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Lead and its compounds

Lead is a heavy dull grey soft ductile metallic element belonging to group 14 (formerly IVB) of the periodic table. The main ore is the sulphide galena (PbS); other minor sources include anglesite (PbSO₄), cerussite (PbCO₃) and litharge (PbO). The metal is extracted by roasting the ore to give the oxide, followed by the reduction with carbon. Silver is also recovered from the ores. Lead has a variety of uses including building construction, lead-plate accumulators, bullets, and shot, and is a constituent of such alloys as solder, pewter, bearing metals, type metals, and fusible alloys. Chemically, it forms compounds with the +2 and +4 oxidation states, the lead(II) state being the more stable.

Lead-acid accumulator is an accumulator in which the electrodes are made of lead and the electrolyte consists of dilute sulphuric (V1) acid. The electrodes are usually cast from a lead alloy containing 7-12% of antimony (to give increased hardness and corrosion resistance) and a small amount of tin (for better casting properties). The electrodes are coated with a paste of lead (II) oxide (PbO) and finely divided lead; after insertion into theelectrolyte a forming current is passed through the cell to convert the PbO on the negative plate into a sponge of finely divided lead. On the positive plate the PbO is converted to lead (IV) oxide (PbO₂). The equation for the overall reaction during discharge is:

PbO₂ + 2H₂SO₄ + Pb ® 2PbSO₄ + 2H₂O.

The reaction is reversed during charging. Each cell gives an e.m.f. of about 2 volts and in motor vehicles a 12-volt battery of six cells is usually used. The lead-acid battery produces 80-120 kJ per kilogram.

Lead (ll) carbonate is a white solid, PbCO₃, insoluble in water; rhombic; r.d. is 6.6. It occurs as the mineral cerussite, which is isomorphous with aragonite and may be prepared in the laboratory by the addition of cold ammonium carbonate solution to a cold solution of a lead (II) salt (acetate or nitrate). It decomposes at 315°C to lead (II) oxide and carbon dioxide.

Lead (ll) carbonate hydroxide (white lead; basic lead carbonate) is a

Oxidation-reduction reactions (redox)

Originally, oxidationwas simply regarded as a chemical reaction with oxygen. The reverse process - loss of oxygen -was called reduction. Reaction with hydrogen also came to be regarded as reduction. Later, a more general idea of oxidation and reduction was developed in which oxidation was loss of electrons and reduction was gain of electrons. This wider definition covered the original one. For example, in the reaction:

4Na (s) + O₂ (g) ® 2Na₂O (s).

The sodium atoms lose electrons to give Na⁺ ions and are oxidized. At the same time, the oxygen atoms gain electrons and are reduced. These definitions of oxidation and reduction also apply to reactions that do not involve oxygen. For instance in reaction:

2Na (s) + Cl₂ (g) ® 2NaCl (s).

The sodium is oxidized and the chlorine reduced. Oxidation and reduction also occurs at the electrodes in cells.

This definition of oxidation and reduction applies only to reactions in which electron transfer occurs - i.e. to reactions involving ions. It can be extended to reactions between covalent compounds by using the concept of oxidation number (or state). This is a measure of the electron control that an atom has in a compound compared to the atom in the pure element. An oxidation number consists of two parts:

1. its sign, which indicates whether the control has increased (negative) or decreased (positive);

2. its value which gives the number of electrons over which control has changed.

The change of electron control may be complete (in ionic compounds) or partial (in covalent compounds). For example, in SO₂ the sulphur has an oxidation number +4, having gained partial control over 4 electrons compared to sulphur atoms in pure sulphur. The oxygen has an oxidation number -2, each oxygen having lost partial control over 2 electrons compared to oxygen atoms in gaseous oxygen. Oxidation is a reaction involving an increase in oxidation number and reduction involves a decrease. Thus in reaction:

2H₂ + O₂ ® 2H₂O.

The hydrogen in water is +1 and the oxygen -2. The hydrogen is oxidized and the oxygen is reduced. The oxidation number is used in naming inorganic compounds. Thus in H₂SO₄, sulphuric (VI) acid, the sulphur has an oxidation number of +6. Compounds that tend to undergo reduction readily are oxidizing agents; those that undergo oxidation are reducing agents.

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