9.6 Electrolysis Of Aqueous Solutions Flashcards
Why do the products of electrolysis of aqueous solution differs from those of a molten salt?
The difference arises because water can be oxidised reduced at the electrodes as well as individual ions. Water can be oxidised to oxygen at the anode or reduced to hydrogen at the cathode.
What are the products of electrolysis of an aqueous solution in general?
Anode (positive) - Oxygen or halogen
Cathode (negative) - metal or hydrogen
What are the main three factors that affect the products formed at the electrodes?
The standard electrode potentials of the species in the solution
The concentration of the electrolyte
The material from which the electrodes are made
In general what happens is a metal has a very negative electrode potential?
Hydrogen will be discharged at the cathode.
In general what happens is a metal has a positive electrode potential?
The metal metal will be discharged at the cathode.
What is the effect of concentration of the electrolyte (mainly for solutions donating chloride ions)?
At very low chloride concentrations the product of electrolysis is mainly oxygen (oxidation of water) but with more concentrated solutions chlorine in the major product.
For the electrolysis of copper sulfate how will the products depend on the electrode if it is made of graphite/Pt?
The anode is positive the cathode is negative, the product at the anode is O2 and the product at the cathode is Cu.
The half equation at the anode is:
2H2O (l) —–> O2 (g) + 4H+ (aq) + 4e-
The half equation at the cathode is:
Cu2+ (aq) + 2e- —–> Cu (s)
Observations during electrolysis at the cathode is coated in copper, bubbles of O2 is given off at the anode, blue colours of the solution fades as the concentration of copper ions decreases, the solution gets more acidic since H+ ions are produced.
For the electrolysis of copper sulfate how will the products depend on the electrode if it is made of copper?
The anode is positive the cathode is negative, the anode dissolves and the product at the cathode is Cu.
The half equation at the anode is:
Cu (s) —–> Cu2+ (aq) + 2e-
The half equation at the cathode is:
Cu2+ (aq) + 2e- ——-> Cu (s)
Observations during electrolysis at the anode is that it gets smaller, the cathode is coated with copper and the solution remains the same colour since the concentrations of copper ions does not change.
Why is the oxidation of copper at the copper anode more favourable than the oxidation of water?
Because the electrode potential is less negative (more positive).
What is electroplating?
The process of coating an object with a thin layer of metal using electrolysis.
What should the cathode be?
The object to be coated should be used as the cathode since metal ions are positive.
What should the anode be?
The metal with which the object is to be plated with.
What should the electrolyte be?
A solution contains the ions of the coating metal.
Why does sulphuric acid need to be added to distilled water for its electrolysis?
Since distilled water is a very poor conductor of electricity and so a small amount of sulphuric acid is dully added to increase the concentration of ions present.
What ratio of hydrogen to oxygen should be collected from the electrolysis of water?
Twice as much hydrogen as oxygen. When four electrons are lost at the anode one molecules of O2 is formed but when four electrons are gained at the cathode two molecules of H2 are formed therefore two molecules of H2 are formed for every one molecule of O2 formed.
