SS2 Chemistry Lesson Note on Electrolysis of Acidified Water

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ELECTROLYSIS OF ACIDIFIED WATER (DILUTE TETRAOXOSULPHATE (VI) ACID).

The equation of the reaction that is involved in this process is given as shown below:

H₂SO₄   →   2H⁺ + SO₄^{2 –}        (1)

H₂O          ⇌    H⁺ + OH^–                (2)

At the cathode: H+ ions migrate to the cathode and are reduced by gaining electrons to become neutral hydrogen atoms.

H⁺_(aq)   + e^–       _→      H_(g) reduction.

The hydrogen atoms then combine in pairs to form diatomic hydrogen gas molecules

H_g + H_{(g)          →}            H_2(g).

At the Anode: SO₄^2- and OH^– anode. The ions migrate to the anode. The OH- ions being lower than SO₄^2- in the electrochemical series are preferentially discharged.

OH^–_(aq)    →       OH _(g) + e^–.

The OH groups interact to form water and oxygen molecules;

OH  + OH   →   H₂O + O

O + O    →    O₂

Overall half-equation

4OH^– →   O₂ + 2H₂0 +  4e^–

The net result of the electrolysis is that two volumes of hydrogen are produced at the cathode and one volumes of oxygen is produced at the anode. The migration of  SO₄^2- ions to the anode and the discharge of the H⁺ ions cause a decrease in the concentration of H₂SO₄ acid around the anode disturbs the ionic equilibrium of water. To reverse this, more water ionizes

H₂O_(L)         ⇌    H⁺ + OH-_(aq).

This produces an access of H⁺ ions and with the incoming SO₄^2- ions, an increase in the concentration of H₂SO₄ acid is obtained at the anode. The overall result of the electrolysis is that the total amounts of the acid in the solution remain unchanged at the end of the electrolysis. Since two volumes of hydrogen are obtained at the cathode and one volume of oxygen is obtained at the anode, the entire process is equivalent to the electrolysis of water.

ELECTROLYSIS OF COPPER (II) CHLORIDE USING COPPER – CARBON OR PLATINUM ELECTRODE

A copper rod is used as the cathode and carbon rod as the anode. The equations of the reaction involved were given as shown below:

CuCl_{2 (aq     →}     Cu²⁺ _(aq) +  2Cl^–_(aq) 

 

H₂O_(l)    →         H⁺_(aq) + OH^–_(aq) 

When electric currents is passed through the electrolytic solution, Cu²⁺ and H⁺ ions migrate to the cathode, and Cl^– with OH^– migrate to the anode. At the cathode: Cu²⁺ being lower than H⁺ in the electrochemical series is preferentially discharged. The Cu²⁺ gains two electrons from the cathode; which are in turn deposited as neural metallic copper at the cathode.

Cu²⁺_(aq) + 2e-    →   Cu _(s)

At the Anode: Cl^– for its higher concentration than OH^– is preferentially discharged as it loses an electron to the anode and becoming chlorine atoms.

Cl^– _(aq)    →      Cl _(g) + e^–

The chlorine atoms then combine to form chlorine molecules. The discharge of copper and chlorine causes the solution to become progressively dilute as evidenced by the fading away of the light green colour of the electrolyte. If electric current is passed continuously without adding mire electrolyte, at a stage, the hydroxyl ions begin to discharge, because at this stage, the influence of concentration of Cl  ion is no longer the overall controlling factor. In the end, a mixture of chlorine and oxygen is obtained at the anode.

ELECTROLYSIS OF CUSO₄ USING COPPER –COPPER ELECTRODES (I.E COPPER AS CATHODE AND COPPER AS ANODE)

The equation of the reaction of the electrolysis is given as shown below.

CuSO_{4 aq)} →     Cu²⁺ + SO₄^2-_(aq)

H₂O _{(l)         →}        H⁺ _(aq) + OH^–_(aq)

In this case, because both the cathode and anode are made up of copper metal, at anode three possible mechanisms occurs.

1. Discharge of SO₄^2- ion by loss of electrons

SO₄^2-(aq)  →     SO_{4 (g)} + 2e^–

2. Discharge of OH- ions by loss of electrons

OH^–_(aq)        →     OH_(g) + e^–

3. Copper metal loses electrons become Cu²⁺ (aq)

Anode:  Cu (s)      →    Cu²⁺ _(aq) + 2e^–

Cathode:   Cu²⁺_(aq  + 2e^–  →   Cu_(s)

Out of these possibilities, (3) occurs most readily because it requires the least energy. as a result, no ions are discharge at the anode. For each copper atom deposited, at the cathode, one atom of copper is dissolved from the anode to form Cu²⁺ ion in solution. Therefore, there is no change in the concentration of the electrolyte. The electrolysis merely produces copper ions at the anode and deposits same at the cathode. The colour of the electrolyte does not change.

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