Checkpoint 2

• Q1 — C: The NESA activity series from most to least reactive: K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Ag, Au. Potassium (K) is the most reactive metal on this list.
• Q2 — B: Iron is above copper in the activity series → Fe displaces Cu from its salt solution. Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). Observations: red-brown copper metal deposits on the nail; the blue colour of the CuSO₄ solution fades as Cu²⁻ is removed from solution.
• Q3 — B: Magnesium goes from oxidation number 0 (in Mg metal) to +2 (in MgO). Increase in oxidation number = oxidation = loss of electrons (OIL). Magnesium is the reducing agent (it is oxidised and causes O₂ to be reduced). Option A incorrectly applies the definition — gaining oxygen IS oxidation, not reduction.
• Q4 — B: In NO₃⁻: charge = −1. O = −2 each × 3 = −6. N + (−6) = −1 → N = +5.
• Q5 — C: Fe²⁻ loses one electron to become Fe³⁻ — this is oxidation (OIL). A species that is oxidised acts as the reducing agent (it reduces something else by donating electrons). Oxidation occurs at the anode in a galvanic cell, not at the cathode.
• Q6 — B: At the anode (Zn), oxidation occurs: Zn → Zn²⁻ + 2e⁻. Electrons released at the zinc anode travel through the external wire to the copper cathode, where reduction occurs: Cu²⁻ + 2e⁻ → Cu. Electrons always flow from anode to cathode through the external circuit.
• Q7 — D: E°cell = E°cathode − E°anode = +0.34 − (−0.76) = +0.34 + 0.76 = +1.10 V. Never reverse the sign of the anode reduction potential before subtracting.
• Q8 — D: Inert electrodes (Pt, graphite) are used when the electrode material itself would otherwise react — for example, when the half-reaction involves only aqueous ions (no solid metal electrode species) or when an active metal electrode would dissolve into the electrolyte and interfere with the cell reaction.
• Q9 — B: A positive E°cell indicates the reaction is spontaneous under standard conditions. In a spontaneous galvanic cell, electrons flow spontaneously from the anode (oxidation) to the cathode (reduction) through the external circuit.
• Q10 — A: For Ag to be oxidised and Cu²⁻ to be reduced, E°cell = E°(Cu²⁻/Cu) − E°(Ag⁺/Ag) = +0.34 − 0.80 = −0.46 V. A negative E°cell means the reaction is non-spontaneous — no reaction occurs. Silver is BELOW copper in the activity series (less reactive), so it cannot displace copper from solution.

SA1: (a) Zinc is above silver in the NESA activity series → Zn will react with AgNO₃. Copper is also above silver → Cu will also react with AgNO₃. (b) Zn(s) + 2AgNO₃(aq) → Zn(NO₃)₂(aq) + 2Ag(s); Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s). In both cases, a silver-grey solid deposits on the metal strip and the metal ions in solution change: Zn²⁻ replaces Ag⁺ (colourless solution remains); Cu²⁻ replaces Ag⁺ (solution turns blue as Cu²⁻ forms).

SA2: (a) In MnO₄⁻: O = −2 × 4 = −8; overall charge = −1; Mn = −1 − (−8) = +7. In Mn²⁻: Mn = +2. (b) Mn decreases from +7 to +2 → reduced. Fe increases from +2 to +3 → oxidised. (c) Fe²⁻(aq) → Fe³⁻(aq) + e⁻ [Note: the permanganate half-reaction requires H⁺ and H₂O to balance in acidic solution; that full balancing is beyond this checkpoint scope, but the Fe²⁻ half-equation is straightforward].

SA3: (a) Anode = Zn (lower reduction potential, E° = −0.76 V; Zn is oxidised). Cathode = Ag (higher reduction potential, E° = +0.80 V; Ag⁺ is reduced). (b) E°cell = E°cathode − E°anode = +0.80 − (−0.76) = +1.56 V. E°cell > 0 → spontaneous. (c) Half-equations: Zn → Zn²⁻ + 2e⁻; 2 × (Ag⁺ + e⁻ → Ag). Overall: Zn(s) + 2Ag⁺(aq) → Zn²⁻(aq) + 2Ag(s).

SA4: (a) Sacrificial anode protection (a form of cathodic protection). (b) Mg has a more negative standard reduction potential (E° = −2.37 V) than Fe (E° = −0.44 V). This means Mg is more readily oxidised than Fe. When both metals are in electrical contact in the moist soil (acting as an electrolyte), Mg acts as the anode (is preferentially oxidised: Mg → Mg²⁻ + 2e⁻) and Fe is protected as the cathode — electrons flow from Mg to Fe rather than Fe being oxidised. E°cell = −0.44 − (−2.37) = +1.93 V > 0 confirms the process is spontaneous. (c) The magnesium blocks gradually corrode and dissolve as Mg is continuously oxidised. Over time they shrink and must be periodically inspected and replaced to maintain protection.