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Electrolysis of Aqueous Solutions

Instructor: Saranya Chatterjee

Saranya has a masters degree in Chemistry and in Secondary Education. She has taught high school, AP chemistry for 2 years and is teaching undergraduate college chemistry for 3 years.

This lesson will talk about electrolysis of aqueous solutions of some single ionic compounds with their half cell reactions. It will also discuss an example of electrolysis with inert electrodes.

Electrolysis

Did you know that when electricity is passed through water, it decomposes into hydrogen and oxygen. Can you believe this? This process of decomposition of a substance by an electric current is called electrolysis. The simplest kind of decomposition reaction is the decomposition of a binary compound into its elements. Passing an electric current through water will decompose the water into its constituent elements, hydrogen and oxygen.

2H2 O(l) → 2H2 (g) + O2 (g)


Electrolysis of water


So, electrolysis is the process of passing a current through a cell for which the cell potential is negative and causing an oxidation reduction reaction to occur. Here, electrical energy is used to force a nonspontaneous chemical reaction to occur. It is an electro-chemical process where current is passed between two electrodes through an ionized solution (electrolyte) to deposit positive ions (anions) on the negative electrode (cathode) and negative ions (cations) on the positive electrode (anode). The system is called an electrolytic cell which is used in several industries such as electroplating, refining bauxite into aluminum, producing chlorine and caustic soda from table salt (sodium chloride), and in analytical techniques such as polarography. Principles of electrolysis were first discovered by the British scientist Michael Faraday (1791-1867) and were developed by the Swedish scientist Svante Arrhenius (1859-1927) who is the winner of the Nobel Prize in 1903 in chemistry.

Electrolysis of water

The electrolysis of water leads to the cell reaction in which water is broken down into its elements, H2 and O2. We all know that hydrogen gas and oxygen gas combine spontaneously to form water and are used to power fuel cells, which in turn produce electricity. So, the reverse process (electrolysis of water) is nonspontaneous and requires energy. The reactions taking place at the electrodes are called cathodic and anodic half reactions and adding these half cell reactions, we get the overall oxidation reduction reaction. In case of electrolysis of water, the following half reactions occur at the anode and cathode.

At the anode, 6H2 O (l) → 4e + O2 (g) + 4 H3 O+ (aq)

At the cathode, 4H2 O (l) + 4e → 2H2 (g) + 4OH- (aq)

Electrolysis of Some Single Ionic Compounds

Electrolysis of Aqueous solution of sodium chloride

Sodium chloride ionizes to give sodium and chloride ions and water ionizes to produce hydrogen ions and hydroxide ions

NaCl(aq) → Na+ (aq) + Cl-(aq)

H2 O (l) → H+(aq) + OH-(aq)

So, at the cathode either sodium ions or hydrogen ions can migrate and at the anode, either chloride ions or hydroxide ions can migrate. Based on our previous knowledge of the electrochemical series, we know that the metal will be produced if it is less reactive than hydrogen. Hydrogen will be produced if the metal is more reactive than hydrogen.


Electrochemical series
activity series


During electrolysis, Hydrogen ions H+ (from the water) are discharged at the negative electrode as hydrogen gas, H2. Chloride ions Cl- are discharged at the positive electrode as chlorine gas, Cl2. Sodium ions Na+ and hydroxide ions OH- (from the water) stay behind — they form sodium hydroxide solution, NaOH.

Cathodic half cell reaction : 2H+ (aq) +2e → H2 (g)

Anodic half cell reaction : 2Cl- (aq) -2e → Cl2

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