Chemical Reactions and Equations

The balanced equation is
2 ext{Mg}+ ext{O}_2
ightarrow2 ext{MgO}
. One
ext{O}_2
molecule contains two oxygen atoms, so two Mg atoms form two units of MgO. Therefore, two magnesium atoms are required. Using only one magnesium atom would leave the equation unbalanced.

According to the law of conservation of mass, a balanced chemical equation has the same number of atoms of each element on the reactant and product sides. If the number of atoms of even one element differs, the equation is unbalanced. Options C and D only state the usual arrangement for writing an equation; they do not by themselves establish that it is balanced.

A subscript is part of a chemical formula and specifies the ratio of atoms in one molecule or formula unit. Changing H₂O to H₂O₂, for instance, changes water into hydrogen peroxide, a different substance. During balancing, only coefficients placed before formulae may be changed because they alter quantities without changing identities. Hence, changing subscripts causes the problem stated in option A.

When two or more reactants combine to form a single product, the reaction is a combination reaction. A hot container indicates that heat is released during the reaction, so it is also exothermic. In contrast, in a decomposition reaction, one compound breaks down into two or more substances.

The reaction is \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\). Calcium oxide and water combine to form only one product, calcium hydroxide, so it is a combination reaction. Heat is released, making it exothermic. In a decomposition reaction, one compound breaks down into two or more products, which does not occur here.

On heating, calcium carbonate undergoes thermal decomposition to form calcium oxide and carbon dioxide:
\(\mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}\).
Therefore, option A is correct. Option B is incorrect because calcium hydroxide and oxygen are not the products formed by heating calcium carbonate.

Lead nitrate undergoes thermal decomposition: \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). Here, \(\mathrm{NO_2}\) is the brown gas and \(\mathrm{PbO}\) is the solid residue. Option B is incorrect because calcium oxide is not formed during the decomposition of lead nitrate.

When lead nitrate is heated, it undergoes thermal decomposition: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. Heat is supplied as an external condition to provide the energy needed for decomposition; it is not one of the products. The reaction produces lead oxide, brown nitrogen dioxide gas, and oxygen. Therefore, option B correctly describes the role of heat.

The balanced electrolysis equation is \(2H_2O(l) \rightarrow 2H_2(g) + O_2(g)\). Thus, 2 moles of water produce 2 moles of hydrogen and 1 mole of oxygen. At the same temperature and pressure, gas volume is proportional to the number of moles, so the volume ratio of hydrogen to oxygen is \(2:1\). Option B is incorrect because, at the same temperature and pressure, gas volume depends on the number of moles, not on molecular mass.

Iron is more reactive than copper, so it displaces copper from copper sulphate: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). The iron sulphate solution is light green, and the reddish-brown deposit on the nail is copper. Unlike a combination reaction, two substances do not combine to form one product; iron replaces copper.

In sunlight, silver chloride undergoes photodecomposition: \(2\mathrm{AgCl}\xrightarrow{\text{sunlight}}2\mathrm{Ag}+\mathrm{Cl}_2\). The metallic silver formed is grey, so silver chloride appears grey. Chlorine is also produced, but it does not cause the grey colour.

Silver chloride is photosensitive. In sunlight, it undergoes photodecomposition to form silver and chlorine:
\[2\mathrm{AgCl}(s)\xrightarrow{\text{sunlight}}2\mathrm{Ag}(s)+\mathrm{Cl_2}(g)\]
A dark-coloured bottle reduces exposure to light and therefore helps prevent this decomposition. Option B refers to thermal decomposition, whereas light is the cause of decomposition in this case.

Silver chloride decomposes in sunlight to form silver and chlorine gas: \(2AgCl \xrightarrow{sunlight} 2Ag + Cl_2\). Therefore, it is a photochemical decomposition reaction. In options B and D, heat provides the energy for decomposition, whereas in option C, electricity does so.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution. The balanced reaction is Fe + CuSO₄ → FeSO₄ + Cu. The sulphate ion remains associated with iron, producing iron sulphate, while copper is deposited as a reddish-brown solid. Copper sulphate is the reactant, and sodium or barium sulphate is not formed. Therefore, option A is correct.

A displacement reaction occurs only when the metal placed in the solution is more reactive than the metal ion present in it. Zinc is above copper in the reactivity series, so copper is less reactive and \(\text{Cu} + \text{ZnSO}_4\) gives no reaction. Unlike option B, copper cannot displace zinc ions from zinc sulphate solution to form zinc metal.

The reaction is \(\mathrm{BaCl_2(aq)+Na_2SO_4(aq)\rightarrow BaSO_4(s)+2NaCl(aq)}\). In solution, \(\mathrm{Ba^{2+}}\) and \(\mathrm{SO_4^{2-}}\) ions combine to form insoluble \(\mathrm{BaSO_4}\), which separates as a solid precipitate. \(\mathrm{NaCl}\) remains soluble in water, so it does not form the precipitate.

In a double displacement reaction, the ions of two ionic compounds exchange partners. In option A, \(\mathrm{Na^+}\) and \(\mathrm{Ba^{2+}}\) form new ionic pairs with the anions, producing insoluble \(\mathrm{BaSO_4}\) as a precipitate. Option B is a single displacement reaction because zinc displaces copper from its salt.

In reaction A, Ba^{2+} and Na^+ exchange partners, so it is a double displacement reaction. Insoluble white BaSO_4 forms as a precipitate, so it is also a precipitation reaction. In contrast, option B is a combination/oxidation reaction of magnesium, not an exchange of ions.

The correct answer is copper. On heating, hydrogen removes oxygen from black copper(II) oxide, reducing it to copper: \(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\). The copper formed is reddish-brown. Copper(II) sulphate, chloride, or carbonate cannot form because no source of sulphate, chloride, or carbonate ions is present in the reaction.

The reaction is CuO + H₂ → Cu + H₂O. Hydrogen combines with oxygen from copper(II) oxide to form water, so hydrogen gains oxygen and is oxidised. Copper(II) oxide loses oxygen and becomes copper, so it is reduced. The copper and water shown among the options are products rather than the substance undergoing oxidation. Therefore, option B is correct.