Electrolysis of Aqueous Solutions Using Inert Electrodes

in electrolysis - when more than 1 type of cation / anion → is present in a solution → only one cation and one anion → is preferentially discharged → although both cations and anions would be attracted to cathode and anode respectively.

• this is known as selective discharge of ions

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there are more than 1 type of cations/ anions → because $H_{2}O$ can also ionise → and there will be hydrogen ions, $H^{+}$ and $O{H}^{-}$ ions from water.

• so in electrolyte → aqueous sodium chloride → there will be 2 types of cations/positive anions and anions/negative ions.

• $N{a}^{+}$ from salt and $H^{+}$ from water → move towards cathode (negative electrode)

• $C{l}^{-}$ from salt and $O{H}^{-}$ from water → move towards anode (positive electrode) (Unlike molten binary ionic compounds → where this is only 1 type of cation and anion)

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1. Selective Discharge of Cations

• metal ions of element → lower in reactivity seriesReactivity Series Main Note → are discharged at cathode in preference to other cations in the solution → since less energy is required.
  • this is because → cations of less reactive element gain electrons more readily → thus greater tendency to undergo reduction. (position of ions is similar to metal reactivity series)

Summary

Cation that is selected for discharge →less reactive → has higher tendency to undergo reduction → as it gains electrons more readily → hence is preferentially discharged.

As we go down reactivity series of metals → the ease of discharge of cation increases

Important

metal above hydrogen - hydrogen ions preferentially discharged. vice versa

1. Selective Discharge of Anions

• For electrolytes that are dilute aqueous solutions & concentrated solutions → that do not contain halide ions (negative ions from group 17, Group Properties - Group 17 Elements (Halogens))

• AKA for solutions that contain sulfate and nitrate ions, regardless of concentration → hydroxide ions will be selectively discharged.

  • Usually hydroxide ions ($O{H}^{-}$ ) are selected for discharge → in preference to other anions (such as sulfate, nitrate, halide ions etc.) → since less energy is required.
  • This is because → hydroxide ions → lose electrons more readily → thus greater tendency to undergo oxidation

• For electrolytes that are concentrated solutions → that contain halide ions (negative ions from group 17 (Group Properties - Group 17 Elements (Halogens))

  • Halide ions are selected for discharge in preference to hydroxide ions → in solution
  • This is because → concentration of halide ions → is higher → than concentration of hydroxide ions.

• For electrolytes that are dilute solutions → that contain halide ions

  • hydroxide ions will be selectively discharged in preference to halide ions based on the ease of discharge of anions.

• In concentrated sodium chloride solution → concentration of $C{l}^{-}$ is higher than concentration of $O{H}^{-}$ → as there is plenty of $NaCl$ → $C{l}^{-}$ will be preferentially discharged.

• In dilute sodium chloride solution → concentration of $O{H}^{-}$ → is higher than concentration of $C{l}^{-}$ because there is plenty of water → $O{H}^{-}$ will be preferentially discharged as it loses electrons more readily → thus greater tendency to undergo oxidation.

• Sulfates, Nitrates → not discharged → remain in the solution → as more energy is required. they are not affected by concentration, be it high or low.

• less easily oxidised*

• polyatomic anions**

Summary

Ease of Discharge of Anions and Cations:

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Important

when hydroxide is discharged, this is the standard equation for the products produced. $4O{H}^{-}(aq)\to 2H_{2}O(l)+O_2(g)+4e^{-}$

Note

According to these rules:

• to obtain reactive metals via electrolysis → ionic compounds in molten state need to be used

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Electrolysis of Dilute sodium chloride → results in decomposition of water → into components. as water is removed from solution during electrolysis → the concentration of sodium chloride solution increases

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• if both hydroxide and oxide are removed, all the time there will be no change in acidity or alkalinity.
• this is due to the balancing between hydrogen and hydroxide ions.

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Important

ions are selectively discharged, not the substance itself!

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