Mr Cole Chemistry

Writing Equations From the Electrochemical Series Questions

EWriting Equations From the Electrochemical Series

Electrochemical Series Worksheet

You need to use the Electrochemical Series Data to answer these questions.

1. Combinations of Half-Cells

For the following combinations of half-cells, write the equation that will occur.
a) F₂ / F⁻ half cell and Ni²⁺ / Ni
F₂ + Ni → 2F⁻ + Ni²⁺
b) V³⁺ / V²⁺ and MnO₄⁻ / Mn²⁺
MnO₄⁻ + 8H⁺ + 5V²⁺ → Mn²⁺ + 4H₂O + 5V³⁺
c) Al³⁺ / Al and Cu²⁺ / Cu
2Al + 3Cu²⁺ → 2Al³⁺ + 3Cu
d) MnO₄⁻ / Mn²⁺ and Pb⁴⁺ / Pb²⁺
2Mn²⁺ + 8H₂O + 5Pb⁴⁺ → 5Pb²⁺ + 16H⁺ + 2MnO₄⁻
e) I₂ / I⁻ and Cr₂O₇²⁻ / Cr³⁺
Cr₂O₇²⁻ + 14H⁺ + 6I⁻ → 2Cr³⁺ + 7H₂O + 3I₂

2. Will the reaction occur?

Will the following reactions occur? If so, write the ionic equation for the reaction.
a) Sn⁴⁺ and MnO₂

No.

Sn⁴⁺ can only be reduced. For a reaction to occur, there would need to be a MnO₂ half-equation where MnO₂ acts as a reducing agent (is oxidised) and has an Eo value lower than +0.15V. No such half-cell exists in the data.

b) Cl₂ and V²⁺

Yes.

V²⁺ acts as a reducing agent. Chlorine is a very strong oxidising agent (+1.36V), which is sufficient to oxidise V²⁺ all the way to dioxovanadium(V) ions (VO₂⁺), which has an electrode potential of +1.00V.

2V²⁺ + 2Cl₂ + 2H₂O → 4Cl⁻ + 2VO₂⁺ + 4H⁺
c) VO₃⁻ and Cr

Yes.

Chromium is a strong reducing agent. It reduces VO₃⁻ (vanadate(V)) down to V²⁺, while the Chromium is oxidised to Cr³⁺.

VO₃⁻ + 6H⁺ + Cr → V²⁺ + 3H₂O + Cr³⁺
d) [Ni(NH₃)₆]²⁺ and Mn²⁺

No reaction.

[Ni(NH₃)₆]²⁺ has an Eo of -0.51V. If it acts as an oxidising agent (is reduced), the Mn²⁺ half cell would need to provide electrons. However, Mn²⁺ is very stable and cannot be oxidised by such a weak oxidising agent.

e) Ag⁺ and Cr₂O₇²⁻

No reaction.

Both Ag⁺ and Cr₂O₇²⁻ are oxidising agents. Neither can act as a reducing agent for the other.

3. Reagent Choices

Which acid(s) out of HCl, H₂SO₄ and HNO₃ can oxidise Fe²⁺ to Fe³⁺?

HNO₃ only.

  • HCl: No. Cl⁻ is a reducing agent, not an oxidising agent.
  • H₂SO₄: No. Although sulfate(VI) can be reduced, its Eo is lower than the +0.77V required to oxidise Fe²⁺.
  • HNO₃: Yes. NO₃⁻ can be reduced to NO₂ or HNO₂ as these half cells have Eo values higher than +0.77V.
Suggest a reagent that could oxidise V to V²⁺ but not to V³⁺.

You need a reagent with an electrode potential between -1.20V and -0.26V.

  • Anything higher than -1.20V will oxidise V to V²⁺.
  • Anything higher than -0.26V would further oxidise V²⁺ to V³⁺.

Examples: Cr³⁺ (-0.41V) or Fe(OH)₃.

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