Q1) Choose the correct option from the bracket and explain

Options: (Oxidation, displacement, electrolysis, reduction, zinc, copper, double displacement, decomposition)

1. a) To prevent rusting, a layer of zinc metal is applied on iron sheets. (Galvanization: Zn is more reactive and protects Fe.)
2. b) Conversion of ferrous sulphate to ferric sulphate is an oxidation reaction. (\(\mathrm{Fe^{2+} \to Fe^{3+} + e^-}\): loss of electron.)
3. c) When electric current is passed through acidulated water, electrolysis of water takes place. (\(\mathrm{2H_2O \to 2H_2 + O_2}\))
4. d) Mixing \(\mathrm{ZnSO_4(aq)}\) with \(\mathrm{BaCl_2(aq)}\) gives white \(\mathrm{BaSO_4}\) ppt — a double displacement reaction.
   \(\mathrm{Ba^{2+} + SO_4^{2-} \rightarrow BaSO_4(s)\downarrow}\)

Q2) Write answers

a) Name the reaction with simultaneous oxidation and reduction. Example.

Redox reaction — oxidation and reduction occur together.

Here, \(\mathrm{CuO}\) is reduced to Cu (loss of O) and \(\mathrm{H_2}\) is oxidized to \(\mathrm{H_2O}\) (gain of O).

b) How to increase the rate of decomposition of \(\mathrm{H_2O_2}\)?

• Add a catalyst like \(\mathrm{MnO_2}\) or KI (most effective & common in lab).
• Increase temperature (within safe limits).
• Use light (photochemical decomposition).

c) Reactant and product (with examples)

Reactants are starting substances; products are substances formed.

Reactants: \(\mathrm{AgNO_3}\), \(\mathrm{NaCl}\). Products: \(\mathrm{AgCl}\), \(\mathrm{NaNO_3}\).

d) Types of reactions with reference to oxygen & hydrogen

• Oxidation: addition of O / removal of H / loss of e\(^-\).
  \(\mathrm{C + O_2 \rightarrow CO_2}\), \(\ \mathrm{MgH_2 \rightarrow Mg + H_2}\) (dehydrogenation)
• Reduction: addition of H / removal of O / gain of e\(^-\).
  \(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\)

e) Similarity & difference: adding \(\mathrm{NaOH}\) to water vs. adding \(\mathrm{CaO}\) to water

• Similarity: Both are exothermic (temperature rises).
• Difference:
  • \(\mathrm{NaOH + H_2O}\): dissolution (physical process) with heat.
  • \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\): chemical reaction (combination) with heat.

Q3) Explain terms with examples

• a) Endothermic reaction: absorbs heat.
  \(\mathrm{CaCO_3(s) \xrightarrow{\ \Delta\ } CaO(s) + CO_2(g)}\)
• b) Combination reaction: two or more reactants \(\to\) one product.
  \(\mathrm{2Mg + O_2 \rightarrow 2MgO}\)
• c) Balanced equation: atoms of each element equal on both sides.
  \(\mathrm{2NaOH + H_2SO_4 \rightarrow Na_2SO_4 + 2H_2O}\)
• d) Displacement reaction: a more reactive element displaces a less reactive one.
  \(\mathrm{CuSO_4(aq) + Fe(s) \rightarrow FeSO_4(aq) + Cu(s)}\)

Q4) Scientific reasons

1. a) Heating limestone (\(\mathrm{CaCO_3}\)) gives \(\mathrm{CO_2}\). Passing it through fresh lime water forms insoluble \(\mathrm{CaCO_3}\) (milky).
   \(\mathrm{CaCO_3(s) \xrightarrow{\ \Delta\ } CaO(s) + CO_2(g)}\)
   \(\mathrm{Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s)\downarrow + H_2O(l)}\)
2. b) Powder of Shahabad tile (fine \(\mathrm{CaCO_3}\)) reacts faster with HCl than pieces due to greater surface area (higher rate).
   \(\mathrm{CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + CO_2(g) + H_2O(l)}\)
3. c) Diluting conc. \(\mathrm{H_2SO_4}\) is highly exothermic. Adding acid slowly to water allows heat to dissipate safely; adding water to acid can cause splashing/boiling.
4. d) Oils become rancid by air oxidation. Airtight containers limit oxygen contact (and often light), slowing oxidation — longer shelf life.

Q5) Rusting picture — write the reaction with explanation

Under a water drop on iron, tiny regions act as electrodes:

Then:

Explanation: Differential aeration inside the water drop makes a tiny electrochemical cell; iron is oxidized and hydrated iron oxides (rust) deposit.

Q6) Identify oxidation & reduction

1. a) \(\mathrm{Fe + S \rightarrow FeS}\): Fe is oxidized (0→+2), S is reduced (0→−2).
2. b) \(\mathrm{2Ag_2O \rightarrow 4Ag + O_2}\): In \(\mathrm{Ag_2O}\), \(\mathrm{Ag^+}\) is reduced to Ag(0); \(\mathrm{O^{2-}}\) is oxidized to \(\mathrm{O_2}\).
3. c) \(\mathrm{2Mg + O_2 \rightarrow 2MgO}\): Mg is oxidized; \(\mathrm{O_2}\) is reduced.
4. d) \(\mathrm{NiO + H_2 \rightarrow Ni + H_2O}\): \(\mathrm{NiO}\) (Ni\(^{2+}\)) is reduced to Ni; \(\mathrm{H_2}\) is oxidized to \(\mathrm{H_2O}\).

Q7) Balance the equations stepwise

a) \(\mathrm{H_2S_2O_7(l) + H_2O(l) \rightarrow H_2SO_4(l)}\)

1. Count atoms (LHS): H=4, S=2, O=8.
2. Make RHS S=2 by coefficient 2: \(\mathrm{2H_2SO_4}\).
3. Check H (4) and O (8): balanced.

b) \(\mathrm{SO_2(g) + H_2S(aq) \rightarrow S(s) + H_2O(l)}\)

1. Balance S using inspection: put 2 before \(\mathrm{H_2S}\) and 3 before S.
2. Balance H by putting 2 before \(\mathrm{H_2O}\).

c) \(\mathrm{Ag(s) + HCl(aq) \rightarrow AgCl + H_2}\)

d) \(\mathrm{NaOH(aq) + H_2SO_4(aq) \rightarrow Na_2SO_4(aq) + H_2O(l)}\)

1. Balance Na with coefficient 2 on \(\mathrm{NaOH}\).
2. Balance H/O by putting 2 on \(\mathrm{H_2O}\).

Q8) Identify endothermic vs exothermic

• a) \(\mathrm{HCl + NaOH \rightarrow NaCl + H_2O + \text{heat}}\) — Exothermic (neutralization).
• b) \(\mathrm{2KClO_3(s) \xrightarrow{\ \Delta\ } 2KCl(s) + 3O_2}\) — Endothermic (requires heat).
• c) \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2 + \text{heat}}\) — Exothermic.
• d) \(\mathrm{CaCO_3(s) \xrightarrow{\ \Delta\ } CaO(s) + CO_2}\) — Endothermic.

Q9) Match the columns — Correct pairing