Chemistry

Chemical reactions and compounds

A chemical reaction rearranges atoms into new substances, and reactions are classified by the pattern of bonds broken and formed. In a combination reaction, two or more reactants unite to form a single product, as when calcium oxide reacts with water to give slaked lime, \mathrm{CaO + H_2O \rightarrow Ca(OH)_2}.

A decomposition reaction is the reverse pattern, in which one compound breaks into simpler substances. Thermal decomposition of lead nitrate absorbs heat, so it is endothermic rather than exothermic.

In a displacement reaction, a more reactive element replaces a less reactive element from its compound. Magnesium and zinc both release hydrogen from dilute sulphuric acid, so hydrogen is the characteristic gas of metal-acid reactions.

In a double displacement reaction, two compounds exchange ions, and an insoluble product such as silver chloride precipitates out of solution. Neutralization is a special case in which an acid and a base give a salt and water.

Combustion is the rapid reaction of a substance with oxygen with release of heat and light. Ethanol burns completely to give carbon dioxide and water.

Redox reactions involve changes in oxidation state. Reduction is a decrease in oxidation state, the oxidant is the species that is reduced, and the reductant is the species that is oxidised. When a sulphide is oxidised, its sulphur rises in oxidation state while the metal ion that is reduced acts as the oxidant. Oxygen usually stays at -2 in oxides and is often a spectator in such changes.

Amphoteric oxides react with both acids and bases. Aluminium oxide is the standard example, since it dissolves in hydrochloric acid and also in sodium hydroxide to form sodium aluminate. Basic oxides such as those of calcium, sodium and potassium react only with acids, so they are not amphoteric.

Sodium hydrogencarbonate, known as baking soda, is used as a leavening agent because it releases carbon dioxide on heating or on reacting with an acid in dough. The gas bubbles expand the dough, and it is the gas, not the salt or the water, that produces the soft and spongy texture.

Hydrogenation converts an alkene into an alkane by adding hydrogen across the carbon-carbon double bond, and it needs a metal catalyst. Finely divided nickel is the classic catalyst for this process, and palladium is also used to catalyse the hydrogenation of alkenes.

An exothermic process releases heat to the surroundings. Dissolution of calcium oxide in water, dilution of sodium hydroxide and the decomposition of vegetable matter in composting are all exothermic.

Methane reacts with chlorine in sunlight by free-radical substitution, giving chloromethanes and hydrogen chloride, not hydrogen gas. Unsaturated hydrocarbons burn with a sooty flame because their carbon content is high relative to hydrogen.

Nitrolim is calcium cyanamide, \mathrm{CaCN_2}. Ammonium nitrate, urea and potassium nitrate are \mathrm{NH_4NO_3}, \mathrm{CO(NH_2)_2} and \mathrm{KNO_3} respectively, so each fertiliser compound must be matched to its own formula.

Among carbon compounds, the number of multiple bonds determines the class of a hydrocarbon. 1-Propyne has one triple bond and not three double bonds. Acetic acid boils far above methane because hydrogen bonding in carboxylic acids is much stronger than the weak forces between nonpolar methane molecules. Ethanol reacts with sodium metal to release hydrogen gas. Esters do not react with sodium bicarbonate, whereas carboxylic acids do and give effervescence.

Brine is a concentrated aqueous solution of common salt, sodium chloride, and is used in food preservation and the chlor-alkali industry.

Minerals, ores and alloys

An ore is a naturally occurring mineral from which a metal can be extracted economically. Cinnabar, which is mercury(II) sulfide, is the principal ore of mercury, and roasting it in air releases mercury vapour.

Borax, which is sodium tetraborate decahydrate, is made by reacting boric acid with sodium carbonate. The sodium in borax therefore comes from the carbonate, not from calcium or magnesium salts.

White gold is a jewellery alloy in which gold is mixed with whitening metals that remove its yellow colour. Nickel and palladium are the standard whitening constituents, and cobalt, titanium, platinum and magnesium are not the usual components.

Gypsum is calcium sulphate dihydrate, \mathrm{CaSO_4 \cdot 2H_2O}. On moderate heating it loses water and forms the hemihydrate \mathrm{CaSO_4 \cdot \tfrac{1}{2}H_2O}, which is plaster of Paris, and it does not become a monohydrate on heating. Plaster of Paris sets back to gypsum when mixed with water, which is why it is used to immobilise fractured bones. Gypsum is a crystalline solid at room temperature even though it contains water of crystallisation.

Fuels and atmospheric pollutants

Biogas is produced by the anaerobic digestion of organic matter such as cattle dung and plant residues. Its main combustible constituent is methane, which burns with a clean, smokeless flame. Carbon dioxide is also present in large amounts but does not burn, while hydrogen and hydrogen sulphide occur only in small quantities.

Acid rain forms when oxides of sulphur and nitrogen, released mainly by the combustion of fossil fuels, dissolve in atmospheric water to give sulphuric and nitric acids. Carbon dioxide yields only weak carbonic acid, and solar heating or electrical charges in clouds do not produce acidic compounds.