Electrolysis

Prior Knowledge:
1. Atomic Structure
2. Redox reactions

You will learn the following in this chapter:

(a) describe electrolysis as the conduction of electricity by an ionic compound (an electrolyte), when molten or dissolved in water, leading to the decomposition of the electrolyte
(b) describe electrolysis as evidence for the existence of ions which are held in a lattice when solid but which are free to move when molten or in solution
(c) describe, in terms of the mobility of ions present and the electrode products, the electrolysis of molten sodium chloride, using inert electrodes
(d) predict the likely products of the electrolysis of a molten binary compound


Terminologies:

Electrolyte: Molten ionic compounds or aqueous solution of ionic compounds that allows electricity to pass through and are decomposed in the process.
Electrode: Metal plates or graphite rods that conduct electricity into the electrolyte.
Anode: Electrode that is connected to the positive terminal of the battery. Negatively charged ions, anions, moved towards the anode.
Cathode: Electrode that is connected to the negative terminal of the battery. Postively charged ions, cations, moved towards the cathode.

The Process:

When an electric current pass through the electrolyte, ions in the solution migrate towards the oppositely charged electrode.
At the anode, negatively charged ions lose their electron(s) to the anode (connected to positive terminal of battery) to form neutral atoms. The negatively charged ions are said to be oxidised and discharged at the anode. Oxidation occured at the anode.
At the cathode, positively charged ions gain electron(s) from the cathode (connected to negative terminal of battery) to form neutral atoms. The positively charged ions are said to be reduced and discharged at the cathode. Reduction occured at the cathode.

This discharge of ions at the electrodes results in the chemical decomposition of the electrolyte to form its elements.

Watch this introductory video about electrolysis:




Electrolysis of Molten Compounds

A small quantity of solid lead bromide is taken in a silica crucible and two graphite electrodes are inserted. A battery consisting of two dry cells is connected to the electrodes through a key and an ammeter.
electrolysis of molten lead bromide
electrolysis of molten lead bromide
Electrolysis of molten lead bromide
When the key is pressed, no current flows through the system. This is because solid lead bromide does not conduct electricity. But, when the crucible containing lead bromide is heated the solid lead bromide melts.
At the cathode
Pb2+ ions gain electrons from the electrodes to become lead atoms. The Pb2+ are reduced.
In this reaction, lead(II) ions have been discharged and molten greyish globules of lead metal are formed below the electrolyte.
Electrode reaction at the cathode: Pb2+(l) + 2e- ---------> Pb(l)
At the anode

Br- ions lose electrons to electrode to become bromine molecules. The Br- are oxidised.
In this reaction, bromine ions are discharged forming an effervescence of pungent, red-brown bromine gas.
Electrode reaction at the anode: 2Br-(l) ----------> Br2 (g) + 2e-

The overall equation for the reaction: PbBr
2(l) ----------> Br2(g) + Pb(l)


Electrolysis of Aqueous Solutions


Electrolysis can be used to decompose molten compounds or compounds in aqueous solutions. The reactions that occur at the electrodes are rather easy to work out for the electrolysis of molten compounds. It is slightly more complex to predict the reactions for the electrolysis of aqueous solutions.

An aqueous solution of a compound is really a mixture of 2 electrolytes. For example, an aqueous solution of copper(II) sulfate contains 2 electrolytes, namely copper(II) sulfate and water. The aqueous solution, therefore, contains copper(II) ions, sulfate ions, and also hydrogen ions and hydroxide ions from the dissociation of water. These ions compete for discharge at the electrodes.

Electrochemical Series and Selective Discharge of ions

In electrolysis, when more than one type of cation or anion is present in a solution, only one cation or anion are preferentially discharged. This is known as the selective discharge of ions.


Selective Discharge of cations

In the electrochemical series, the metals are arranged in order of their reactivity. Reactive metals form ions which are stable and NOT readily discharged. On the other hand, less reactive metals which are lower down in the electrochemical series form ions which are easily discharged. For eg. if a solution containing Na+ and H+ ions is electrolysed, H+ ions are discharged in preference to Na+ ions.
external image electrochemicalseries.jpg

Selective discharge of anions

external image electrochemicalseries%28anions%29.jpg
Generally, at the Anode,
SO42- and NO3- are NOT discharged as they are very stable, OH- from water is preferentially discharged to form oxygen gas.
The reaction is : 4OH- (aq) --> O2 (g) + H2O (l) + 4e-
Note: for all the examples mentioned so far, inert electrodes were used.
external image moz-screenshot-7.jpg

Effect of concentration on selective discharge of anions

An increase in the concentration of an anion tends to promote its discharge. For eg. in the electrolysis of concentrated NaCl solution, 2 types of ions are attracted to the anode: Cl- and OH- ions. According to their position in the electrochemical series, OH- ions should be discharged preferentially. However, in concentrated NaCl solution, Cl- ions are far more numerous than OH- ions and so are discharged at the anode instead.

2Cl-(aq) --------> Cl2(g) + 2e


Factors affecting electrolysis of aqueous solutions


Products produced at the end of the electrolysis are affected by:

1. Position of the ions in Electrochemical Series.
2. Concentration of Ions
3. Nature and Types of Electrodes used



Remember the following key points when dealing with aqueous electrolytes:
  1. In predicting the products of electrolysis, always identify the metal and non-metal ions in the electrolyte. Remember that an aqueous solution also contains H+ and OH- from the dissociation of water molecules.
  2. Ions of very reactive metals (Na+, K+, Ca2+) are not discharged.
  3. SO42- and NO3- are NOT discharged
  4. Discharge of halide ions (Cl-, Br-, I-) is influenced by concentration of anions. e.g. when concentrated aqueous hydrochloric acid is electrolyte, chloride ion is preferentially discharged.

Electrolysis of Dilute Sulphuric Acid

Dilute sulphuric acid can be electrolysed using the apparatus shown below:

external image electr4.jpg

Ions present in dilute sulphuric acid:
external image ions%20in%20dilute%20sulphuric%20acid.jpg

At the cathode, H+ ions attracted to the cathode.
Electrode reaction at the cathode (-) : 2H+(aq) + 2e- --> H2(g)

At the anode, both OH- and SO42- will be attracted to the anode but OH- ions are preferentially discharged.(Recall SO42- are not discharged when electrolyte is in aqueous form)

Electrode reaction at the anode (+) : 4OH-(aq) --> O2(g) + H2O(l) + 4e-

Overall Reaction: 2H2O(l) --> 2H2(g) + O2(g)
This reaction is sometimes known as electrolysis of acidified water.
Sulphuric acid is added to increase the number of mobile ions to help conduct electricity.
NB: In this process, the amount of acid remains the same, but amount of water decreases.
Hence the concentration of sulphuric acid increases.
Looking at the overall equation, since the number of hydrogen molecules produced is twice as that of oxygen
so the volume of hydrogen gas collected is twice as that of oxygen gas!



Assignment:


To test your understanding, pls do this assignment and submit hard copy to me next Mon 8th Feb.