Ch 3Metals and Non-metals

Chapter 3: Metals and Non-metals

1. Physical Properties

Metals

• Lustre: metals have a shining surface (metallic lustre) in pure state.
• Hardness: generally hard; hardness varies (Na, K are so soft they can be cut with a knife).
• Malleability: can be beaten into thin sheets; gold and silver are the most malleable.
• Ductility: can be drawn into thin wires; gold is the most ductile (1 g of gold gives a wire about 2 km long).
• Conductivity: good conductors of heat and electricity; best conductors of electricity are silver and copper; poor conductors among metals: lead and mercury. High melting points generally.
• Sonorous: produce a ringing sound when struck (hence used for bells).

Non-metals

Either solids or gases (only bromine is a liquid). Generally non-lustrous (except iodine), not malleable or ductile (solid non-metals are brittle), poor conductors of heat and electricity, low melting and boiling points, not sonorous.

Important exceptions

• Mercury (Hg): a metal that is liquid at room temperature.
• Gallium and caesium: metals with very low melting points; melt on the palm of the hand.
• Graphite (carbon): a non-metal that conducts electricity and has a very high melting point.
• Iodine: a lustrous (shiny) non-metal.
• Diamond (carbon): a non-metal, but the hardest natural substance with a very high melting point.
• Alkali metals (Li, Na, K): soft metals, cut with a knife, low densities and melting points.

2. Chemical Properties of Metals

(a) Reaction with air (oxygen)

Metal + oxygen → metal oxide.

Amphoteric oxides (react with both acids and bases): Al₂O₃ and ZnO.

Na and K react so vigorously that they catch fire in open air; hence stored under kerosene. Mg burns with a dazzling white flame. Surfaces of Mg, Al, Zn, Pb get a protective oxide layer. Ag and Au do not react with oxygen even at high temperatures. Na₂O and K₂O dissolve in water to form alkalis:

Anodising: electrolytic process that thickens the protective oxide layer on aluminium, improving corrosion resistance.

(b) Reaction with water

Metal + water → metal oxide + hydrogen; soluble oxides dissolve to give hydroxides.

Ca and Mg float because bubbles of hydrogen stick to their surfaces (Mg reacts only with hot water). Al, Fe, Zn react only with steam:

Pb, Cu, Ag, Au do not react with water at all.

(c) Reaction with dilute acids

Metal + dilute acid → salt + hydrogen (Cu, Ag, Au do not displace hydrogen from dilute acids).

Hydrogen is not usually evolved with dilute HNO₃ because nitric acid is a strong oxidising agent — it oxidises the H₂ to water and is itself reduced to oxides of nitrogen (NO, NO₂, N₂O). Exception: very dilute HNO₃ with Mg and Mn gives H₂. Rate of bubble formation: Mg > Al > Zn > Fe.

Aqua regia: a freshly prepared 3 : 1 mixture of concentrated HCl and concentrated HNO₃; highly corrosive and fuming; it can dissolve even gold and platinum, which no single acid can.

(d) Displacement reactions (reaction with solutions of other metal salts)

A more reactive metal displaces a less reactive metal from its salt solution.

(Blue colour of copper sulphate fades; reddish-brown copper deposits on the iron/zinc.)

3. The Reactivity (Activity) Series

Metals arranged in decreasing order of reactivity:

Metals above hydrogen displace H₂ from dilute acids; those below (Cu, Hg, Ag, Au) cannot.

4. Reaction of Metals with Non-metals: Ionic Bonding

Atoms react to achieve a completely filled outermost shell (noble gas configuration). Metals lose electrons to form cations; non-metals gain electrons to form anions. The electrostatic attraction between oppositely charged ions is the ionic (electrovalent) bond.

Formation of NaCl (electron-dot idea in words): Sodium (2,8,1) loses its one outer electron to become Na⁺ (2,8 — neon configuration). Chlorine (2,8,7) gains that electron to become Cl⁻ (2,8,8 — argon configuration). The Na⁺ and Cl⁻ ions attract each other strongly and pack into a crystal lattice; NaCl exists as ions in a lattice, not as discrete molecules.

Similarly MgCl₂: Mg (2,8,2) loses two electrons to become Mg²⁺, each of two Cl atoms gains one to become Cl⁻.

Properties of ionic compounds

• Physical nature: hard, brittle crystalline solids (break under pressure).
• High melting and boiling points (large energy needed to break strong inter-ionic attraction), e.g. NaCl melts at 1074 K.
• Solubility: generally soluble in water, insoluble in solvents like kerosene and petrol.
• Electrical conduction: conduct electricity in molten state and in aqueous solution (ions free to move) but NOT in solid state (ions fixed in lattice).

5. Occurrence and Extraction of Metals (Metallurgy)

Minerals are naturally occurring elements/compounds in the earth's crust; ores are minerals from which metal can be extracted profitably. K, Na, Ca, Mg, Al are so reactive they never occur free; Cu and Ag occur both free and combined; Au and Pt occur in the free state. Gangue = impurities (soil, sand) in ores, removed before extraction. Extraction method depends on position in the reactivity series.

(a) Metals low in the series — heating alone

Oxides of Hg and Cu are reduced simply by heating in air:

(b) Metals in the middle (Zn, Fe, Pb, Cu) — roasting/calcination + reduction

• Roasting: heating a sulphide ore strongly in excess air to convert it to oxide.
  2ZnS(s) + 3O₂(g) + heat → 2ZnO(s) + 2SO₂(g)
• Calcination: heating a carbonate ore strongly in limited air.
  ZnCO₃(s) + heat → ZnO(s) + CO₂(g)
• Reduction with carbon (coke):
  ZnO(s) + C(s) → Zn(s) + CO(g)
• Displacement by highly reactive metals / Thermit reaction (highly exothermic; iron obtained molten — used to join railway tracks and machine parts):
  Fe₂O₃(s) + 2Al(s) → 2Fe(l) + Al₂O₃(s) + heat
  3MnO₂(s) + 4Al(s) → 3Mn(l) + 2Al₂O₃(s) + heat

Extraction of a metal from its oxide is a reduction; obtaining the oxide from the ore is often an oxidation step.

(c) Metals high in the series (K, Na, Ca, Mg, Al) — electrolytic reduction

These cannot be reduced by carbon (they have more affinity for oxygen than carbon does). They are obtained by electrolysis of their molten chlorides/oxides. For molten NaCl:

Aluminium is obtained by electrolytic reduction of molten aluminium oxide.

(d) Refining — electrolytic refining of copper

Impure metal is made the anode, a thin strip of pure metal the cathode, and a solution of the metal salt (acidified CuSO₄ for copper) the electrolyte. On passing current, pure copper dissolves from the anode and equivalent pure copper deposits on the cathode. Soluble impurities pass into solution; insoluble impurities (including traces of silver and gold) settle below the anode as anode mud. Cu, Zn, Sn, Ni, Ag, Au are refined this way.

6. Corrosion

Attack of moisture, air, acids etc. on metal surfaces. Examples: black coating on silver (Ag₂S), green coating on copper [basic copper carbonate, CuCO₃·Cu(OH)₂], reddish-brown rust on iron (hydrated iron(III) oxide, Fe₂O₃·xH₂O).

Conditions for rusting (test-tube experiment): iron rusts only when both air (oxygen) and moisture (water) are present. Nails in boiled water + oil layer (no air) or with anhydrous CaCl₂ (no moisture) do not rust.

Prevention of corrosion

• Painting, oiling, greasing.
• Galvanisation: coating iron/steel with a thin layer of zinc. The article stays protected even if the zinc coating is broken, because zinc is more reactive than iron and is oxidised preferentially (sacrificial protection).
• Chrome plating, tin plating, anodising.
• Alloying (e.g. stainless steel).

7. Alloys

An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal, made by melting the primary metal and dissolving the other elements in it. Alloying improves properties: usually lower electrical conductivity and lower melting point than pure metals, better hardness and corrosion resistance.

• Stainless steel = iron + nickel + chromium (hard, does not rust). Iron + small amount of carbon = hard, strong steel.
• Brass = copper + zinc.
• Bronze = copper + tin. (Brass and bronze are poor conductors of electricity compared with pure copper.)
• Solder = lead + tin; low melting point, used for welding electrical wires.
• Amalgam = an alloy containing mercury.
• Gold: pure gold is 24 carat, too soft for jewellery; in India it is alloyed with silver or copper to make 22 carat gold (22 parts gold : 2 parts Cu/Ag) which is harder.