Chemical Reactions and Equations

In the standard representation of a chemical reaction, the arrow points from reactants to products. Substances formed after the reaction are therefore written on the right side and are called products. Option C is correct. Reactants are the starting substances on the left, while solvents and catalysts may assist a reaction but are not the general name for substances produced by it.

A chemical equation represents reactants and products by their chemical formulae. The coefficients in a balanced equation also show the mole ratio of the substances. In contrast, a word equation usually gives only the names of the substances, so a chemical equation is more precise and useful.

To balance a chemical equation, coefficients are placed before chemical formulae. For example, in \(2H_2 + O_2 \rightarrow 2H_2O\), 2 is a coefficient and shows the number of molecules. Subscripts inside a formula, such as 2 in \(H_2\), must not be changed because doing so changes the formula and identity of the substance.

Chemical formulae show the fixed composition and identity of substances. If a subscript is changed while balancing, the ratio of atoms within a molecule changes and a different substance may be represented. Therefore option A is correct. A chemical equation must be balanced by changing only coefficients placed before formulae, because coefficients alter the number of particles without changing the substances themselves.

The governing classification depends on the number of reactants and products. In a combination reaction, two or more substances join to form a single product, represented generally as A + B → AB. Hence option B is correct. Decomposition has one reactant and several products; displacement replaces an element in a compound, while precipitation specifically involves formation of an insoluble solid from solutions.

The governing concept is decomposition: one compound is split into two or more simpler substances. Its general pattern is AB → A + B, and the change may be caused by heat, light, or electricity. Therefore option C is correct. Combination shows the opposite pattern, double displacement exchanges ions between compounds, and combustion is a reaction with oxygen that commonly releases heat and light.

On heating, calcium carbonate undergoes thermal decomposition. The balanced reaction is CaCO₃ → CaO + CO₂, with heat supplied as the condition. The gaseous product is therefore carbon dioxide, so option C is correct. Calcium oxide remains as the solid product. Hydrogen, oxygen, and nitrogen are not formed in this decomposition because none appears among the products of the balanced equation.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). The reddish-brown coating on the nail is copper. This is a displacement reaction, whereas a combination reaction forms one product from two or more reactants.

Calcium oxide reacts with water to form calcium hydroxide: CaO + H₂O → Ca(OH)₂ + heat. The important evidence is that heat is released and the mixture becomes warm, so the reaction is exothermic and option A is correct. An endothermic reaction would absorb heat and cool the surroundings. The light-driven and electricity-driven decomposition choices do not describe this heat-releasing reaction.

The governing concept is the classification of reactions by the number and nature of reactants and products. In \(2Mg + O_2 \rightarrow 2MgO\), magnesium and oxygen are two reactants, but they form only one compound, magnesium oxide. Therefore it is a combination reaction. It is not displacement or double displacement because no element or pair of ions is exchanged, and it is not decomposition because no single compound breaks apart.

On heating, lead nitrate breaks down into simpler substances:

eq

2 ext{Pb(NO}_3 ext{)}_2
ightarrow 2 ext{PbO} + 4 ext{NO}_2 + ext{O}_2

. Since this decomposition is caused by heating, it is thermal decomposition. Photolytic decomposition requires light energy, such as the decomposition of silver chloride in sunlight.

Silver chloride is a photosensitive compound. In sunlight, it undergoes photodecomposition: \(2\mathrm{AgCl}(s) \xrightarrow{\text{sunlight}} 2\mathrm{Ag}(s) + \mathrm{Cl}_2(g)\). The metallic silver formed is grey, so white silver chloride gradually appears grey. This change is not due to dissolving in water.

Silver chloride and silver bromide are light-sensitive salts. On exposure to light, they decompose to form metallic silver, producing a dark or grey appearance. This light-sensitive property is used to form images in traditional photographic films. Water purification generally uses methods such as chlorination or filtration, not the photolytic decomposition of these silver salts.

The balanced equation for electrolysis of water is \(2H_2O(l) \rightarrow 2H_2(g) + O_2(g)\). At the same temperature and pressure, gas volume is proportional to the number of moles, so the volume ratio of hydrogen to oxygen is \(2:1\). Therefore, hydrogen is obtained in the greater volume, while oxygen is produced in half that volume.

Iron is more reactive than copper in the reactivity series, so it displaces copper from copper sulphate: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). The iron sulphate formed makes the solution green, and the displaced copper deposits as a reddish-brown coating on the nail. In option B, an unchanged blue solution would not indicate this displacement reaction.

Iron is more reactive than copper. Therefore, iron displaces copper from copper sulphate solution to form iron sulphate: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). A reaction in which a more reactive metal replaces a less reactive metal from its salt solution is a displacement reaction. It is not a combination reaction because the reactants do not form a single product.

Iron is more reactive than copper, so it displaces copper from copper sulphate. The reaction is \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). The ferrous sulphate formed gives the solution a green colour. The reddish-brown colour belongs to copper deposited on the nail, not to the solution.

Zinc is more reactive than copper, so it displaces copper from copper sulphate solution. Copper is deposited on the zinc surface. The reaction is \(\mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}\). Zinc does not form the coating; it enters the solution as zinc sulphate.

In a displacement reaction, a more reactive metal can displace a less reactive metal from its salt solution. Zinc is above copper in the reactivity series, so copper cannot displace zinc from zinc sulphate solution. Option B is incorrect because copper is less reactive than zinc, not more reactive.

The defining concept of a double displacement reaction is the exchange of ions between two compounds, usually ionic compounds in aqueous solution. Its general pattern is \(AB + CD \rightarrow AD + CB\): the positive or negative ions change partners and two new compounds may form. Burning a metal is combustion, while melting and liquefaction are physical changes rather than ionic chemical reactions.

This is a double displacement reaction: \(\mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\). The \(\mathrm{Ba^{2+}}\) and \(\mathrm{SO_4^{2-}}\) ions combine to form insoluble barium sulphate, a white precipitate. Sodium chloride remains dissolved in water, so it does not form the precipitate.

A precipitation reaction commonly occurs when two aqueous solutions are mixed and one of the possible products is insoluble in water. That insoluble product separates from the solution as a solid and is called a precipitate. It may make the mixture cloudy or settle at the bottom. A gas, a liquid, or a substance that remains completely dissolved does not meet this definition.

The reaction is \(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\). Oxygen is removed from copper(II) oxide to form water, leaving copper behind. Removal of oxygen is reduction; therefore, \(\mathrm{CuO}\) is reduced. In contrast, hydrogen gains oxygen and is oxidised.

On exposure to air, magnesium ribbon develops a thin layer of magnesium oxide on its surface. This layer prevents direct contact between the metal and oxygen and hinders burning. Rubbing with sandpaper removes the layer and exposes fresh magnesium for combustion. It does not lower the ribbon’s melting point.

The governing concept is that oxidation and reduction are complementary processes. If one substance is oxidised, another substance in the same reaction is reduced; electrons, oxygen, hydrogen, or oxidation state may be used to describe the change. Such a reaction is called a redox reaction. Combustion and precipitation are particular reaction situations, while condensation is generally a change of state, so none of those names gives the required definition.