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Difficulty: EasyGalvanic Cells and Standard Electrode Potentials

Given the standard reduction potentials (E∘E^\circ) below, arrange the metals in order of increasing reducing strength (from weakest reducing agent to strongest reducing agent):

Ag(aq)++e−→Ag(s)E∘=+0.80 VCu(aq)2++2e−→Cu(s)E∘=+0.34 VFe(aq)2++2e−→Fe(s)E∘=−0.44 VZn(aq)2++2e−→Zn(s)E∘=−0.76 V\begin{aligned} \text{Ag}^+_{(\text{aq})} + \text{e}^- &\rightarrow \text{Ag}_{(\text{s})} \quad E^\circ = +0.80\text{ V} \\ \text{Cu}^{2+}_{(\text{aq})} + 2\text{e}^- &\rightarrow \text{Cu}_{(\text{s})} \quad E^\circ = +0.34\text{ V} \\ \text{Fe}^{2+}_{(\text{aq})} + 2\text{e}^- &\rightarrow \text{Fe}_{(\text{s})} \quad E^\circ = -0.44\text{ V} \\ \text{Zn}^{2+}_{(\text{aq})} + 2\text{e}^- &\rightarrow \text{Zn}_{(\text{s})} \quad E^\circ = -0.76\text{ V} \end{aligned}

Which sequence represents the correct order of increasing reducing strength?

  1. 1Silver (Ag\text{Ag})
  2. 2Copper (Cu\text{Cu})
  3. 3Iron (Fe\text{Fe})
  4. 4Zinc (Zn\text{Zn})

Answer

The correct order of increasing reducing strength is Silver (Ag\text{Ag}), Copper (Cu\text{Cu}), Iron (Fe\text{Fe}), and Zinc (Zn\text{Zn}).
Reducing power increases as standard reduction potential (E∘E^\circ) becomes more negative, because metals with negative reduction potentials readily lose electrons. Silver has the most positive reduction potential (+0.80 V+0.80\text{ V}), followed by copper (+0.34 V+0.34\text{ V}), iron (−0.44 V-0.44\text{ V}), and zinc (−0.76 V-0.76\text{ V}). Therefore, the order from weakest to strongest reducing agent is Silver, Copper, Iron, and Zinc.

Step-by-Step Solution

1
Understand the relationship between standard reduction potential (E∘E^\circ) and reducing strength.
A species with a more negative standard reduction potential has a greater tendency to undergo oxidation (lose electrons) and is therefore a stronger reducing agent. A species with a more positive E∘E^\circ is a weaker reducing agent.
Reducing strength is inversely proportional to standard reduction potential.
2
Compare the given E∘E^\circ values.
E∘(Ag+/Ag)=+0.80 V>E∘(Cu2+/Cu)=+0.34 V>E∘(Fe2+/Fe)=−0.44 V>E∘(Zn2+/Zn)=−0.76 VE^\circ(\text{Ag}^+/\text{Ag}) = +0.80\text{ V} > E^\circ(\text{Cu}^{2+}/\text{Cu}) = +0.34\text{ V} > E^\circ(\text{Fe}^{2+}/\text{Fe}) = -0.44\text{ V} > E^\circ(\text{Zn}^{2+}/\text{Zn}) = -0.76\text{ V}
Listing potentials from most positive to most negative orders the elements from weakest to strongest reducing agent.
3
Arrange the metals in increasing order of reducing power.
Silver (Ag\text{Ag}) < Copper (Cu\text{Cu}) < Iron (Fe\text{Fe}) < Zinc (Zn\text{Zn})
Silver has the highest E∘E^\circ and is the weakest reducing agent, while zinc has the lowest E∘E^\circ and is the strongest reducing agent.

Key Concept

Reducing Strength and Standard Electrode Potentials
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Topics

Galvanic Cells and Standard Electrode Potentials