Chemical Reactions and Equations — Important Questions

Answer: (b) $Fe + CuSO_4 \rightarrow FeSO_4 + Cu$

Iron is more reactive than copper, so it displaces copper from copper sulphate solution. Option (a) is combination, (c) is decomposition, and (d) is double displacement.

Answer: (c) Exothermic combination and redox reaction

Respiration releases energy (exothermic), combines glucose with oxygen, and involves oxidation of glucose, so it is a redox reaction as well.

Answer: (c) Copper ($Cu$) and copper oxide ($CuO$)

Copper is a shiny brown metal. On heating in air it reacts with oxygen to form a black layer of copper(II) oxide: $2Cu + O_2 \xrightarrow{\Delta} 2CuO$.

Answer: (a) $HCl$ is oxidised and $MnO_2$ is reduced

Chlorine in $HCl$ goes from the $-1$ state to $0$ in $Cl_2$ (loses electrons, oxidised), while manganese in $MnO_2$ goes from $+4$ to $+2$ in $MnCl_2$ (gains electrons, reduced). Hence $MnO_2$ is the oxidising agent and $HCl$ the reducing agent.

Answer: (c) A is true but R is false

Decomposition of $CaCO_3 \xrightarrow{\Delta} CaO + CO_2$ requires continuous heating, so it is endothermic (heat is absorbed, not released). The assertion is correct but the reason states the opposite.

Balanced equation:

$3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2$

Check: Fe = 3 on each side; H = 8 on each side; O = 4 on each side.

Balancing is based on the Law of Conservation of Mass, which states that mass can neither be created nor destroyed in a chemical reaction, so the total number of atoms of each element must be equal on both sides.

A redox reaction is one in which oxidation and reduction occur simultaneously — one substance is oxidised (gains oxygen / loses hydrogen or electrons) while another is reduced.

In $CuO + H_2 \rightarrow Cu + H_2O$:

• $H_2$ is oxidised to $H_2O$ (it gains oxygen).
• $CuO$ is reduced to $Cu$ (it loses oxygen).

Thus hydrogen is the reducing agent and copper oxide is the oxidising agent.

Corrosion: The slow deterioration of a metal surface by the action of air, moisture or chemicals around it. Example: rusting of iron to form a reddish-brown coating of hydrated iron(III) oxide, $Fe_2O_3 \cdot xH_2O$.
Prevention: painting, oiling/greasing, galvanisation (coating with zinc), or making alloys like stainless steel.

Rancidity: The oxidation of fats and oils in food on exposure to air, which produces an unpleasant smell and taste.
Prevention: adding antioxidants, flushing the packet with nitrogen gas, refrigeration, or storing in airtight containers.

(Both are consequences of oxidation reactions occurring in daily life.)

(a) Electrolysis of water — electrolytic decomposition (also endothermic):

$2H_2O \xrightarrow{\text{electricity}} 2H_2 + O_2$

(b) Quick lime with water — combination reaction (exothermic):

$CaO + H_2O \rightarrow Ca(OH)_2 + \text{heat}$

(c) Sunlight on silver chloride — photolytic decomposition:

$2AgCl \xrightarrow{\text{sunlight}} 2Ag + Cl_2$

White silver chloride turns grey as silver metal is set free; this reaction is used in black-and-white photography.

Observations:

1. The green colour of ferrous sulphate crystals changes to a brown solid (ferric oxide) as the water of crystallisation is first lost.
2. A characteristic smell of burning sulphur is noticed due to the gases $SO_2$ and $SO_3$ evolved.

Balanced equation:

$2FeSO_4 \xrightarrow{\Delta} Fe_2O_3 + SO_2 + SO_3$

Type of reaction: Thermal decomposition reaction (one reactant breaks down into several products on heating).

(a) A double displacement reaction is one in which two compounds exchange their ions to form two new compounds. When one of the products is insoluble, it separates out as a precipitate.

Example: $Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s)\downarrow + 2NaCl(aq)$

Here $BaSO_4$ is a white precipitate.

(b) Balanced equations:
(i) $2Pb(NO_3)_2 \rightarrow 2PbO + 4NO_2 + O_2$
(ii) $2NaOH + H_2SO_4 \rightarrow Na_2SO_4 + 2H_2O$

(c) In $Pb(NO_3)_2 + 2KI \rightarrow PbI_2\downarrow + 2KNO_3$, an insoluble yellow solid (lead iodide) is formed. Because an insoluble solid (a precipitate) separates out from the solution, it is called a precipitation reaction. It is also a double displacement reaction.

(a) Combination reaction — two or more substances combine to form a single product:
$2H_2 + O_2 \rightarrow 2H_2O$

(b) Thermal decomposition — a single compound breaks up on heating:
$CaCO_3 \xrightarrow{\Delta} CaO + CO_2$

(c) Displacement reaction — a more reactive metal displaces a less reactive one:
$Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$

(d) Double displacement reaction — exchange of ions between two compounds:
$Na_2SO_4 + BaCl_2 \rightarrow BaSO_4\downarrow + 2NaCl$

(e) Exothermic reaction — a reaction that releases heat:
$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat}$
(Combustion, respiration and the reaction of quick lime with water are common exothermic examples.)

(a) $CaO + H_2O \rightarrow Ca(OH)_2$
This is a combination reaction (two reactants form a single product).

(b) The reaction is exothermic — a large amount of heat is released as slaked lime is formed, so the bucket and mixture become warm.

(c) $Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$

(d) The shine is due to the thin layer of calcium carbonate ($CaCO_3$) formed on the walls.