Table of Contents
IB Chemistry: Electrolysis
Electrolysis
1. Predict and explain the products of electrolysis of aqueous solutions.
Cathode:
Cations are reduced when they arrive at the anode.
Therefore, the positive ions with the higher Eo (i.e. reduction potential) in the electrolyte will be drawn to the cathode and be reduced, as they will be more readily be reduced to form the metal product at the cathode.
Anode:
Anions are oxidized when they arrive at the anode.
Therefore, negative ions that are less electronegative will be drawn to the anode as they are more willing to the lose the extra electrons they carry. In terms of Eo, anions with a smaller Eo value will be oxidized at the anode as it represents a greater oxidation potential.
Examples:
Aqueous sodium chloride
Ions present: (H+, OH-, Na+, Cl-)
Cathode: H= ions will be reduced to form H2 gas as H+ ions have a higher Eovalue.
Anode: OH- will be oxidized to form O2 gas and water as OH‑ ions are less electronegative than Cl- and have a lower Eo value.
However, depending on the concentration of the ions in the electrolyte, if one is significantly greater than the other , then it may be produced instead. In the above example, Chlorine gas may be formed if the concentration of Cl- ions is significantly greater.
2. Determine the relative amounts of the products formed during electrolysis
Strength of Current flowing the in circuit. Current simply represents the number of electrons flowing past a given point in the circuit in a second. If you have a higher current, then more products will be formed in a given amount of time as electrons are flowing at a faster rate.
Time is pretty self-explanatory. More products will be formed as time goes on.
Charge on the ions will determine the amount of product formed. Ions with higher charges will take longer to form e.g. Fe2+ will require twice as much electrons to be reduced to Fe as compared to Na+.
3. Describe the use of electrolysis in electroplating.
The process of electrolysis can be used to electroplate objects i.e. coat the object with a thin layer metal.
In electroplating, the cathode will be the object being electroplated whilst the anode forms the coating. The electrolyte will contain the same cations as produced from the anode. Cations formed from the anode will be drawn to the cathode and be reduced, thus forming a thin layer of metal around the cathode.
This industrial process has many uses. Copper is purified in the same manner as electroplating with two copper electrodes. This is key as one of the main uses of copper is electrical wiring, which impure copper can affect the resistance of the wires.
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