Chemistry: Matter and Reactions

Types of chemical reactions

A chemical reaction is a process in which reactants are converted into products with a different composition and different properties. Reactions are grouped by the pattern in which atoms or ions are rearranged, and this classification is one of the most frequently tested topics in the NDA General Ability Test. A physical change alters only the state or form of a substance and does not form a new substance. Milk turning sour is a chemical change, because bacteria convert lactose into lactic acid, a new substance that cannot be reversed by simple cooling or mixing. Burning magnesium ribbon in air forms magnesium oxide, so it is also a chemical change, whereas evaporation, dissolution and compression of a gas are physical changes.

• Combination reaction — two or more reactants combine to give a single product, e.g. \mathrm{C + O_2 \rightarrow CO_2}. Burning of coal is a combination reaction. Quick lime combines with water to form slaked lime, and the large amount of heat released makes this reaction exothermic. Metals combining with oxygen, such as iron forming iron(III) oxide, are also combination reactions.
• Decomposition reaction — one compound breaks down into two or more simpler substances. The breakdown may be triggered by heat (thermal decomposition), light (photolysis) or electricity (electrolysis). Silver chloride splits into silver and chlorine in sunlight. Electrolysis of water is also a decomposition reaction. Silver oxide decomposes on heating, whereas zinc oxide and magnesium oxide are thermally stable and do not decompose under ordinary heating.
• Displacement reaction — a more reactive element replaces a less reactive element from its compound. An iron nail turns brown in copper sulphate solution because iron displaces copper. Zinc displaces copper from copper sulphate in the same way. Aluminium displaces iron from iron(III) oxide in the thermite reaction, which is highly exothermic and is used for joining railway tracks.
• Double displacement reaction — the ions of two compounds exchange partners. When barium chloride reacts with a sulphate solution, an insoluble precipitate of barium sulphate forms.
• Redox reaction — oxidation and reduction occur together, so the oxidation states of elements change. A reaction in which every element keeps its oxidation state, such as the hydrolysis of aluminium chloride, is not a redox reaction. In the thermite reaction, iron(III) oxide is reduced to iron, so it acts as the oxidising agent.
• Addition reaction — atoms add across a multiple bond. Hydrogen adds across the carbon-carbon double bonds of vegetable oils in the presence of finely divided nickel, converting them into saturated fats such as vanaspati ghee. Elimination is the reverse process, in which atoms are removed to create a multiple bond.

Classification by counting reactants and products is the key skill. Many reactants forming one product is combination, one reactant forming several products is decomposition, an element replacing another in a compound is displacement, and two compounds exchanging partners is double displacement.

Metallurgy and alloys

Metallurgy is the science and technology of extracting metals from their ores and refining them. An ore is a mineral from which a metal can be extracted economically, and the method of extraction depends on the reactivity of the metal. Less reactive metals are obtained by simple heating, metals of medium reactivity are reduced by carbon, and highly reactive metals are obtained by electrolysis of their molten compounds.

• Ore — the mineral is named after the metal it yields. Cinnabar, whose chief mineral is mercuric sulphide (HgS), is the ore of mercury, not of copper, zinc or manganese.
• Carbon reduction — carbon is a suitable reducing agent for metals of medium reactivity such as zinc, iron and lead, because their oxides are reduced at high temperature, e.g. \mathrm{ZnO + C \rightarrow Zn + CO}. Zinc is therefore extracted by this method. Aluminium is too reactive to be reduced by carbon, and silver and gold are too unreactive to need carbon reduction.
• Electrolytic refining — impure metal is made the anode and a thin sheet of pure metal the cathode. For copper, the electrolyte is acidified copper sulphate solution, so copper dissolves from the anode and is deposited as pure copper on the cathode.

An alloy is a homogeneous mixture of a metal with another metal or a non-metal, and alloying changes properties such as hardness, strength and melting point.

• Bronze — an alloy of copper and tin. Brass is a different copper alloy, made with zinc.
• Solder — a low-melting alloy of lead and tin used for joining metals.
• Steel — an alloy of iron with a small amount of carbon, which is a non-metal. Steel is therefore the common alloy that contains a non-metal, whereas brass, bronze and amalgam (mercury alloyed with other metals) contain only metals.
• Stainless steel — iron alloyed with chromium, which gives corrosion resistance, and often nickel. Tin is not a standard constituent of stainless steel.
• Tungsten filament — a filament must have a very high melting point. Tungsten has the highest melting point of all metals, about 3422 degrees Celsius, which is why it is used in incandescent bulbs.
• Galvanisation — iron or steel is protected from rusting by a thin coating of zinc. Zinc corrodes in preference to iron, so the iron is protected even where the coating is scratched.

Chemical formulas and compounds

The formula of an ionic compound is written so that the total positive charge equals the total negative charge. For ammonium carbonate, the ammonium ion \mathrm{NH_4^+} carries +1 and the carbonate ion \mathrm{CO_3^{2-}} carries -2, so two ammonium ions balance one carbonate ion, giving (\mathrm{NH_4})_2\mathrm{CO_3}. Neutral \mathrm{NH_3} is a molecule and is not the ammonium ion.

The oxyacids of halogens are named with the prefixes hypo- and per- according to the oxidation state of the central atom. Hypobromous acid, with bromine at +1, has the formula \mathrm{HOBr}. Bromic acid, \mathrm{HBrO_3}, has bromine at +5, and perbromic acid, \mathrm{HBrO_4}, has bromine at +7. Hydrogen bromide, \mathrm{HBr}, is a binary acid containing no oxygen.

Dihydrogen (\mathrm{H_2}) is a light gas that is insoluble in water. Its H-H bond is strong, so it is unreactive at room temperature, but it reacts with alkali metals on heating to form metal hydrides. A mixture of carbon monoxide and hydrogen is called syngas, whereas a mixture of nitrogen dioxide and hydrogen is not syngas.

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt, and such salts are called hydrates. Ferrous sulphate crystals have the formula \mathrm{FeSO_4 \cdot 7H_2O}, so each formula unit contains seven water molecules.

Plaster of Paris is calcium sulphate hemihydrate, \mathrm{CaSO_4 \cdot \frac{1}{2}H_2O}, which is equivalent to 2\mathrm{CaSO_4 \cdot H_2O}. It is made by heating gypsum, the dihydrate \mathrm{CaSO_4 \cdot 2H_2O}, to about 120 degrees Celsius, which drives off most of its water of crystallisation. Writing the dihydrate as the formula of plaster of Paris is therefore incorrect.

Milk of magnesia is an aqueous suspension of magnesium hydroxide, \mathrm{Mg(OH)_2}, used as an antacid and a mild laxative. Magnesium sulphate, which is Epsom salt, and magnesium carbonate are different compounds.

Borax is sodium tetraborate decahydrate, \mathrm{Na_2B_4O_7 \cdot 10H_2O}, and its constituent elements are sodium, boron, oxygen and hydrogen. It contains no magnesium, potassium or nitrogen.

Washing soda, \mathrm{Na_2CO_3 \cdot 10H_2O}, removes the permanent hardness of water by precipitating calcium and magnesium ions as carbonates. The clouds of Venus consist largely of concentrated sulphuric acid.

Properties of carbon compounds

Carbon has four valence electrons and shows tetravalency, forming four covalent bonds. It also shows catenation, the ability to link with other carbon atoms into long chains and rings. Carbon forms single, double and triple bonds with itself and with other elements, but it does not form a tetra bond.

Bonding in organic compounds is covalent, with electrons shared between atoms and no free ions or mobile electrons. Most carbon compounds are therefore poor conductors of electricity.

Diamond and graphite are allotropes of carbon, meaning different forms of the same element. Graphite is used as a lubricant because its layers slide easily over one another.

• Organic compound — generally a compound of carbon, although simple oxides, carbonates and cyanides are classed as inorganic. Ammonium cyanate is inorganic, although Wöhler converted it into urea in 1828, a historic result that showed inorganic material can yield an organic compound. Methane (marsh gas), urea and sucrose (cane sugar) are organic.
• Hydrogenation — hydrogen adds across the double bonds of unsaturated oils in the presence of a nickel catalyst, converting liquid oils into solid fats such as margarine.
• Homologous series — a series of compounds differing by a CH_2 unit with a common general formula. Alkanes have \mathrm{C_nH_{2n+2}}, alkenes have \mathrm{C_nH_{2n}} and alkynes have \mathrm{C_nH_{2n-2}}. Since \mathrm{C_4H_8} fits the alkene formula with n = 4, it is an alkene such as but-1-ene or but-2-ene.
• Structural isomers — compounds with the same molecular formula but different carbon skeletons. Pentane, \mathrm{C_5H_{12}}, has three structural isomers: n-pentane, 2-methylbutane and 2,2-dimethylpropane.
• Boiling point trend — in a homologous series of straight-chain alkanes, boiling point rises with increasing molecular mass, because larger molecules have stronger van der Waals forces. Methane, the smallest, boils lowest, and octane boils higher than propane or butane.

Allotropes of carbon

An allotrope is one of two or more different structural forms of the same element in the same physical state. Carbon is the classic example, and its allotropes differ greatly in properties even though every form is pure carbon. Diamond, graphite, graphene and fullerenes are all allotropes of carbon, whereas materials such as fly ash are not, because fly ash is a mixture of oxides such as silica, alumina and iron oxide left after coal is burnt.

Diamond is crystalline, and each carbon atom is bonded tetrahedrally to four neighbours in a rigid three-dimensional covalent network. This structure is the source of diamond's great hardness, and diamond is the hardest naturally occurring substance, with a Mohs hardness of 10.

Because diamond has a cubic lattice in which every atom is covalently bonded to four neighbours, it is isomorphous with crystalline silicon, which has the same diamond cubic structure. Coke and coal are amorphous forms of carbon and have no such lattice.

Graphite has a layered hexagonal structure. Within each layer, each carbon atom contributes one delocalised electron that moves freely, which makes graphite a good conductor of electricity. The layers are held together only weakly, so they slide over one another easily, which makes graphite soft, slippery and a good solid lubricant. Graphite is therefore not the second hardest substance, since its Mohs hardness is only about 1 to 2.

Graphene is a single layer of carbon atoms in a hexagonal lattice. It is the thinnest known material and is also among the strongest.

Fullerenes are cage-shaped molecules. The best-known example, Buckminster fullerene C_{60}, consists of 60 carbon atoms arranged in 12 pentagonal and 20 hexagonal rings, a pattern that resembles a football. Fullerenes are very light and strong and are recognised as an allotrope of carbon.

Diamond is not a good conductor of electricity, because all four valence electrons of each carbon atom are used in covalent bonds and no free electrons remain. Diamond is therefore an electrical insulator, in contrast to graphite.

Acids, bases and pH values

The pH scale runs from 0 to 14 and measures the hydrogen ion concentration of a solution. A pH below 7 indicates an acidic solution, a pH of 7 indicates a neutral solution, and a pH above 7 indicates a basic or alkaline solution. Values near 0 are strongly acidic and values near 14 are strongly alkaline.

Milk of magnesia is a suspension of magnesium hydroxide, Mg(OH)_2, in water. Magnesium hydroxide is a mild base that dissolves only slightly, so the suspension is alkaline with a pH of about 10. It is not strongly basic, so its pH is far below 14.

Goldsmiths clean gold and silver articles with nitric acid, which removes surface tarnish and base-metal impurities. Sulphuric, hydrochloric and phosphoric acids are not the standard cleaning agents for these articles.

Litmus is an indicator that changes colour only in aqueous acidic or basic solutions, in which it turns red in acid and blue in base. Ethanol is a neutral compound that does not release significant H^+ or OH^- ions, so it causes no change in the colour of either red or blue litmus. Bleaching is a different process from indicator behaviour and does not describe the action of ethanol.

Human blood keeps a pH of about 7.35 to 7.45 through buffer systems. The normal working range of the human body is therefore taken as 7.0 to 7.8. A pH below this range is called acidosis and a pH above it is called alkalosis, and both are harmful.

Indigestion is often caused by excess hydrochloric acid in the stomach. Antacids are mild bases, such as magnesium hydroxide or sodium bicarbonate, that neutralise this excess acid and relieve the discomfort. Analgesics relieve pain, antiseptics destroy microorganisms on living tissue, and antibiotics act against bacterial infections, so none of these treats acidity.

Separation techniques for mixtures

The choice of separation method depends on the physical properties of the components. Sublimation separates a solid that passes directly from the solid to the vapour state on heating from a non-volatile solid. Anthracene sublimes, while common salt does not, so the two can be separated by sublimation, with the anthracene vapour collected on a cold surface.

Fractional distillation separates liquids that have different boiling points. Crude petroleum is a mixture of hydrocarbons with different boiling ranges, and in a refinery its vapours are passed into a fractionating column, where the components condense at different heights. Kerosene and petrol are miscible liquids with different boiling points, so fractional distillation is the appropriate method for separating them, whereas a separating funnel cannot be used.

A separating funnel separates two immiscible liquids of different densities. Benzene and water form two distinct layers, and the lower water layer is drained off through the stopcock. A round bottom flask is used for heating and reactions, and a conical flask is used for holding and mixing liquids, so neither separates layers. A Dean and Stark apparatus removes water from a reaction mixture by azeotropic distillation.

Acetone and water are miscible, and their boiling points differ, so distillation separates them. Cream is separated from milk by centrifugation, which uses centrifugal force to separate components of different density. Plant pigments are separated by chromatography, which relies on differential adsorption, and kerosene and water are separated by a separating funnel because they are immiscible.

Simple evaporation removes a volatile solvent but cannot separate several components of different boiling points. Chromatography is a laboratory technique used mainly for small quantities of components.

Atomic structure and electron shells

The maximum number of electrons in a shell is given by 2n^2, where n is the principal quantum number. For the M shell, n = 3, so the capacity is 2 \times 3^2 = 18 electrons. The N shell, with n = 4, holds 2 \times 4^2 = 32 electrons.

An isoelectronic group is a set of atoms and ions that have the same number of electrons. Fluoride ion F^- has 9 protons and 10 electrons. The oxide ion O^{2-} (8 + 2 = 10), the sodium ion Na^+ (11 - 1 = 10) and neon (10) are all isoelectronic with it. The nitride ion N^-, which has 7 + 1 = 8 electrons, is not.

In a neutral atom, the atomic number Z equals both the number of protons and the number of electrons. The mass number A is the sum of protons and neutrons, so the number of neutrons is A - Z. For argon, with A = 40 and Z = 18, there are 18 protons, 18 electrons and 40 - 18 = 22 neutrons. The mass number is not equal to the sum of protons and electrons.

In the Bohr-Bury scheme, electrons are distributed in shells that fill in order. An element with atomic number 35, bromine, has the distribution 2, 8, 18, 7. The valence shell is the outermost shell, which therefore contains 7 electrons.

Solutions and mixtures

A solution is a homogeneous mixture of two or more substances, and it may be solid, liquid or gaseous. An alloy such as brass is a solid solution of metals, and air is a gaseous solution of nitrogen, oxygen and other gases. A pure compound such as sugar is not a solution by itself and forms one only when it dissolves in a solvent such as water. Milk is a colloidal dispersion rather than a true solution.

A solution conducts electricity only when it contains freely moving ions. Electrolytes such as copper sulphate, sodium chloride and sodium hydroxide dissociate into ions when dissolved, so their solutions conduct. Sugar dissolves as intact neutral molecules and does not ionise, so its solution contains no charge carriers and is a non-electrolyte.

A colloidal solution is a heterogeneous system in which particles of about 1 to 1000 nm in size are dispersed throughout a dispersion medium. Because the particles are small, they do not settle when the system is left undisturbed, and they scatter light, which produces the Tyndall effect. A colloid is nevertheless heterogeneous, since it contains two distinct phases, and it is not homogeneous.

A heterogeneous mixture has visibly distinct components or phases. Sand particles in water, undissolved sulphur in water and separate sugar and salt crystals all form heterogeneous mixtures. Sulphur dissolves completely in carbon disulphide to form a uniform single phase, so that mixture is homogeneous.

Valency of elements

Valency is the combining capacity of an element, and for main group elements it is generally related to the number of valence electrons. Elements in the same group usually have the same valency. Magnesium, calcium and barium are all Group 2 elements with two valence electrons, so each has a valency of 2. Sodium and potassium, by contrast, have valency 1.

In a covalent compound, each shared pair of electrons forms one covalent bond. The molecular formula of methanol is CH_3OH, which contains three C–H bonds, one C–O bond and one O–H bond. The total number of covalent bonds is therefore 3 + 1 + 1 = 5.

An element with a complete outer shell, such as neon, has a valency of 0. Magnesium has valency 2, nitrogen has valency 3 and silicon has valency 4. Arranging these in increasing order gives Ne < Mg < N < Si.

Natural acids in food

Many foods contain natural organic acids that give them their characteristic taste. Each food is associated with a particular acid. Acetic acid is the main acid of vinegar, tartaric acid is characteristic of tamarind and grapes, and lactic acid is the acid of sour milk and of curd. Standard textbook tables list oxalic acid as the acid associated with tomato.

Cut fruit such as apple turns brown because enzymes such as polyphenol oxidase convert phenolic compounds into quinones in the presence of oxygen. Lemon juice contains ascorbic acid (vitamin C), which is a reducing agent and an antioxidant. It reduces the coloured quinones back to colourless phenols and slows the enzyme, so lemon juice minimises browning. Table sugar and milk of magnesia have no such effect.

Curd is acidic because lactic acid is produced when milk ferments. Copper reacts with this acid and can form harmful copper compounds, so curd should not be kept in a copper vessel. Stainless steel, plastic and glass are chemically inert toward curd and are suitable containers.

Metal reactivity with water

A reactivity series arranges metals in decreasing order of their tendency to react with water, steam or dilute acids. Zinc and iron react with steam at high temperature, lead reacts only very slowly, and copper does not react with water. The order of decreasing reactivity is therefore zinc, iron, lead and copper.

Density determines whether a metal floats or sinks in water. Potassium has a density of about 0.86 g/cm³, which is less than that of water, so it floats while it reacts vigorously with cold water to liberate hydrogen. Magnesium, calcium and copper have densities greater than 1 g/cm³ and sink.

Potassium, calcium and sodium are highly reactive metals that react with cold water to liberate hydrogen and form the corresponding metal hydroxide. Iron is only moderately reactive and does not react with cold water to liberate hydrogen. It reacts only with steam at high temperature.

Soap and detergent cleansing action

Soaps are sodium or potassium salts of long-chain carboxylic acids, that is, fatty acids. They are not ammonium salts. Each soap molecule has a hydrophobic hydrocarbon tail and an ionic carboxylate head.

In water, soap molecules aggregate into micelles, with the hydrophobic tails directed inward and the ionic heads facing outward. Oil and dirt are trapped in the hydrophobic centre and are carried away with the water. Micelles are colloidal in size and scatter light, which is the Tyndall effect.

In hard water, soap reacts with calcium and magnesium ions to form insoluble salts, which appear as scum. This is why soap performs poorly in hard water.

Soap dissolved in water forms lyotropic liquid crystal phases, whose structure depends on the concentration of the solution. Thermotropic liquid crystals form on heating a pure substance within a temperature range, and metallotropic liquid crystals are metal-containing mesophases, so neither applies to soap in water. Soap and water give a colloidal system with aggregated micelles, not a homogeneous solution.

Common laboratory reactions and tests

A laboratory test identifies a substance by a characteristic observable change, such as a colour change, a precipitate, or the evolution of a gas. The classic test for carbon dioxide uses lime water, which is a clear aqueous solution of calcium hydroxide, \text{Ca(OH)}_2. When carbon dioxide is passed through it, insoluble calcium carbonate forms and the solution turns milky: \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3\downarrow + \text{H}_2\text{O}. The precipitate is a fine white solid, and this colour is the expected observation for this test.

The milkiness is temporary if carbon dioxide is passed in excess. The excess gas converts the insoluble carbonate into soluble calcium hydrogencarbonate, \text{Ca(HCO}_3)_2, and the solution becomes clear again. This is a separate second reaction and should not be confused with the first step.

Green, blue and brown precipitates are typical of certain transition metal compounds and do not arise from calcium carbonate.

• Lime water : clear solution of calcium hydroxide used to detect \text{CO}_2. Example: a gas that turns lime water milky is most likely carbon dioxide.
• Milky : describes the white precipitate of \text{CaCO}_3 suspended in the solution.

Physical states of elements

At ordinary room temperature and normal pressure, most elements are solids, a few are gases, and only two elements are liquids: mercury and bromine. Mercury is a metal that remains liquid at room temperature, while bromine is a non-metal liquid. The state of an element is decided by its melting and boiling points relative to room temperature, so an element with a melting point just above room temperature is still solid under ordinary conditions.

Gallium and caesium have melting points only slightly above room temperature, so they are solid in ordinary conditions. Students often lose marks by assuming that a metal with a low melting point is a liquid.

Compounds also have states. A compound can be a solid while its decomposition products are a liquid and a gas. For example, mercuric oxide, \text{HgO}, is a solid. When it is heated, it decomposes as 2\text{HgO} \xrightarrow{\Delta} 2\text{Hg} + \text{O}_2, giving liquid mercury and gaseous oxygen. The states of reactants and products must be judged separately.

• Liquid element at room temperature : mercury and bromine.
• Gas at room temperature : hydrogen, oxygen, nitrogen and the noble gases.

Isotopes and average atomic mass

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, and so have different mass numbers. Most naturally occurring elements are mixtures of isotopes, so the atomic mass given in the periodic table is an average. The average atomic mass is the abundance-weighted mean of the isotopic masses:

\bar{M} = \sum_i (\text{mass}_i \times \text{fractional abundance}_i)

If two isotopes occur in the ratio a : b, their fractional abundances are \dfrac{a}{a+b} and \dfrac{b}{a+b}. For example, if isotopes of mass 10 and 12 occur in the ratio 1 : 4, the average is \dfrac{1 \times 10 + 4 \times 12}{5} = 11.6. The average always lies closer to the mass of the more abundant isotope.

When the percentage abundance is unknown, let the percentage of the lighter isotope be x, so that the heavier isotope has 100 - x. For isotopes of mass 20 and 22 with an average of 20.4, the equation is 20x + 22(100 - x) = 2040. Solving gives x = 80, so the lighter isotope is 80% abundant and the heavier is 20% abundant.