Chemical Bonding and Reactions: Types, Rules and Facts

The correct answer is B. In a double displacement reaction two compounds exchange their ions, so both reactants must be compounds. Here hydrogen and oxygen are elements combining to form a single product, water, which makes it a combination reaction and also a combustion and redox reaction, but certainly not a double displacement. Option A is wrong as a choice because barium chloride and sulphuric acid swap partners to give a white precipitate of barium sulphate and hydrochloric acid, the classic precipitation type of double displacement. Option C is wrong because copper sulphate and hydrogen sulphide exchange ions to give black copper sulphide and sulphuric acid. Option D is wrong because sodium hydroxide and hydrochloric acid exchange ions to give salt and water, which is neutralisation, a special case of double displacement. The test to apply is simple: look for two compounds exchanging partners.

The correct answer is D, ethylene. Ethylene, also called ethene, has the formula C2H4, and its two carbon atoms share two pairs of electrons, which is a carbon-carbon double bond made of one sigma and one pi bond; it is the simplest alkene. Option A is wrong because ethane, C2H6, is an alkane in which the two carbons are joined by a single bond only. Option B is wrong because methane, CH4, has just one carbon atom joined to four hydrogens by single bonds and can have no carbon-carbon bond at all. Option C is wrong because acetylene, or ethyne, C2H2, has a triple bond, three shared pairs made of one sigma and two pi bonds, not a double bond. Learn the family in order: ethane single, ethylene double and acetylene triple, and remember that the double and triple bonded compounds are unsaturated and decolourise bromine water.

The correct answer is A, 12 and 3. Benzene has the formula C6H6 and a ring of six carbon atoms, each carrying one hydrogen. Count the sigma bonds first: six carbon-carbon bonds around the ring and six carbon-hydrogen bonds give twelve sigma bonds. The ring also carries three carbon-carbon double bonds in the Kekule structure, and since every double bond is one sigma plus one pi, the extra component is three pi bonds. Option B is wrong because three counts only the pi bonds twice over and ignores the twelve sigma framework. Option C is wrong because six counts only the carbon-carbon sigma bonds and leaves out the six carbon-hydrogen bonds. Option D is wrong because nine is simply a distractor with no basis in the structure. The general rule to carry into the exam is that a single bond is one sigma, a double bond one sigma and one pi, and a triple bond one sigma and two pi.

The correct answer is A, aluminium oxide. An amphoteric oxide reacts with both acids and bases to give a salt and water. Aluminium oxide dissolves in hydrochloric acid to give aluminium chloride and in sodium hydroxide to give sodium aluminate, so it behaves as a base in one case and as an acid in the other. Zinc oxide behaves the same way. Option B is wrong because iron oxide is a basic oxide, reacting with acids but not with alkalis. Option C is wrong because magnesium oxide is strongly basic; dissolved in water it gives an alkaline solution and it reacts only with acids. Option D is wrong because carbon dioxide is a typical acidic oxide of a non-metal, giving carbonic acid with water and reacting with bases such as lime water. The two amphoteric oxides most often asked for are those of aluminium and zinc.

The correct answer is B, Ni, that is, nickel. Vegetable oils contain unsaturated fatty acids with carbon-carbon double bonds, and when hydrogen is passed through the oil at about 60 degrees Celsius in the presence of finely divided nickel, the double bonds add hydrogen and become saturated, turning the liquid oil into semi-solid vanaspati. Option A is wrong because lead is not a hydrogenation catalyst; it is in fact used as a catalyst poison in some reactions. Option C is wrong because hydrogen is the reactant being added across the double bond, not the catalyst, and the question asks for the substance that speeds the reaction without being consumed. Option D is wrong because helium is a noble gas, chemically inert, and takes no part in any such reaction. Remember the set of industrial catalysts: nickel for hydrogenation, iron for the Haber process and vanadium pentoxide for the contact process.

The correct answer is B, ionic bond. In an ionic or electrovalent bond a metal atom loses one or more electrons to a non-metal atom; the metal becomes a positive ion and the non-metal a negative ion, and the electrostatic attraction between the two holds the compound together, as in sodium chloride and magnesium oxide. Option A is wrong because a covalent bond is formed by the sharing of electron pairs between two atoms, typically two non-metals, with no transfer at all. Option C is wrong because a coordinate bond is a special covalent bond in which both shared electrons come from one atom, as in the ammonium ion; the electrons are still shared, not transferred. Option D is wrong because a hydrogen bond is a weak attraction between a hydrogen atom already bonded to a very electronegative atom and a lone pair on another such atom, and it is not a true chemical bond of this kind.

The correct answer is C, three. A nitrogen atom has five valence electrons and needs three more to complete its octet, so two nitrogen atoms share three pairs of electrons and form a triple bond, written N to N with three lines. That triple bond, made of one sigma and two pi bonds, is very strong, which is why nitrogen gas is so unreactive that it is used to fill food packets and to provide an inert atmosphere. Option A is wrong because one shared pair would make a single bond, as in the hydrogen or chlorine molecule. Option B is wrong because two shared pairs make a double bond, as in the oxygen molecule or in ethylene. Option D is wrong because no ordinary covalent molecule of nitrogen shares four pairs; the octet rule allows at most three here. Remember the set: hydrogen single, oxygen double, nitrogen triple.

The correct answer is B, hydrogen. Zinc is above hydrogen in the reactivity series, so it displaces hydrogen from a dilute acid: zinc and dilute hydrochloric acid give zinc chloride and hydrogen gas. This is a displacement reaction, and the gas is tested by bringing a burning splinter to the mouth of the tube, where it burns with a pop sound. Option A is wrong because oxygen is not produced by a metal-acid reaction; it comes from the decomposition of substances such as hydrogen peroxide or potassium chlorate. Option C is wrong because carbon dioxide is released when an acid reacts with a carbonate or a bicarbonate, not with a metal. Option D is wrong because chlorine is not set free here; the chloride ion goes into the salt, zinc chloride. The general rule to remember is that a metal above hydrogen in the reactivity series plus a dilute acid gives a salt and hydrogen.

The correct answer is B, oxidation. Rusting is a slow oxidation in which iron combines with oxygen in the presence of moisture to form hydrated iron oxide, the reddish-brown flaky substance we call rust. Both air and water must be present, which is why iron does not rust in dry air or in boiled water free of dissolved oxygen, and why it is protected by painting, oiling, galvanising with zinc or alloying into stainless steel. Option A is wrong because reduction is the removal of oxygen or the gain of electrons, the opposite of what happens to iron here. Option C is wrong because neutralisation is the reaction between an acid and a base to give salt and water, which has nothing to do with rusting. Option D is wrong because decomposition means one compound splitting into simpler substances, whereas rusting builds a new compound. Rusting is one form of corrosion.

The correct answer is C, tetrahedral. In methane the carbon atom is joined to four hydrogen atoms by four single bonds and has no lone pair, so the four bonding pairs push one another as far apart as possible and point to the corners of a regular tetrahedron, with bond angles of about 109.5 degrees. Option A is wrong because a linear shape, with a bond angle of 180 degrees, belongs to molecules such as carbon dioxide and acetylene. Option B is wrong because a trigonal planar shape, with angles of 120 degrees, belongs to molecules such as boron trifluoride, which has only three bonding pairs. Option D is wrong because a pyramidal shape belongs to ammonia, where one lone pair on nitrogen pushes the three bonds down to an angle of about 107 degrees. Water, with two lone pairs, is bent at about 104.5 degrees; learn these four shapes together.