in 1811 by the French chemist Bernard Courtois in the ash of seaweed: when sulfuric acid was added to the ash, a violet vapour rose. The name of the element comes from the Greek word “iodes” — “violet-coloured”.
Bromine was isolated by two chemists independently: Carl Löwig (1825) from mineral spring water, and Antoine Balard (1826) from seaweed ash from the salt marshes of Montpellier. The name comes from the Greek word “bromos” — “stench”.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

More experiments in this section

Oxygen and ozone — Grade 7
Hydrogen — the lightest gas — Grade 7
Water: composition and properties — Grade 7
Mineral resources — Grade 7
Lime and classes of compounds — Grade 8
Chlorine and hydrochloric acid — Grade 8
The oxygen group and sulfur — Grade 8
Sulfuric acid and the contact process — Grade 8
Nitrogen, ammonia and nitric acid — Grade 8
Phosphorus and mineral fertilisers — Grade 8