Reactivity, extraction, electrolysis MCQs

Q1. The two metals X and Y form oxides XO and YO respectively. ΔG° for the formation of XO at 1500 K is −300 kJ/mol of O2, whereas ΔG° for YO at the same T is −500 kJ/mol of O2. Which of the following deductions is correct?

• A.Y can reduce XO to X spontaneously at 1500 K.
• B.X can reduce YO to Y spontaneously at 1500 K.
• C.Both reductions are equally favourable.
• D.Neither reduction is thermodynamically possible.

Answer: A. Y can reduce XO to X spontaneously at 1500 K.

Diagnose the question type - a typical chemistry numerical. The data on the table: 1500 K, 300 kJ, 500 kJ. We are after the quantity the stem asks for. Tool of choice - ΔG° = ΔG°(YO) − ΔG°(XO) = −500 − (−300) = −200 kJ/mol < 0, so it is spontaneous. Rearrange it for the unknown before substituting. Numbers in: ΔG° = ΔG°(YO) − ΔG°(XO) = −500 − (−300) = −200 kJ/mol < 0, so it is spontaneous. Common-sense check: The metal whose oxide has the MORE negative ΔG° (i.e. Lock in option A) Y can reduce XO to X spontaneously at 1500 K.

Q2. Which is true about the electrolysis of brine (concentrated aqueous NaCl)?

• A.Cl2 at anode, H2 at cathode, NaOH in solution
• B.O2 at anode, Na at cathode
• C.Cl2 at anode, Na at cathode
• D.H2 at anode, Cl2 at cathode

Answer: A. Cl2 at anode, H2 at cathode, NaOH in solution

What's the underlying rule? Concentrated NaCl(aq) → at anode Cl- preferentially oxidised over OH- (overpotential effect) giving Cl2; at cathode H2O reduced to H2 (Na is too reactive). Solution becomes basic with NaOH (chlor-alkali process). Pick: A) Cl2 at anode, H2 at cathode, NaOH in solution.

Q3. From the reactivity series K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Hg > Ag > Au, which of the following reactions will NOT occur spontaneously?

• A.Cu + 2AgNO3 → Cu(NO3)2 + 2Ag - and Fe + CuSO4 → FeSO4 + Cu - and Zn + Pb(NO3)2 → Zn(NO3)2 + Pb are all spontaneous, but Cu + ZnSO4 → CuSO4 + Zn does NOT
• B.Cu + 2AgNO3 → Cu(NO3)2 + 2Ag
• C.Fe + CuSO4 → FeSO4 + Cu
• D.Zn + Pb(NO3)2 → Zn(NO3)2 + Pb

Answer: A. Cu + 2AgNO3 → Cu(NO3)2 + 2Ag - and Fe + CuSO4 → FeSO4 + Cu - and Zn + Pb(NO3)2 → Zn(NO3)2 + Pb are all spontaneous, but Cu + ZnSO4 → CuSO4 + Zn does NOT

Which principle settles this? A metal displaces another only if it is higher (more reactive) in the series. Cu is below Zn, so Cu cannot displace Zn from ZnSO4. The other three obey the reactivity order. Pick: A) Cu + 2AgNO3 → Cu(NO3)2 + 2Ag - and Fe + CuSO4 → FeSO4 + Cu - and Zn + Pb(NO3)2 → Zn(NO3)2 + Pb are all spontaneous, but Cu + ZnSO4 → CuSO4 + Zn does NOT.