Problem: Magnesium, the element, is produced commercially by electrolysis from a molten salt (the "electrolyte") using a cell similar to the one shown below.What half-reaction occurs at the cathode?

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We’re being asked to determine the cathode half-reaction for the electrolysis of a molten salt given a similar cell diagram containing liquid Magnesium, Mg and Cl2 (g)


Recall that during electrolysis, the cation is reduced/gains electrons while the anion is oxidized/loses electrons:


  • cathode is where reduction occurs 
  • anode is where oxidation occurs


For a redox reaction, we will use the mnemonics LEO GER:

Lose               Gain

Electron         Electrons

Oxidation       Reduction


This means that:


  • cation (+) → reduction → cathode
  • anion (-) → oxidation  → anode


We will first write the equation for the formation of the cation and anion during electrolysis. 


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Problem Details

Magnesium, the element, is produced commercially by electrolysis from a molten salt (the "electrolyte") using a cell similar to the one shown below.

A diagram shows a box containing electrolyte with two electrodes in it, one steel and one carbon.  The negative end of a voltage source is attached to the steel electrode while the positive end is attached to the carbon electrode.  There is a thin layer of liquid Mg near the steel electrode.  Bubbles from around the carbon electrode are Cl2 (gas), which escapes through an outflow tube.

What half-reaction occurs at the cathode?

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