# The Hall Héroult process is used to produce what?

Mar 26, 2017

Aluminium.

#### Explanation:

The Hall-Héroult process is probably the most common method of smelting aluminium industrially.

Alumina (or Aluminium Oxide, $A {l}_{2} {O}_{3}$), the most common aluminium oxide, undergoes electrolysis hrough the Hall-Héroult process, where alumina is dissolved in molten cryolite ($N {a}_{3} A l {F}_{6}$) for electrolysis, at around ${940}^{\circ} - {980}^{\circ}$ celsius.

At the cathode:
$A {l}^{3 +} + 3 {e}^{-} \to A l$

At the anode:
${O}^{2 -} + C \to C O + 2 {e}^{-}$

Full Equation:
$A {l}_{2} {O}_{3} + 3 C \to 2 A l + 3 C O$

As to why the alumina must be dissolved in cryolite rather than just aqueous $A {l}_{2} {O}_{3}$: hydronium ions in the water will oxidise the elemental aluminium. You may think, "why not use molten alumina?" Well, while this would work, alumina has a melting point of around ${2000}^{\circ}$C. Thus it is dissolved in molten cryolite (MP $\approx {1000}^{\circ}$C).

PS: natural cryolite is far too rare to be used for electrolysis, so a synthetic version is created from fluorite, a far more common material.

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