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.
Did you know that when electricity is passed through water, it decomposes into hydrogen and oxygen? Can you believe this? The 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.
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's an electro-chemical process where current is passed between two electrodes through an ionized solution (the 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, who lived from 1791 to 1867, and were developed by the Swedish scientist Svante Arrhenius, who lived from 1859 to 1927 and who was the winner of the Nobel Prize in 1903 in chemistry.
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 (liquid) → 4e + O2 (gas) + 4 H3 O+ (aqueous)
At the cathode, 4H2 O (liquid) + 4e → 2H2 (gas) + 4OH- (aqueous)
Electrolysis of Ionic Compounds
Let's take a closer look at some examples of electrolysis of some single ionic compounds, starting with electrolysis of an 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.
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
Now let's take a closer look at electrolysis of a copper chloride solution. The electrolysis will only take place when electricity is passed through the copper chloride (CuCl2) solution. Copper chloride solution will ionize giving copper ions and chloride ions, and water will ionize giving hydrogen and hydroxyl ions as follows:
CuCl2 (aq) → Cu2+ (aq) + 2Cl- (aq)
H2 O (l) → H+ (aq) + OH- (aq)
The positive copper(II) ions Cu2+ (from copper chloride) and the H+ ions (from water) are attracted to the negative cathode. According to the electrochemical series, only the copper(II) ion is discharged (preferentially) at the cathode. At the anode, chloride ions are discharged giving off chlorine gas:
Cathodic half-cell reaction: Cu2+ (aq) + 2e → Cu(s)
Anodic half-cell reaction: 2Cl- (aq) - 2e → Cl2
Electrolysis With Inert Electrodes
Let's start looking at examples of electrolysis with inert electrodes by looking at electrolysis of a copper sulphate solution.
1. Using normal copper electrodes
The products of electrolyzing copper sulfate solution with copper electrodes are copper metal and copper ions (the copper anode dissolves). The copper deposit on the negative cathode electrode and dissolves at the positive anode electrode. This copper anode reaction differs from when you use an inert graphite electrode for the anode.
When copper(II) sulphate is electrolyzed with a copper anode electrode (the cathode can be carbon or copper), the copper deposit on the cathode (-) equals the copper dissolved at the anode (+). Therefore the blue color of the Cu2+ ions stays constant because Cu deposited = Cu dissolved. Both involve a two electron transfer so it means mass of Cu deposited is equal to mass of Cu dissolving for the same quantity of current flowing (flow of electrons).
At the cathode, a reduction electrode reaction occurs:
Cu2+ + 2e- → Cu(s) (copper deposit, reduction—2 electrons gained, positive ion reduction by electron gain)
At anode, an oxidation electrode reaction occurs:
Cu(s) - 2e- → Cu2+ (copper dissolves, oxidation—2 electrons lost)
or Cu(s) → Cu(s) + 2e- (atom oxidation by electron loss)
So, a balancing act goes on.
a) Copper atoms oxidized to copper(II) ions: dissolving of copper in its electrolytic purification or electroplating (must have positive copper anode). The change involves two electrons per copper atom.
b) Copper(II) ion reduced to copper atoms: deposition of copper in its electrolytic purification or electroplating using copper(II) sulphate solution. The change involves two electrons per copper ion.
This means for every copper atom that gets oxidized, one copper ion is reduced; therefore, when copper electrodes are used in the electrolysis of a copper sulfate solution, the mass loss of copper from the positive anode electrode should equal the mass of copper gained and deposited on the negative cathode electrode.
2. Using inert carbon/graphite electrode
Using the simple electrolytic cell and inert carbon (graphite) electrodes, we can see that the products of the electrolysis of copper sulfate solution are a copper deposit on the negative cathode electrode and oxygen gas at the positive anode electrode.
All right, let's take a moment to review what we've learned. This lesson talked about electrolysis of single ionic compounds, which is basically the decomposition of the ionic compound into its constituent elements by the passage of electric current. As we saw, electrical energy is used to force a nonspontaneous chemical reaction to occur. It's an electro-chemical process where current is passed between two electrodes through an ionized solution (the electrolyte) to deposit positive ions (anions) on the negative electrode (cathode) and negative ions (cations) on the positive electrode (anode). Since this has been shown with some specific examples, you can always go back in the lesson to look them over in more detail.
To unlock this lesson you must be a Study.com Member.
Create your account
Register to view this lesson
Unlock Your Education
See for yourself why 30 million people use Study.com
Become a Study.com member and start learning now.Become a Member
Already a member? Log InBack