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| 1.2 |
Standard Electrode Potential |
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| Definition of Electrode Potential |
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Potential difference produced when an equilibrium is established between metal M and the aqueous solution containing metal \(M^{n+}\)ions in a half-cell.
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| The Electrode Potential of A Cell |
- Electrode potential cannot be measured directly.
- The value is determined based on the difference of electrode potential between two half-cells.
- The electrode potential can be measured by pairing up the electrode to the standard reference electrode system.
- The international consensus is the standard hydrogen electrode.
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| Standard Hydrogen Electrode |
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- Function of platinum(IV) oxide, \(PtO_2\): To increase the surface area of platinum, Pt to adsorb hydrogen gas, \(H_2\).
- This is done so that hydrogen molecules are in closer contact with hydrogen ions, \(H^+\) in the solution.
- Half equation of the hydrogen half-cell;
- \(2H^+ (aq) +2e^- \rightleftharpoons H_2(g)\)
- The standard hydrogen electrode potential, Eº is given the value of 0.00 V:
- \(H^+ (aq) +e^- \rightleftharpoons \frac{1}{2}H_2(g) \hspace{1cm}E^0=0.00V\)
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| Standard Electrode Potential Value, Eº |
- Apparatus set-up to determine standard electrode potential, Eº of zinc is as below:
- Since the standard hydrogen potential is 0.00 V, the voltmeter reading of 0.76 V shows the electrode potential of zinc.
- Zinc, Zn has a greater tendency to release electrons compared to hydrogen.
- Hence, zinc, Zn becomes the negative terminal.
- Electrons move from zinc electrode, Zn (negative terminal) to platinum electrode, Pt (positive terminal) through the connecting wires.
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| Electrode |
Zinc |
Hydrogen |
| Terminal |
Negative |
Positive |
| Half equation |
\(Zn(p) \rightarrow Zn^{2+} (aq) +2e^-\) |
\(2H^{+} (aq) +2e^-\rightarrow H_2(g)\) |
| Reaction |
Oxidation |
Reduction |
| Standard electrode potential for half cell |
\(Zn^{2+} (aq) +2e^- \rightleftharpoons Zn(s)\\ \,\\ E^0=-0.76V\)
The negative symbol shows that zinc electrode is the negative terminal when connected to the standard hydrogen electrode
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| Oxidising Agents and Reducing Agents Based on The Value of Standard Electrode Potential |
- The standard electrode potential, Eº is also known as the standard reduction potential.
- All half-cell equations are written as reduction.
- Eº value is a measure of the tendency of a substance to accept or donate electrons.
- \(\text{Oxidising agent }+ \text{electron} \rightleftharpoons \text{Reducing agent}\)
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| Standard Electrode Potential Series |
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| Comparison of standard electrode potential, Eº value |
- Is used to determine whether silver, Ag or magnesium, Mg is an oxidising agent or reducing agent.
| When Eº Value of Ag is More Positive |
When Eº Value of Ag is More Negative |
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Silver ion, 𝐴𝑔+ on the left side is a stronger oxidising agent.
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Magnesium atom, Mg on the right side is a stronger reducing agent.
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It is easier for 𝐴𝑔+ ion to receive electrons and undergo reduction.
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It is easier for magnesium atom, Mg to release electrons and undergo oxidation.
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Conversely, silver atom, Ag on the right side is difficult to release electrons.
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Conversely, magnesium ion, 𝑀𝑔2+Mg2+ on the left side is difficult to accept electrons.
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| The Relationship between Eº value to The Strength of a Substance as an Oxidizing Agent and a Reducing Agent |
| Molecules or Ions with a More Positive or Less Negative Standard Electrode Potential Value Eº |
Atoms or Ions with a More Negative or Less Positive Standard Electrode Potential Value Eº |
| Has a greater tendency to accept electrons. |
Has a greater tendency to release electrons. |
| Easier to undergo reduction reaction. |
Easier to undergo oxidation reaction. |
| A stronger oxidising agent. |
A stronger reducing agent. |
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