Thursday, 8 January 2015

6.3 Electrolysis of aqueous solutions

Cations and anions in aqueous solutions
■ Electrolysis of molten salt

Molten salt At anode (+) At cathode (-)
Movements of ion to electrode Anion of salt Cation of salt
■ Electrolysis of aqueous solution

Water molecules in an aqueous solution dissociate partially to form H+ and OH- .
Aqueous solution At anode (+) At cathode (-)
Movements of ion to electrode Anion of salt and hydroxide ion, OH- Cation of salt and hydrogen ion, H+

Factors affecting the selective discharge of ion:
Positions of ions in the electrochemical series
Concentration of ions in the solution
Types of electrodes
■ This video contains contains information about electrolysis of aqueous solutions.


Positions of ions in the electrochemical series
■ Positions of ions in the electrochemical series

The lower the ion in electrochemical series, the easier to be discharged.

The following mnemonic method can be used to memorize the sequence of electrochemical series.
For cation:
Potassium, sodium, calcium, magnesium, aluminium, zinc, iron, tin, lead, hydrogen, copper, argentum
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For anion:
Fluoride, sulphate, nitrate, chloride, bromide, iodide, hydroxide
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Laboratory Activity 6.3.1: Positions of ions in the electrochemical series
Worked-example 6.3(a)
Predict the products of electrolysis of lead (II) nitrate solution at anode and cathode.

Solution  
Aqueous solution At anode (+) At cathode (-)
Movements of ion to electrode NO3-, OH- Pb2+, H+
Ion selected for discharged Ion OH- selected for discharged as it is lower position in electrochemical series. Ion H+ selected for discharged as it is lower position in electrochemical series.
Half-equation 4OH- → 2H2O + O2 + 4e-
Oxygen gas is collected
2H+ + 2e- → H2
Hydrogen gas is collected


Concentration of ions in electrolyte solution
■ Concentration of ions in electrolyte solution

An ion will be selected for discharge, when the concentration of a particular type of ion is high.

Usually, in the electrolysis of concentrated halide solution [Cl-, Br-, I-], the concentration of the halide ion is always higher than the hydroxide ion, OH-. Thus, halide ions will be discharged at the anode.

Laboratory Activity 6.3.2: Concentration of ions in electrolyte solution

Worked-example 6.3(b)
Predict the products of electrolysis of concentrated sodium chloride solution at anode and cathode.

Solution  
Aqueous solution At anode (+) At cathode (-)
Movements of ion to electrode Cl-, OH- Na+, H+
Ion selected for discharged Ion Cl- selected for discharged as it has a higher concentration than hydroxide ion. Ion H+ selected for discharged as it is lower position in electrochemical series.
Half-equation 2Cl- → Cl2 + 2e-
Chlorine gas is collected
2H+ + 2e- → H2
Hydrogen gas is collected



Types of electrode
■ Types of electrod

Commonly used electrode: Carbon and platinum [Inert to chemical reactions with the electrolyte or the products of electrolysis.]

Others electrodes: Copper plates or mercury.

Laboratory Activity 6.3.3: Types of electrodes

Worked-example 6.3(c)
Predict the products of electrolysis of concentrated sodium chloride solution with
(a) carbon electrodes
(b) mercury electrodes

Solution
(a) carbon electrodes
Aqueous solutionAt anode (+)At cathode (-)
Movements of ion to electrodeCl-, OH-Na+, H+
Ion selected for dischargedIon Cl- selected for discharged as it has a higher concentration than hydroxide ion.Ion H+ selected for discharged as it is lower position in electrochemical series.
Half-equation2Cl- → Cl2 + 2e-
Chlorine gas is collected
2H+ + 2e- → H2
Hydrogen gas is collected


(b) mercury electrodes
Aqueous solutionAt anode (+)At cathode (-)
Movements of ion to electrodeCl-, OH-Na+, H+
Ion selected for dischargedIon Cl- selected for discharged as it has a higher concentration than hydroxide ion.Ion Na+ is selected for discharged as mercury forms an amalgam with sodium..
Half-equation2Cl- → Cl2 + 2e-
Chlorine gas is collected
Na+ + e- → Na
Sodium is produced



Summary of problems solving involving electrolysis
■ Summary of problems solving involving electrolysis

The animation below shows the summary of problem involving electrolysis.
■ Electrolysis of molten Ax By electrolyte

Molten salt At anode (+) At cathode (-)
Movements of ion to electrode Bx- Ay+
Half equation Bx- → B + xe- Ay+ + ye- → A
■ Summary of problems involving electrolysis of aqueus solution

Aqueous solution At anode (+) At cathode (-)
Movements of ion to electrode anion of salt and hydroxide ion,OH- cation of salt and hydrogen ion, H+
Ion selected for discharged
For dilute solution, use electrochemical series to judge which ion to be discharged.
For concentrated solution, the ions with high concentration will more likely to be discharged.
For active electrode, anode dissolves to form ions.
Use electrochemical series to judge which ion to be discharged.



⇲ For exercise(objective and subjective), download for free on Android OS.

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