4.3 Electrolysis
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How is a soluble salt formed?
● React the excess acid with some insoluble chemical (e.g. metal oxide) ● Filter off the leftovers ● Crystallise the product
How are metals more reactive than carbon extracted?
By electrolysis
What do acids and alkalis produce in aqueous solutions?
Acids produce hydrogen ions, alkalis produce hydroxide ions
What are bases, acids and alkalis?
Bases are compounds that neutralise acids, acids produce hydrogen ions in aqueous solutions, alkalis are soluble bases - produce hydroxide ions in aqueous solutions
What is the pH scale and what does a pH of 7 show?
The measure of acidity/alkalinity of a solution. A pH of 7 is a neutral solution.
State the general equation for a neutralisation reaction in a short, ionic form.
OH− + H+→ H2O
What is electrolysis?
The passing of an electric current through ionic substances that are molten or in solution to break them down into elements; ions are discharged (they lose/gain electrons) at electrodes to produce these.
What is an electrolyte?
The liquid/solution which conducts electricity
What is a cathode and what is an anode?
Cathode is the negative electrode, anode is the positive electrode
What occurs at the cathode and what occurs at the anode during electrolysis?
Reduction occurs at the cathode Oxidation occurs at the anode
In aqueous electrolysis, which element is discharged at the cathode?
The less reactive element discharges at the cathode. Hydrogen is produced unless there is a less reactive metal, in which case the said metal is produced.
Oxygen is produced at the anode unless what?
Oxygen is produced at the anode unless the solution contains halide ions, in which case halogen molecules are produced.
How is aluminium manufactured? Why is it expensive?
Aluminium is made through the electrolysis of aluminium oxide and cryolite. Lots of energy is needed to produce the current in electrolysis which makes this process expensive.
What are the half equations in the extraction of aluminium?
Al3+ + 3 e−→ Al (cathode) 2 O2− → O2+ 4 e− (anode) Oxygen reacts with C of the anode producing CO2
Why is cryolite used in this process?
It lowers the melting point of aluminium oxide, reducing energy costs
What are the half equations in the electrolysis of the aqueous Na2SO4? (Higher)
2 e− + 2H+→ H2 (cathode) 4 OH−→ 2 H2O + O2+ 4 e− (anode)
What are the half equations in electrolysis of the molten KCl? (Higher)
Molten: e− + K+→ K (cathode) 2 Cl−→ Cl2+ 2 e−(anode)
What are the half equations in electrolysis of the aqueous CuBr2? (Higher)
Cu2+ + 2 e−→ Cu (cathode) 2 Br−→ Br2+ 2 e− (anode)