■ Electrolysis of molten salt
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► | Water molecules in an aqueous solution dissociate partially to form H+ and OH- .
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► | Factors affecting the selective discharge of ion:
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■ Positions of ions in the electrochemical series
► | The lower the ion in electrochemical series, the easier to be discharged.
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► | The following mnemonic method can be used to memorize the sequence of electrochemical series.
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Laboratory Activity 6.3.1: Positions of ions in the electrochemical series |
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
► | 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 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
► | The animation below shows the summary of problem involving electrolysis.
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⇲ For exercise(objective and subjective), download for free on Android OS. | ||
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