Electrolysis of Aqueous Solutions of Compound

Electrolysis of Aqueous Solutions of Compound

Selective Discharge of Ions and Reactivity Series

If inert electrodes are used during electrolysis, the ions discharged and the products formed depend on three factors:

1. Selective discharge of cations

2. selective discharge of anions

3. effect of concentration on the selective discharge of anions

1. Selective discharge of cations

*ions of reactive metals will remain as ions and will not be discharged

*ions of hydrogen and less reactive metal will accept electrons more readily and get discharged .

Some discharge of cations:

potassium ion, K+

sodium ion, Na+

calcium ion, Ca+

magnesium, Mg2+

zinc ion, Zn2+

iron ion, Fe2+

lead ion, Pb2+

hydrogen, H+

copper (II) ion, Cu2+ —>

silver ion, Ag+ —> ions of metals below hydrogen will be discharged during electrolysis

2. Selective discharge of anions

Sulfate ion, SO^2-₄

_{Nitrate ion, NO}^-₃

_{are not discharged during electrolysis}

_{chloride ion, Cl}-

bromide ion, Br^-

^{iodide, I-}

^{hydroxide, OH- ; 4OH (aq) —> 2H}₂^{O (l) + O}2 ^{(g) + 4e-}

The general rules for predicting the product of electrolysis of an aqueous solution

1. Identify the cations and anions in the electrolyte

2. determine the anion that is discharged at the anode

3. determine the cation that is discharged at the cathode

4. Identify the cations and anions that remain in the solution after electrolysis

Electrolysis of dilute sodium chloride solution

Rule 1; ions present in dilute sodium chloride solution

ions from sodium chloride

ions from water

Rule 2 at the anode

OH- and Cl- ions are attracted to the platinum anode

OH - ions give up electrons more readily than Cl- ions

the Cl- ions remain in solution

Rule 3 at the cathode

H+ and Na+ ions are attracted to the platinum cathode

Sodium is higher in the reactivity series

The Na+ ions remain in solution

Rule 4

The overall reaction: 2H2O(l) —> 2H2 (g) + O2 (g)

Electrolysis of concentrated NaCl solution

Rule 1

ions present in concentrated sodium chloride solution

Rule 2

OH- and Cl- are attracted to the carbon ( inert) anode

Cl- ions are discharged as chloride gas

the OH- ions remain in solution

Rule 3

H+ and Na+ ions are attracted to the carbon / inert cathode

The Na+ ions remain in solution

Rule 4

One volume of hydrogen gas is given off at the cathode and one volume of chlorine gas is produced at the anode

Electrolysis of copper (II) solution using inert electrodes

Rule 1

ions from copper (II) sulfate

ions from water

Rule 2, at the anode

OH- and SO^2-₄ ions are attracted to the anode

SO^2-₄ ions remain in solution

Rule 3, at the cathode

H⁺ and Cu ²⁺ ions attracted to the cathode

Copper ; Cu²⁺ (aq) + 2e^{- —>} Cu (s)

The H⁺

Rule 4, summary

*Cathode: reddish brown copper metal

Anode: oxygen gas

*blue colour

* the resulting electrolyte becomes increasingly acidic

Open Ended

Predict the product obtained at the anode and cathode when a concentrated solution of copper (II) chloride is electrolysed using carbon electrodes