6.3 Electrolysis of aqueous solutions

Cations and anions in aqueous solutions
■ Electrolysis of molten salt

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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 Pop star can make all zebras in town like his car again ○ For anion: Fluoride, sulphate, nitrate, chloride, bromide, iodide, hydroxide Fabulous superman notifies climate becomes increasingly hot

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 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

(b) mercury electrodes

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 Na+ is selected for discharged as mercury forms an amalgam with sodium..
Half-equation	2Cl- → Cl2 + 2e- Chlorine gas is collected	Na+ + e- → Na Sodium is produced

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

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The animation below shows the summary of problem involving electrolysis.

■ Electrolysis of molten A_x B_y electrolyte

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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

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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.

← 6.2 Electrolysis of Molten Compounds

6.4 Electrolysis in Industry →

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