Sunday, December 9, 2007

Chapter 13A. Halogens

The Halogens are non-metals and form the 7th Group in the Periodic Table.

'Halogens' means 'salt formers' and the most common compound is sodium chloride which is found from natural evaporation as huge deposits of 'rock salt' or the even more abundant 'sea salt' in the seas and oceans.

Physical properties

• typical non-metals with relatively low melting points and boiling points.
• They are all poor conductors of heat and electricity - typical of non-metals.
• When solid they are brittle and crumbly e.g. iodine.

F Fluorine--- pale yellow gas

Cl Chlorine--- green gas

Br Bromine--- dark red liquid, brown vapour

I Iodine---- dark crumbly solid, purple vapour

At Astatine--- black solid, dark vapour

important trends down the Group with increasing atomic number
• The melting points and boiling increase steadily down the group (so the change in state at room temperature from gas ==> liquid ==> solid), this is because the weak electrical intermolecular attractive forces increase with increasing size of atom or molecule.
• They are all coloured non-metallic elements and the colour gets darker down the group.
• The size of the atom gets bigger as more inner electron shells are filled going down from one period to another.
Chemical Properties
• The atoms all have 7 outer electrons,
o they form singly charged negative ions e.g. chloride Cl- because they are one electron short of a noble gas electron structure. They gain one negative electron (reduction) to be stable and this gives a surplus electric charge of -1. These ions are called the halide ions, the bromide Br- and iodide I- ions.
o they form ionic compounds with metals e.g. sodium chloride Na+Cl-. (ionic bonding revision page)
o they form covalent compounds with non-metals and with themselves.
o The bonding in the molecule involves single covalent bonds e.g. hydrogen chloride HCl or H-Cl.
• The elements all exist as X2 or X-X, diatomic molecules where X represents the halogen atom.
• A more reactive halogen can displace a less reactive halogen from its salts .
• The reactivity decreases down the group .
• they are all TOXIC elements .
• Astatine is very radioactive, so difficult to study

Reaction with hydrogen H-2

• Halogens readily combine with hydrogen to form the hydrogen halides which are colourless gaseous covalent molecules. e.g. hydrogen + chlorine ==> hydrogen chloride
• H-2(g) + Cl-2(g) ==> 2HCl(g)
• The hydrogen halides dissolve in water to form very strong acids with solutions of pH1 e.g. hydrogen chloride forms hydrochloric acid in water HCl(aq) or H+Cl-(aq) because they are fully ionised in aqueous solution even though the original hydrogen halides were covalent. An acid is a substance that forms H+ ions in water.
• Bromine forms hydrogen bromide gas HBr(g), which dissolved in water forms hydrobromic acid HBr(aq). Iodine forms hydrogen iodide gas HI(g), which dissolved in water forms hydriodic acid HI(aq).

Reaction with Group 1 Alkali Metals Li, Na, K etc.

• Alkali metals burn very exothermically and vigorously when heated in chlorine to form colourless crystalline ionic salts e.g. NaCl or Na+Cl-.
• e.g. sodium + chlorine ==> sodium chloride
• 2Na(s) + Cl2(g) ==> 2NaCl(s)
• The sodium chloride is soluble in water to give a neutral solution pH 7, universal indicator is green. The salt is a typical ionic compound i.e. a brittle solid with a high melting point. Similarly potassium and bromine form potassium bromide KBr, or lithium and iodine form lithium iodide LiI.

Reaction with other metals
• If aluminium or iron is heated strongly in a stream of chlorine (or plunge the hot metal into a gas jar of chlorine carefully in a fume cupboard) the solid chloride is formed.
• aluminium + chlorine ==> aluminium chloride(white):
o 2Al(s) + 3Cl2(g) ==> 2AlCl3(s)
• iron + chlorine ==> iron(III) chloride(brown):
o 2Fe(s) + 3Cl2(g) ==> 2FeCl3(s)
• If the iron is repeated with bromine the reaction is less vigorous, with iodine there is little reaction, these also illustrate the halogen reactivity series.

No comments: