Chemical Reactions and Equations — CBSE Class 10 Science Important Questions

Answer: (b) $Fe + CuSO_4 \rightarrow FeSO_4 + Cu$Fe + CuSO₄ → FeSO₄ + 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 redox reaction

Respiration releases energy (exothermic), and glucose gains oxygen, so it is oxidised (redox). It is not a combination reaction, because it forms two products, not one.

Answer: (c) Copper ($Cu$Cu) and copper oxide ($CuO$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$2Cu + O₂ → (Δ) 2CuO.

Answer: (a) $HCl$HCl is oxidised and $MnO_2$MnO₂ is reduced

$HCl$HCl loses hydrogen to form $Cl_2$Cl₂, so it is oxidised. $MnO_2$MnO₂ loses oxygen to form $MnCl_2$MnCl₂, so it is reduced. Hence $MnO_2$MnO₂ is the oxidising agent and $HCl$HCl the reducing agent.

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

Decomposition of $CaCO_3 \xrightarrow{\Delta} CaO + CO_2$CaCO₃ → (Δ) CaO + CO₂ 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$3Fe + 4H₂O → Fe₃O₄ + 4H₂

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 $CuO + H_2 \rightarrow Cu + H_2O$CuO + H₂ → Cu + H₂O:

• $H_2$H₂ is oxidised to $H_2O$H₂O (it gains oxygen).
• $CuO$CuO is reduced to $Cu$Cu (it loses oxygen).

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

(a) To remove the layer of magnesium oxide on its surface, which would stop the ribbon from burning readily.

(b) The ribbon burns with a dazzling white flame, and a white powder (magnesium oxide) is left behind.

(c) $2Mg + O_2 \rightarrow 2MgO$2Mg + O₂ → 2MgO

This is a combination reaction (two reactants form a single product). It is also exothermic, and magnesium is oxidised.

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

$2H_2O \xrightarrow{\text{electricity}} 2H_2 + O_2$2H₂O → (electricity) 2H₂ + O₂

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

$CaO + H_2O \rightarrow Ca(OH)_2 + \text{heat}$CaO + H₂O → Ca(OH)₂ + heat

(c) Sunlight on silver chloride — photolytic decomposition:

$2AgCl \xrightarrow{\text{sunlight}} 2Ag + Cl_2$2AgCl → (sunlight) 2Ag + Cl₂

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

Observations:

1. The green crystals first lose their water of crystallisation and turn white; on stronger heating they decompose, leaving a reddish-brown solid (ferric oxide, $Fe_2O_3$Fe₂O₃).
2. A characteristic smell of burning sulphur is noticed due to the gases $SO_2$SO₂ and $SO_3$SO₃ evolved.

Balanced equation:

$2FeSO_4 \xrightarrow{\Delta} Fe_2O_3 + SO_2 + SO_3$2FeSO₄ → (Δ) Fe₂O₃ + SO₂ + SO₃

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: $Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s)\downarrow + 2NaCl(aq)$Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq)

Here $BaSO_4$BaSO₄ is a white precipitate.

(b) Balanced equations:
(i) $2Pb(NO_3)_2 \rightarrow 2PbO + 4NO_2 + O_2$2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
(ii) $2NaOH + H_2SO_4 \rightarrow Na_2SO_4 + 2H_2O$2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

(c) In $Pb(NO_3)_2 + 2KI \rightarrow PbI_2\downarrow + 2KNO_3$Pb(NO₃)₂ + 2KI → PbI₂↓ + 2KNO₃, 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$2H₂ + O₂ → 2H₂O

(b) Thermal decomposition — a single compound breaks up on heating:
$CaCO_3 \xrightarrow{\Delta} CaO + CO_2$CaCO₃ → (Δ) CaO + CO₂

(c) Displacement reaction — a more reactive metal displaces a less reactive one:
$Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$Zn + CuSO₄ → ZnSO₄ + Cu

(d) Double displacement reaction — exchange of ions between two compounds:
$Na_2SO_4 + BaCl_2 \rightarrow BaSO_4\downarrow + 2NaCl$Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl

(e) Exothermic reaction — a reaction that releases heat:
$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat}$CH₄ + 2O₂ → CO₂ + 2H₂O + heat
(Combustion, respiration and the reaction of quick lime with water are common exothermic examples.)

(a) $CaO + H_2O \rightarrow Ca(OH)_2$CaO + H₂O → 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) $Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

(d) The shine is due to the thin layer of calcium carbonate ($CaCO_3$CaCO₃) formed on the walls.