Half-equations and charge
Introduced| English | Meaning |
|---|---|
| half-equations/hɑːf ɪˈkweɪʒnz/ | half-equations |
| reduction/rɪˈdʌkʃn/ | A chemical change involving a gain of electrons or a decrease in oxidation state. |
| oxidation/ˌɒksɪˈdeɪʃn/ | A chemical change involving a loss of electrons or an increase in oxidation state. |
Electrons at the electrodes
- At each electrode, ions either gain or lose electrons. We track this with half-equations 半反应式.
- They show exactly what happens to the electrons.
In OIL RIG, oxidation is:
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Oxidation Is Loss of electrons.
OIL RIG
- A handy memory aid: OIL RIG.
- Oxidation Is Loss (of electrons); Reduction Is Gain (of electrons).
In electrolysis, positive ions are reduced at the cathode and negative ions are oxidised at the anode
At the cathode, positive ions:
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Reduction Is Gain — at the cathode.
Reduction is the gain of electrons.
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Reduction Is Gain (RIG).
At the cathode, positive ions ______ electrons.
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Cathode = reduction = gain of electrons.
At the cathode (reduction 还原)
- Positive ions gain electrons at the cathode.
- Example: Na⁺ + e⁻ → Na (sodium ion gains one electron to become sodium metal).
The half-equation 2Cl⁻ → Cl₂ + 2e⁻ shows:
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Chloride ions lose electrons — oxidation at the anode.
At the anode (oxidation 氧化)
- Negative ions lose electrons at the anode.
- Example: 2Cl⁻ → Cl₂ + 2e⁻ (chloride ions lose electrons to form chlorine gas).
Worked example. Electrolysing molten sodium chloride: at the cathode Na⁺ + e⁻ → Na (reduction); at the anode 2Cl⁻ → Cl₂ + 2e⁻ (oxidation). Sodium forms at one electrode, chlorine at the other.
Reduction and oxidation at the electrodes
Cations reach the cathode and GAIN electrons (reduction); anions reach the anode and LOSE electrons (oxidation). Step to the discharge stage to see the half-equations.
You've got it
- half-equations show the electrons gained or lost at each electrode
- OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons)
- cathode = reduction (gain e⁻); anode = oxidation (lose e⁻)