Chapter 5
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electrolysis
the breakdown of an ionic compound, molten or in aqueous solution, by the passage of electricity
Describe the electrode products and the observations made during the electrolysis of molten lead(II) bromide
Product at cathode: lead Product at anode: bromine Silvery solid at the cathode Brown gas at the anode
Describe the electrode products and the observations made during the electrolysis of concentrated hydrochloric acid
Product at cathode: hydrogen Product at anode: chlorine Colourless gas at cathode, which makes a lighted splint go ‘pop’ Pale yellow-green gas at anode, which turns universal indicator red
Describe the electrode products and the observations made during the electrolysis of concentrated aqueous sodium chloride
Product at cathode: hydrogen Product at anode: chlorine Colourless gas at cathode, which makes a lighted splint go ‘pop’ Pale yellow-green gas at anode, which turns universal indicator red
Describe the electrode products and the observations made during the electrolysis of dilute sulfuric acid
Product at cathode: hydrogen Product at anode: oxygen Cathode gas makes a lighted splint go ‘pop’. Anode gas relights a glowing splint
metals or hydrogen are formed at the….
cathode
non-metals (other than hydrogen) are formed at the….
anode
Describe the electroplating of metals
It is used to coat one metal with another metal. It is done by having the ‘coating’ metal as the anode electrode and the ‘coated’ metal as the cathode. So when electrolysis occurs, the anode metal coats the other metal as it moves to the cathode. While the cathode electrode stays the same as it is a metal. This also means that the half equations at the cathode and anode are including the same ‘coating’ metal.
Outline the uses of electroplating
It is usually done to improve the appearance of an object such as coating nickel jewellery with gold or silver, or to protect from corrosion, for example using chromium plating on steel car bumpers. Steel cans are also coated with tin in the manufacture of tinned cans for food.
Describe electrolysis of aqueous copper(II) sulphate using carbon electrodes
the products at the anode would be oxygen and the cathode would be copper, as diatomic molecules almost always take place at the anode in solutions
What happens at the cathode during electrolysis?
The cations gain their electrons to form their elements. This is a reduction reaction
What are the ionic half-equations for reactions at the cathode and the anode?
At the cathode: M n + n e → M Reduction At the anode: 2X → X 2 + 2e Oxidation
Predict the products of electrolysis of a specified halide in dilute or concentrated aqueous solution
in a concentrated solution, if the metal is above hydrogen in the reactivity series, hydrogen will form at the cathode and the non-metal at the anode. In a dilute solution, a similar thing would happen however the non-metal would be O_2 from OH- instead as there is an excess of that substance.
Describe the transfer of charge during electrolysis to include: – the movement of electrons in the metallic conductor – the removal or addition of electrons from the external circuit at the electrodes – the movement of ions in the electrolyte
The electrons move though the metal conductor, meaning the wire, from the positive to the negative electrode. For ions, they move through the electrolyte, with the positive ions moving to the negative cathode and the negative ions moving to the positive anode. The electrons separated from the ions at the anode and move to the cathode to which they are taken by those ions.
Describe the production of electrical energy from simple cells, i.e. two electrodes in an electrolyte.
A current of electrons is gone through in order to produce electrical energy
Describe electrolysis of aqueous copper(II) sulphate using copper electrodes
the products at the anode would be the impure copper ions and the cathode would be pure copper. There will be a sludge at the bottom where all the impurities are.
Describe, in outline, the manufacture of: – aluminium from pure aluminium oxide in molten cryolite – chlorine, hydrogen and sodium hydroxide from concentrated aqueous sodium chloride
aluminium oxide has a high melting point so it would be impractical and expensive to electrolyse molten aluminium oxide. So it needs to be dissolved in molten cryolite in order for electrolysis to occur. During electrolysis, oxygen is formed at the anode and pure aluminium at the cathode. Carbon anodes is also replaced as it burns away for some periods. Brine is electrolysed to form sodium hydroxide, hydrogen and chlorine. At the anode, chlorine is formed and at the cathode, hydrogen is formed. With sodium hydroxide as a waste product that is removed.