Analysing the electrolysis of aqueous solutions

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Malaysia SPM Form 4 Chemistry, Chapter 6: Electrochemistry.

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[edit] Introduction

  • We have learnt that electrolyte can be molten ionic compound or aqueous solution of ionic compound, acid or alkali.
  • An aqueous solution is solution of water of a substance. For example, if you heat sodium chloride until it melts, it is called molten sodium chloride, but if you dissolve sodium chloride in water, it is called aqueous sodium chloride.
  • Electrolysis of aqueous solution is different from electrolysis of molten electrolyte.
  • This is mainly because an aqueous solution contain more types of ions.


  • Let us take the example of molten sodium chloride and sodium chloride aqueous.
  • In molten sodium chloride, the ion present are sodium ion (Na+) and chloride ion (Cl-), due to the decomposition of the solid sodium chloride.
NaCl ---> Na+ + Cl-


  • In sodium chloride aqueous, other than the decomposition of sodium chloride solid to form sodium and chloride ions, some of the water molecule will also disassociates to form hydrogen (H+) and hydroxide (OH-) ions.
NaCl ---> Na+ + Cl-
H2O ---> H+ + OH-
  • Which means in an aqueous solution, it can be more than 1 positive and negative ions.
  • When the ions move to the anode and cathode, only 1 negative ion and 1 positive ion will be selected to be discharged, and this is called selective discharge.
  • There are a few factors that determine which ion will be selected to be discharge, and this will be discussed in next section.



[edit] Factors Affecting Electrolysis

  • There are three main factors that can affect the electrolysis products, there are:
  1. position in the electrochemical series
  2. the concentration and
  3. the type of electrode

[edit] Electrochemical series

  • The chart above lists the ions in order of difficulty of discharge.
  • The ions at the top of the list is more difficult to be discharged, but as we go down the table, they become easier to be discharged. For example, Cu2+ easier to be discharged compare with H+ and OH- is easier to be discharged compare with I-.
  • This series of ions is called the Electrochemical Series.
  • The lower the ion in the electrochemical series, the easier the ion to be discharged during electrolysis.

[edit] Electrolysis of Aqueous Sulphuric Acid

  • As sulphuric acid is aqueous, it is composed not only of hydrogen ions (H+ ) and sulphate ions (SO42-), but also of hydroxide ions (OH-) from the water.
H2SO4 + H2O --> 2H+ + SO42- + H+ + OH-
  • The apparatus used to carry out this electrolysis and collect the gases given off is shown in Figure 9 .8 .
  • When we have more than one type of ion moving to an electrode, selective discharge (or preferential discharge) takes place.
  • This means that the ion which can lose or gain electrons with the greatest ease is discharged, and the other ions, which are harder to discharge, remain in solution .
  • With the electrolyte aqueous sulphuric acid, migration of ions to the electrodes also occurs.
At the Cathode At the Anode
  • Here we have only one ion, the hydrogen, H+ (aq), and each ion gains an electron to become a hydrogen atom.
  • Two of these newly formed atoms then combine to form a hydrogen gas molecule .
  • Here we have a choice of either sulphate, SO42- (aq), or hydroxide OH- (aq) ions.
  • Hydroxide is easier to discharge, so oxygen gas is given off at the anode.
Equation:

2H+ + 2e ---> H2

Equation:

OH- + 4e ---> O2 + H2O

Notes

  • With electrolysis of aqueous solutions of dilute acids or alkalis, the volume of hydrogen given off at the cathode is roughly twice that of the oxygen gas at the anode.
  • Accordingly, the elements of water are lost and as the electrolysis continues, the concentration of the acid or alkali increases .
  • Essentially, the electrolysis of aqueous sulphuric acid is the electrolysis of water, with hydrogen and oxygen gas being given off in a ratio of 2 : 1 .

[edit] Concentration

  • If the concentration of a particular ion is high, it will be selected to be discharged even though it is higher in the electrochemical series compares with another ion present in the solution.
  • For example, if dilute hydrochloric acid is electrolysed, hydrogen gas is given off at the cathode and oxygen gas at the anode.
  • However, when concentrated hydrochloric acid is electrolysed, hydrogen gas is still given off at the cathode, but chlorine rather than oxygen gas will be released at the anode, even though chloride is in a higher position in electrochemical series.


[edit] Electrolysis Of Diluted Or Concentrated Hydrochloric Acid



[edit] Electrolysis of Concentrated Sodium Chloride Solution (Brine)

  • The electrolytic cell used for electrolysis of concentrated sodium chloride solution is designed to collect gaseous products at both electrodes as shown in Figure above.
At Cathode At Anode
  • The sodium and hydrogen ions move to the cathode .
  • As the hydrogen ion (H+), is lower in the reactivity series than the sodium ions (Na+ ), it accepts electrons more easily.
  • The hydrogen ions (H+) are discharged.
  • Both the chloride ions (Cl-) and the hydroxide ions (OH-) migrate to the anode .
  • The chloride ions (Cl-) are preferentially discharged because of their higher concentration
Equation:

H+ + e ---> H

Hydrogen atoms join in pairs to give molecules :
H + H ---> H2

Equation:

Cl- ---> Cl + e

Chlorine atoms join in pairs to give molecules:
Cl + Cl ---> Cl2

Changes in Solution

  • As the hydrogen ions and chloride ions are discharged, sodium ions and hydroxide ions remain in the solution . The solution becomes sodium hydroxide .


[edit] Type of Electrode

  • This is best shown if we consider the electrolysis of aqueous copper(II) sulphate solution.

[edit] Electrolysis of Copper(II) Sulphate by Using Carbon Electrode

Anode

  1. If we use carbon electrodes, they are inert electrodes and do not affect the electrolysis .
  2. Therefore, at the anode, we have a choice of sulphate or hydroxide ions .
  3. The hydroxide ions are easier to discharge, so oxygen gas is given at the anode :

Partial equation 40H- (aq)  O2(g) + 2H2O (l) + 4e- (oxygen gas given off)

Cathode

  • At the cathode, we have a choice of copper or hydrogen ions .
  • The copper ions are easier to discharge, so we see a pink deposit of copper metal on the carbon electrode.

Partial equation Cu2+ (aq) + 2e-  Cu (s) (copper metal deposited)

[edit] Electrolysis of Copper(II) Sulphate by Using Copper Electrode

  • However, if we use copper electrodes, these are active electrodes and do affect the electrolysis.

Anode At the anode, the copper electrode dissolves into solution :

Partial equation Cu(s)  Cu 2+ (aq) + 2e (copper electrode dissolves )

Cathode At the cathode, the copper ions are deposited as pink copper metal:

Partial equation Cu2+ (aq) + 2e-  Cu (s) (copper metal deposited)


The youtube video below shows that a small explosion happen and a "pop" sound is heard when hydrogen gas is brought close to a flame.

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