Writing Half Equations

• Used to represent reaction that happens during electrolysis
• Shows what happens when ions gain/lose electrons

In Half Equations

• Electrons shown as $e^{-}$
• Number of atoms of each element must be same on both sides
• Total charge on each side must be same

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oxidation reaction using chlorine

• One mole of chloride ions, $C{l}^{-}$ → will lose 1 mole of electrons from their outermost shell → to form 1 mole of chlorine atoms, $Cl$
  • $C{l}^{-}\to Cl+e^{-}$ Charge - 1- 0 1- E.C - 2.8.8 2.8.7
• Chlorine atom → cannot exist → by itself → as it is not stable like noble gas > it will combine with another chlorine atom → to form chlorine molecule, $C{l}_2$
• Multiply whole equation by 2:
  • $2C{l}^{-}\to C{l}_2+2e^{-}$
• (this is also the way you show how oxide ion, and other halide ions are oxidised)

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reduction reaction using Magnesium

• One mole of magnesium ions, $M{g}^{2+}$, will gain 2 moles of electrons from 2 moles of chloride Ions to form one mole of magnesium atoms.

  • $M{g}^{2+}+2e^{-}\to Mg$ Charge - 2+ 2- 0 E.C - 2.8 2.8.2

• These electrons gained by the magnesium ion are the electrons lost from the chloride ions.

• (This is also the way you show how other cations are reduced.)

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Writing Overall Equation for Electrolysis

• combine the 2 half equations with an equal number of electrons.