| Meaning of electrode potential |
| 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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- 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 condition for the standard electrode potential measurement |
- The concentration of ions in an aqueous solution is \(\text{1 mol dm}^{-3}\)
- The gas pressure of 1 atm or 101 kPa
- The temperature at 25°C or 298K
- Platinum is used as an inert electrode when a half-cell is not a metal electrode
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Standard Hydrogen Electrode
| 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 |
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- 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 |
\(Zn(s) \rightarrow Zn^{2+} (aq) +2e^-\) |
\(2H^{+} (aq) +2e^-\rightarrow H_2(g) \) |
| 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
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- 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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- A 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 |
Eº value of Mg is more negative |
- Silver ion, \(Ag^+\) on the left side is a stronger oxidising agent.
- It is easier for \(Ag^+\) ion to receive electrons and undergo reduction.
- Conversely, silver atom, Ag on the right side is difficult to release electrons.
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- Magnesium atom, Mg on the right side is a stronger reducing agent.
- It is easier for magnesium atom, Mg to release electrons and undergo oxidation.
- Conversely, magnesium ion, \(Mg^{2+}\) 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 |
