Standard electrode potentials
Introduced| English | Meaning |
|---|---|
| electrode potential/ɪˈlektrəʊd pəˈtenʃl/ | electrode potential |
| half-cell/hɑːf sel/ | half-cell |
| cell potential/sel pəˈtenʃl/ | The potential difference between the electrodes of an electrochemical cell. |
| standard hydrogen electrode/ˈstændəd ˈhaɪdrədʒn ɪˈlektrəʊd/ | The hydrogen reference electrode assigned a standard electrode potential of zero volts. |
| oxidising agent/ˈɒksɪdaɪzɪŋ ˈeɪdʒənt/ | A substance that accepts electrons from another substance and is itself reduced. |
| reducing agent/rɪˈdjuːsɪŋ ˈeɪdʒənt/ | A substance that donates electrons to another substance and is itself oxidised. |
Ranking the electron-grabbers
- The electrode potential 电极电势 $E$ shows how easily a half-cell 半电池 is reduced.
- We measure it against a fixed reference under standard conditions.
- Combining two half-cells gives a cell potential 电池电势.
Electrode potential lab
Ecell = Eright - Eleft
Change cell potential and see oxidising power increase.
A more positive standard electrode potential means a stronger reducing agent.
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False. A more negative E° means a stronger reducing agent, because that species loses electrons more readily. A more positive E° means a stronger oxidising agent.
The reference and measuring E°
- The standard hydrogen electrode 标准氢电极 is the zero reference: $\text{H}_2$ at 1 atm over platinum in $1\ \dfrac{\text{mol}}{\text{dm}^3}$ $\text{H}^+$, defined as 0.00 V.
- To measure a half-cell's $E^{\ominus}$, connect it to the hydrogen electrode and read the voltage (always written as a reduction).
The electrochemical series of standard electrode potentials
The standard hydrogen electrode is defined as having a potential of:
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It is the reference half-cell, set at exactly 0.00 V; all other E° values are measured against it.
Match each electrode-potential idea.
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Each item links the term to its correct meaning.
Combining half-cells
- Polarity: the more negative electrode is the negative terminal; electrons flow from it through the circuit.
- Reactivity: a more positive $E^{\ominus}$ → better oxidising agent 氧化剂; a more negative $E^{\ominus}$ → better reducing agent 还原剂.
Electrolysis electroplates an object with a thin, shiny layer of metal such as chromium.
The standard cell potential is:
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Subtract the less positive electrode potential from the more positive one.
A half-cell with a more positive E° is:
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A more positive E° means the species is readily reduced — a stronger oxidising agent.
In a cell, electrons flow through the external circuit from the:
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The more negative electrode is the negative terminal; electrons flow from it to the more positive electrode.
Reading the electrode-potential table
- The more positive E°, the more easily the species is reduced (a stronger oxidising agent).
- The more negative, the stronger the reducing agent.
The standard hydrogen electrode reference
Worked examples
1. $E^\circ(\text{Zn}^{2+}/\text{Zn}) = -0.76\ \text{V}$ and $E^\circ(\text{Cu}^{2+}/\text{Cu}) = +0.34\ \text{V}$. $E^\circ_{\text{cell}} = E^\circ_{\text{positive}} - E^\circ_{\text{negative}} = 0.34 - (-0.76) = +1.10\ \text{V}$.
2. The zinc half-cell is more negative, so zinc is oxidised and electrons flow from zinc to copper in the external circuit.
You've got it
- $E^{\ominus}$ shows how easily a half-cell is reduced (written as a reduction)
- the standard hydrogen electrode = 0.00 V reference
- $E^{\ominus}_{\text{cell}} = E^{\ominus}(\text{more positive}) - E^{\ominus}(\text{less positive})$
- more positive $E^{\ominus}$ = stronger oxidising agent; more negative = stronger reducing agent