Cell Potential (Standard Conditions)
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Topic Summary & Highlights
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Core Concept
Cell potential (EcellE_{\text{cell}}Ecell) is the voltage produced by a voltaic cell due to the spontaneous redox reaction occurring in the cell.
Unit: Volts (V\text{V}V).
Standard Conditions:
Temperature: 25∘C25^\circ \text{C}25∘C (298 K298 \, \text{K}298K).
Pressure: 1 atm1 \, \text{atm}1atm for gases.
Concentration: 1.0 M1.0 \, \text{M}1.0M for solutions.
Practice Tips
Ecell° determines whether a redox reaction is spontaneous.
Use the standard reduction potential table to calculate Ecell°.
A positive Ecell° indicates a reaction that can generate electric current, while a negative value means it requires external energy.
The Standard Cell Potential (Ecell∘E^\circ_{\text{cell}}Ecell∘)
Represents the cell potential under standard conditions.
Calculated using the reduction potentials of the half-reactions at the cathode and anode: Ecell∘=Ecathode∘−Eanode∘E^\circ_{\text{cell}} = E^\circ_{\text{cathode}} - E^\circ_{\text{anode}}Ecell∘=Ecathode∘−Eanode∘
Ecathode∘E^\circ_{\text{cathode}}Ecathode∘: Standard reduction potential of the reduction half-reaction.
Eanode∘E^\circ_{\text{anode}}Eanode∘: Standard reduction potential of the oxidation half-reaction.
Interpreting Cell Potential
Ecell∘>0E^\circ_{\text{cell}} > 0Ecell∘>0: The reaction is spontaneous, and the cell can generate an electric current.
Ecell∘=0E^\circ_{\text{cell}} = 0Ecell∘=0: The cell is at equilibrium, and no net current flows.
Ecell∘<0E^\circ_{\text{cell}} < 0Ecell∘<0: The reaction is non-spontaneous, and an external voltage is required for it to occur (as in an electrolytic cell).
Significance of Standard Reduction Potentials
The standard reduction potential (E∘E^\circE∘) measures the tendency of a species to gain electrons (be reduced).
Higher E∘E^\circE∘: Stronger oxidizing agent (e.g., F2\text{F}_2F2, +2.87 V+2.87 \, \text{V}+2.87V).
Lower E∘E^\circE∘: Stronger reducing agent (e.g., Li (s)\text{Li (s)}Li (s), −3.04 V-3.04 \, \text{V}−3.04V).
Steps to Calculate Ecell∘E^\circ_{\text{cell}}Ecell∘
Write the Half-Reactions:
Identify the oxidation and reduction half-reactions.
Find Standard Reduction Potentials (E∘E^\circE∘):
Use a standard reduction potential table.
Assign Electrode Roles:
Cathode: The site of reduction (Ecathode∘E^\circ_{\text{cathode}}Ecathode∘).
Anode: The site of oxidation (Eanode∘E^\circ_{\text{anode}}Eanode∘).
Apply the Formula:
Ecell∘=Ecathode∘−Eanode∘E^\circ_{\text{cell}} = E^\circ_{\text{cathode}} - E^\circ_{\text{anode}}Ecell∘=Ecathode∘−Eanode∘
Example Calculation
Reaction:
Zn (s)+Cu2+(aq)→Zn2+(aq)+Cu (s)\text{Zn (s)} + \text{Cu}^{2+} (aq) \rightarrow \text{Zn}^{2+} (aq) + \text{Cu (s)}Zn (s)+Cu2+(aq)→Zn2+(aq)+Cu (s)
Steps:
Write Half-Reactions:
Oxidation (Anode): Zn (s)→Zn2+(aq)+2e−\text{Zn (s)} \rightarrow \text{Zn}^{2+} (aq) + 2e^-Zn (s)→Zn2+(aq)+2e−, E∘=−0.76 VE^\circ = -0.76 \, \text{V}E∘=−0.76V.
Reduction (Cathode): Cu2+(aq)+2e−→Cu (s)\text{Cu}^{2+} (aq) + 2e^- \rightarrow \text{Cu (s)}Cu2+(aq)+2e−→Cu (s), E∘=+0.34 VE^\circ = +0.34 \, \text{V}E∘=+0.34V.
Calculate Ecell∘E^\circ_{\text{cell}}Ecell∘:
Ecell∘=Ecathode∘−Eanode∘E^\circ_{\text{cell}} = E^\circ_{\text{cathode}} - E^\circ_{\text{anode}}Ecell∘=Ecathode∘−Eanode∘ Ecell∘=+0.34 V−(−0.76 V)=+1.10 VE^\circ_{\text{cell}} = +0.34 \, \text{V} - (-0.76 \, \text{V}) = +1.10 \, \text{V}Ecell∘=+0.34V−(−0.76V)=+1.10V
Conclusion:
The positive Ecell∘E^\circ_{\text{cell}}Ecell∘ indicates a spontaneous reaction.
Significance of Standard Reduction Potentials
The standard reduction potential (E∘E^\circE∘) measures the tendency of a species to gain electrons (be reduced).
Higher E∘E^\circE∘: Stronger oxidizing agent (e.g., F2\text{F}_2F2, +2.87 V+2.87 \, \text{V}+2.87V).
Lower E∘E^\circE∘: Stronger reducing agent (e.g., Li (s)\text{Li (s)}Li (s), −3.04 V-3.04 \, \text{V}−3.04V).