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Chemical Reactions and Equations — GK Questions with Answers

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36 questions
  1. Which of these is a chemical change?
    1. Melting of ice
    2. Milk turning into curd
    3. Sublimation of camphor
    4. Dissolving salt
    Show answer

    Answer: (B) Milk turning into curd

    Curd contains a new substance (lactic acid) and the change does not reverse — the other three are physical changes.

  2. On balancing Fe + H₂O ⟶ Fe₃O₄ + H₂, the coefficient of H₂O is?
    1. 2
    2. 3
    3. 6
    4. 4
    Show answer

    Answer: (D) 4

    3Fe + 4H₂O → Fe₃O₄ + 4H₂ — 4H₂O for the 4 oxygens of Fe₃O₄.

  3. Pouring water on quicklime is which type of reaction?
    1. Combination and exothermic
    2. Decomposition
    3. Displacement
    4. Endothermic
    Show answer

    Answer: (A) Combination and exothermic

    CaO + H₂O → Ca(OH)₂ — two substances make one, and a lot of heat comes out.

  4. Which reaction happens in black-and-white photography?
    1. Thermal decomposition of AgBr
    2. Electrolysis of AgNO₃
    3. Photolysis of AgBr
    4. Oxidation of Ag
    Show answer

    Answer: (C) Photolysis of AgBr

    2AgBr → 2Ag + Br₂ in light; black silver grains wherever light fell.

  5. Colour of the precipitate on mixing Pb(NO₃)₂ and KI solutions?
    1. White
    2. Blue
    3. Yellow
    4. Black
    Show answer

    Answer: (C) Yellow

    Pb(NO₃)₂ + 2KI → PbI₂↓ (bright yellow) + 2KNO₃ — double displacement/precipitation.

  6. An iron nail in CuSO₄ solution — colour of the solution?
    1. Blue to pale green
    2. Green to blue
    3. Colourless to blue
    4. No change
    Show answer

    Answer: (A) Blue to pale green

    Fe + CuSO₄ → FeSO₄ (pale green) + Cu — iron is more reactive than copper.

  7. In CuO + H₂ → Cu + H₂O, the oxidising agent is?
    1. H₂
    2. H₂O
    3. Cu
    4. CuO
    Show answer

    Answer: (D) CuO

    CuO gives oxygen and oxidises H₂, and is itself reduced to Cu.

  8. By the electronic definition, oxidation is —
    1. Gain of electrons
    2. Loss of electrons
    3. Gain of protons
    4. Loss of neutrons
    Show answer

    Answer: (B) Loss of electrons

    OIL RIG — Oxidation Is Loss of electrons; reduction is gain.

  9. Oxidation number of Cr in K₂Cr₂O₇?
    1. +6
    2. +3
    3. +7
    4. +12
    Show answer

    Answer: (A) +6

    2(+1) + 2x + 7(−2) = 0 → x = +6.

  10. Which of these is a disproportionation reaction?
    1. 2H₂ + O₂ → 2H₂O
    2. Zn + CuSO₄ → ZnSO₄ + Cu
    3. Cl₂ + 2NaOH → NaCl + NaOCl + H₂O
    4. HCl + NaOH → NaCl + H₂O
    Show answer

    Answer: (C) Cl₂ + 2NaOH → NaCl + NaOCl + H₂O

    Cl goes from 0 to −1 (NaCl) and +1 (NaOCl) — the same element in both directions.

  1. Average oxidation number of iron in Fe₃O₄?
    1. +2
    2. +8/3
    3. +3
    4. +4
    Show answer

    Answer: (B) +8/3

    3x − 8 = 0 → 8/3; really one Fe²⁺ and two Fe³⁺.

  2. Photosynthesis is which type of reaction?
    1. Exothermic
    2. Displacement
    3. Neutralisation
    4. Endothermic
    Show answer

    Answer: (D) Endothermic

    The plant absorbs the Sun’s energy to make glucose (ΔH ≈ +2808 kJ/mol); respiration is its reverse, exothermic.

  3. Why is nitrogen filled in chips packets?
    1. For taste
    2. To add moisture
    3. To add weight
    4. To stop rancidity (oxidation)
    Show answer

    Answer: (D) To stop rancidity (oxidation)

    N₂ is inert; with no oxygen the fat is not oxidised (no staleness).

  4. Unit of rate of reaction?
    1. mol L⁻¹
    2. mol L⁻¹ s⁻¹
    3. s⁻¹
    4. L mol⁻¹
    Show answer

    Answer: (B) mol L⁻¹ s⁻¹

    Change in concentration ÷ time = mol L⁻¹ s⁻¹; s⁻¹ is the unit of k for first order.

  5. The half-life of a first-order reaction is —
    1. Proportional to starting concentration
    2. Inversely proportional to concentration
    3. Independent of starting concentration
    4. Always 1 hour
    Show answer

    Answer: (C) Independent of starting concentration

    t½ = 0.693/k — no concentration in it; radioactive decay is the example.

  6. A first-order reaction has t½ = 20 minutes. How much is left after 80 minutes?
    1. 6.25%
    2. 12.5%
    3. 25%
    4. 50%
    Show answer

    Answer: (A) 6.25%

    80/20 = 4 half-lives → (½)⁴ = 1/16 = 6.25%.

  7. Which of these can be zero or a fraction?
    1. Molecularity
    2. Order
    3. Both
    4. Neither
    Show answer

    Answer: (B) Order

    Order comes from experiment (0, ½, 1.5…); molecularity is always a whole number.

  8. Raising the temperature by 10 °C makes the rate of most reactions —
    1. Half
    2. Unchanged
    3. Ten times
    4. About double
    Show answer

    Answer: (D) About double

    Temperature coefficient 2–3; the molecules with energy above Eₐ nearly double.

  9. Which statement about a catalyst is correct?
    1. It lowers the activation energy
    2. It lowers ΔH
    3. It raises the equilibrium constant
    4. It speeds up only the forward reaction
    Show answer

    Answer: (A) It lowers the activation energy

    A catalyst gives a path of lower Eₐ; forward and backward speed up equally, so K and ΔH stay the same.

  10. Relation of Kp and Kc for N₂ + 3H₂ ⇌ 2NH₃?
    1. Kp = Kc
    2. Kp = Kc(RT)²
    3. Kp = Kc(RT)⁻²
    4. Kp = Kc(RT)
    Show answer

    Answer: (C) Kp = Kc(RT)⁻²

    Δn = 2 − 4 = −2, so Kp = Kc(RT)⁻².

  1. By Le Chatelier, raising the pressure in the Haber process —
    1. Lowers NH₃
    2. Has no effect
    3. Raises NH₃
    4. Raises K
    Show answer

    Answer: (C) Raises NH₃

    The equilibrium shifts to fewer gas moles (2 mol NH₃); K changes only with temperature.

  2. Promoters used with the iron catalyst in the Haber process?
    1. K₂O and Al₂O₃
    2. V₂O₅
    3. Pt
    4. As₂O₃
    Show answer

    Answer: (A) K₂O and Al₂O₃

    K₂O and Al₂O₃ raise the activity of iron; As₂O₃ is a poison in the Contact process.

  3. Correct statement about physical adsorption?
    1. Only one layer
    2. Irreversible
    3. Highly specific
    4. By van der Waals forces, many layers
    Show answer

    Answer: (D) By van der Waals forces, many layers

    Physical adsorption is by weak forces, reversible, multi-layer and falls as temperature rises.

  4. Standard EMF of the Daniell cell?
    1. 0.76 V
    2. 1.10 V
    3. 1.36 V
    4. 2.00 V
    Show answer

    Answer: (B) 1.10 V

    E° = 0.34 − (−0.76) = 1.10 V; Zn anode, Cu cathode.

  5. In a galvanic cell the anode is —
    1. Negative, oxidation
    2. Positive, reduction
    3. Positive, oxidation
    4. Negative, reduction
    Show answer

    Answer: (A) Negative, oxidation

    In a galvanic cell the anode is negative; oxidation is at the anode in every cell.

  6. Strongest reducing agent in water?
    1. Na
    2. K
    3. Li
    4. Cs
    Show answer

    Answer: (C) Li

    Li has the most negative E° (−3.05 V) — the small Li⁺ has a very large hydration energy.

  7. CuSO₄ solution can be kept safely in a vessel of which metal?
    1. Zinc
    2. Silver
    3. Nickel
    4. Iron
    Show answer

    Answer: (B) Silver

    Ag (+0.80 V) cannot displace Cu²⁺ (+0.34 V); Zn, Fe and Ni can.

  8. The factor in the Nernst equation at 298 K is —
    1. 96500/n
    2. 0.693/n
    3. 2.303/n
    4. 0.0591/n
    Show answer

    Answer: (D) 0.0591/n

    E = E° − (0.0591/n) log Q.

  9. On dilution of an electrolyte —
    1. Both κ and Λm rise
    2. Both fall
    3. κ rises, Λm falls
    4. κ falls, Λm rises
    Show answer

    Answer: (D) κ falls, Λm rises

    With fewer ions per cm³ κ falls, but ions move more freely so Λm rises.

  10. How much silver does 1 faraday of charge deposit?
    1. 54 g
    2. 108 g
    3. 216 g
    4. 10.8 g
    Show answer

    Answer: (B) 108 g

    Ag⁺ + e⁻ → Ag; 1 mol e⁻ = 1 F = 1 mol Ag = 108 g.

  1. 2 A for 965 s through CuSO₄ — copper deposited?
    1. 0.3175 g
    2. 1.27 g
    3. 0.635 g
    4. 6.35 g
    Show answer

    Answer: (C) 0.635 g

    Q = 1930 C = 0.02 F → 0.01 mol Cu × 63.5 = 0.635 g.

  2. In the electrolysis of brine, what forms at the cathode?
    1. Hydrogen
    2. Chlorine
    3. Sodium
    4. Oxygen
    Show answer

    Answer: (A) Hydrogen

    In water solution, water is reduced instead of Na⁺: 2H₂O + 2e⁻ → H₂ + 2OH⁻.

  3. Why does a mercury cell keep a steady voltage all its life?
    1. Mercury is a liquid
    2. No ions in the overall reaction
    3. Zinc does not run out
    4. It contains acid
    Show answer

    Answer: (B) No ions in the overall reaction

    Zn + HgO → ZnO + Hg — no ion whose concentration changes, so ≈ 1.35 V stays steady.

  4. As a lead–acid battery discharges —
    1. The acid’s density rises
    2. H₂SO₄ forms
    3. PbO₂ increases
    4. The acid’s density falls
    Show answer

    Answer: (D) The acid’s density falls

    Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O — the acid is used up and water forms.

  5. In the Contact process, SO₃ is absorbed in —
    1. 98% H₂SO₄
    2. Water
    3. Dilute HCl
    4. NaOH
    Show answer

    Answer: (A) 98% H₂SO₄

    SO₃ + H₂SO₄ → H₂S₂O₇ (oleum); water would form a dense acid mist.

  6. Why does galvanising protect iron even when scratched?
    1. Zinc is less reactive
    2. Zinc is shiny
    3. Zinc is more reactive and corrodes first
    4. Zinc absorbs water
    Show answer

    Answer: (C) Zinc is more reactive and corrodes first

    Zinc becomes the sacrificial anode; with tinning the iron rusts fast once scratched.

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