Electrolysis Flashcards
Oxidation
Loses electrons, gains oxygen
Reduction
Gains electrons, loses oxygen
When metals react
They lose electrons group 1 loses 1, group 2 loses 2 etc
Reactivity of metals depends on
How easily they lose electrons
Metals react with oxygen
Form metal oxide
Calcium and oxygen reaction
2Ca + O*2 = 2CaO
If a metal reacts with water
Hydrogen is always produced
Sodium and water reaction
Sodium + water –> sodium hydroxide + hydrogen
Potassium with water
2K+2H2O –> 2KO + H2
Metals react with acid, what gas produced?
Hydrogen
Magnesium and sulfuric acid equation
H2SO4 + Mg –> MgSO4 + H2
Magnesium and hydrochloric acid
Magnesium + hydrochloric acid –> magnesium chloride + hydrogen
Ore
a substance from which it is economically viable to extract a metal
Iron ore
Haematite
Aluminium ore
Bauxite - red mineral made of aluminium oxide + iron oxides
The more reactive a metal
The more extreme the method of extraction
Unreactive metals e.g. Gold Platinum
Found as their uncombined element, no extraction needed
Silver
Can be decomposed by smelting alone
Metals less reactive than carbon
Can be extracted by heating the metal oxide with carbon
Metals less reactive than carbon equation
iron oxide + carbon -> iron + carbon dioxide (displacement and reduction reaction)
Metals more reactive than carbon
Must be extracted by electrolysis as carbon can’t displace them from their compounds
Metals less reactive than hydrogen
Don’t react with acids as they cannot displace them
Electrolysis simple def
The breaking up of an ionic compound using electricity - uses lost of energy so expensive
Displacement reactions
A more reactive element displaces a less reactive element from a compound
Why are displacement reactions redox reactions - oxygen
More reactive metal gains oxygen therefore is oxidised/less reactive loses oxygen so is reduced
Why are displacement reactions redox reactions - electrons
More reactive metal loses electrons therefore is oxidised/less reactive gains electrons so is reduced
Oxidation half equation (Mg
Mg -> Mg2+ + 2e
Reduction half equation (Zn
Zn2+ + 2e -> Zn
Overall equation
Zn2+ + Mg -> Zn + Mg2+
Why can metals not displace other metals
Those metals would also need to be extracted and this would require a more reactive metal
For electrolysis to happen, solid ionic compounds must be
Molten or aqueous (dissolved in water)
Cathode
Negative electrode
Cation
Positive ion, attracted to cathode
Anode
Positive electrode
Anion
Negative ion, attracted to anode
Electrolyte
Substance that has been decomposed/broken up by passing electricity through it
Why is copper being extracted from low grade ores
High grade ores are running out
New extraction methods have been developed which use less energy, making it economically viable
Phytomining process
Plants are grown in soil containing low grade ores
Copper compounds are absorbed through the roots
Copper becomes part of the plant, concentrating it in a smaller volume
Plants are harvested and burned as fuel
The ash contains a high % of copper
Copper is extracted from the ash by traditional methods
Bioleaching
Bacteria which feeds on low grade copper ores absorb copper compounds - they are grown in them to produce a solution of copper compounds known as a leachate
Copper can be extracted from leachate or ash by
Electrolysis
Displacement with scrap iron
Pros of scrap iron
It is plentiful and cheap, and can displace copper as it is more reactive
Pros of phytomining/bioleaching
Uses less energy, able to extract from low grade ores, avoids mining (no need to dig/transport/dispose of rock; plants are used as fuel before copper is extracted and it is classed as CO2 neutral
Cons of phytomining/bioleaching
Slower batch process, may not produce copper when needed, takes up land that could be used for growing food, energy is still needed for electrolysis
During electrolysis at the cathode
Positive ions (cations) move towards the cathode. The positive ions gain electrons (however many they originally lost)
Cathode electrolysis example copper
Cu2+ + 2e -> Cu (reduction)
During electrolysis at the anode
Negative ions (anions) move towards the anode. The negative ions lose however many electrons they originally gained
Anode electrolysis example chlorine
Cl- -> Cl + e or 2Cl- -> Cl2 + 2e (as an element, chlorine exists as diatomic molecules) (this is oxidation)
Electrolysis of molten ionic compounds (simpler as there is only one type of pos/neg ion
Metallic element from ionic compound is formed at the cathode; non metallic element formed at anode
Why is aluminium dissolved in molten Cryolite before electrolysis
To lower the melting point
Aluminium cathode
Whole base of tank - large surface area, Al forms at bottom and is easy to siphon off
Anodes eroding with _ oxide
Oxygen reacts with the hot carbon anode to form carbon dioxide
Electrolysis of aqueous ionic compounds
There are also pos and neg ions from the water present - H+ and OH-. H+ and the metal are attracted to the cathode - one will gain e-s and become an element; the other will stay as an ion in an ionic compound. Same with anode
At the cathode aqueous ionic compounds electrolysis
(metal is MORE reactive than hydrogen)
If the metal is more reactive than hydrogen, it will lose electrons more easily than hydrogen. The hydrogen ion will gain e-s more easily and become reduced (2H+ + 2e -> H2) The metal ion reamins in the solution and we form H2 gas at the cathode
At the cathode aqueous ionic compounds electrolysis
(metal is LESS reactive than hydrogen)
If there ARE halide ions (Cl-, BR-, I-)
They will each lose 1 electron and are oxidised, they are diatomic so the equation is 2Cl- -> Cl2 + 2e- and the OH- ion stay in the solution
If there are NO halide ions
The OH- ion loses an electron. 4 OH- ions give up 4 electrons to form water and oxygen at the anode (4OH- -> 2H2O + O2 +4e)
The other type of anion stays in the solution.
