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Level 134

Electrolysis


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electrolysis
an electric current forces a nonspontaneous redox reaction to occur
Current Type for Electrolysis
A Direct Current E.g. Battery
Electrolyte E.g.HCl
Liquid that conducts electricity
Electrodes
Anode (Positive), Cathode (Negative) this only applies for electrolysis
Electrolysis of HCl
H2 Anode, Cl2 Cathode
Chlorine Gas Test
Bleaches damp litmus paper, may turn red before bleaching a it is an acid.
Hydrogen Test
Lit splint in test tube makes squeaky pop.
Brine/ Seawater
Sodium Chloride (NaCl)
Electrolysis of Brine
Sodium hydroxide (NaOH) = remaining liquid, chlorine (Cl2) anode, hydrogen (H2) cathode
Electrolysis of Water
Gives hydrogen and Oxygen
Electrolyte
A substance which when molten or in an aqueous solution can conduct electricity (and is broken down by it)
Molten lead bromide reaction
Solid does not conduct as the ions are in a fixed lattice
Pb2+ + 2e- = Pb (molten lead)
What is formed at the cathode and what kind of reaction is it? (negative)
(positive)
2Br- - 2e- = Br2 (bromine gas)
Reduction
Cathode is the (oxidation or reduction) part of the reaction?
OIL RIG
Oxidation
Sodium chlorine solution (brine)
Ions present: Na+ Cl- (from NaCl) H+ OH- (from h20)
At cathode
2H+ + 2e- = H2
At anode
2Cl- - 2e- = Cl2
Uses of H2 gas
Rocket fuel, making margarine
Uses of Cl2 gas
making bleach, water sterilisation
Uses of NaOH
Making bleach, oven cleaner, drain cleaner, making soap
Copper sulphate solution
Ions present: Cu2+ So4 2- (from CuSo4) H+ OH- (from H20)
H2SO4
Sulfuric Acid
Sulphuric acid
Ions present: H+ So4 2- (H2So4) H+ OH- (from H2O)
Car batteries
Uses of Sulphuric acid
Test for chlorine
Blue litmus paper blue to white
Test for hydrogen
Lighted splint makes squeaky pop
Test for alkali
Red litmus paper red to blue
Test for acid
Blue litmus paper blue to red
A metal or hydrogen
Generally what is formed at the cathode?
Generally what is formed at the anode?
A non-metal (usually a gas) {-ate = oxygen}
Cheshire
Where is famous for salt deposits?
How is the salt extracted?
Explosives, pumping water down pipes to dissolve salt
Reservoirs
How is the brine stored?
Chlorine gas and sodium hydroxide
What chemicals are produced from the brine?
So they don't react
Why is it important to keep the alkali and chlorine separate?
Mercury cells, diaphragm cells and membrane cells
What are the different cells used in industrial electrolysis?
Potassium
K+1
What's an electrolyte
Name given to the liquid or solution that conducts electricity during electrolysis
Negative electrode
What is the cathode
Why do ionic substances only conduct when molten or in solution
Because the ions are only free to move when molten or in solution
NaCl
Sodium chloride
Sulfuric acid
H2SO4 2-
CuSO4
copper (II) sulfate
PbBr2
Lead bromide
CuNO3
Copper nitrate
LiCl
Lithium chloride
During electrolysis which way do the electrons flow
From the positive electrode to the negative electrode through wires
Which elements go around in pairs
Hydrogen oxygen nitrogen and all of group 7
What is the product at the negative electrode
If it's less reactive than hydrogen and positive it is made, if not it is hydrogen
What's made at the anode
The ion with the least amount of atoms (if its hydroxide oxygen will be made)
H+ and OH-
If the electrolyte is (aq) the two ions that are always present are
Glowing splint relights
The test for oxygen
Use for chlorine
Disinfect drinking water
Use for hydrogen
Fuel and makes margerine
Cryolite
What is aluminium oxide dissolved in to lower its melting point
Lots of electricity is used
Why the extraction of aluminium is so expensive
Abundant
Easily found
Why does the anode wear away
It reacts with the oxygen made
Uses of aluminium
Power lines, alloys used in planes
Why is aluminium good
Resists corrosion, low density
Why is aluminium not electrolysed in water
The aluminium has such a high melting point the water would evaporate
Electrical current
Flow of electrons or ions
(L)
Sign for molten
Why do covalent substances not conduct electricity?
They don't contain ions because they bond by sharing electrons. This means they won have any ions/electrons free to move and carry a charge
Electrode
a metal strip that conducts electricity
Anode
A positive electrode
Cathode
A negative electrode
Strong Electrolyte
A substance which dissociates completely into ions in dilute aqueous solutions.
Weak Electrolyte
A substance that can partially ionize
electricity
The flow of electrons
Cation
(+) Lose an electron
Anion
(-) Gain an electron
Cell
A device which can convert electrical energy to chemical energy or chemical energy to electrical energy
electrochemical cells
a set of chemical species, cathodes, anodes and a salt bridge which uses redox reactions to generate energy.
Electrolytic cell
An electrochemical cell that USES electrical current to drive a non-spontaneous reaction
Reducing agent
A reagent that reduces (adds electrons to) another species
Oxidizing agent
An element/compound that's reduced.
redox reactions
reactions in which electrons are transferred between reactants
Half equations
What type of equations do you use to show the movement of electrons?
Metallic Conduction
1.Current carried by electrons in metal/graphite rod.
Electrolytic Conduction
1.Current carried by +ve & -ve ions of electrolyte.
Selective/Preferential Discharge
The electrode process which takes place most easily when 2 or more ions migrate to the same electrode.
Factors Determining Which Ions Are Discharged
1.Position of the ion in the electrochemical series.
K +
Electrochemical Series (Cations)
SO4 2-
Electrochemical Series (Anions)
lower
electrons that are closer to the nucleus have a _______ energy
greater
The greater the disorder of a system, the _______ its entropy.
electroplating
deposition of a thin layer of a metal on an object in an electrolytic cell; did to protect the surface of the base metal from corrosion or to make it more attractive
Anodizing
The process by which an oxide layer may be thickened.
no salt bridge, one electrolyte, graphite electrode
What are the differences between an electrolytic cell and a Galvanic cell?
Why graphite?
delocalised electrons, inert?
Positive
Anode (Electrolytic Cell)
negative
at the beginning of a redox reaction, the charge of the anode is
oxidation - reduction
What is the Ecell value in an electrolytic cell?
Equation for oxidation of water
O2 + 4H+ + 4e- -> 2H2O
Equation for reduction of water
2H2O + 2e- -> 2OH- + H2
molten NaCl
What does a Down cell use?
porous sheet, separates products/reactants from each other
What is the steel gauze in a Down cell for?
CaCl2, decrease working temperature
What is used in a Down cell? Why?
800 -> 600
What is the decrease in working temperature thanks to CaCl2?
bleach, PVC
Uses of Cl2
remains, water reduced
What happens to Na? in a brine cell? Why?
Uses of H2
ammonia manufacture, margarine
H2 + N2
Reactants for ammonia manufacture
caustic soda
What is the industrial name for NaOH?
paper manufacture, soap industry, ceramic industry
What are the uses of caustic soda?
diaphragm, membrane, mercury
Other names for a cell using brine
water favoured
What happens in a dilute solution of ionic compound?
2H2O -> 2H2 + O2
What is the main equation for a dilute solution of salt being electrolysed?
<7
pH at positive electrode in dilute solution
>7
pH at negative electrode in dilute solution
What are the factors of the rate of electrolysis?
current, voltage, concentration of electrolyte, electrode surface area, distance between electrodes, active electrode
higher current
What is the effect of a higher voltage?
decrease distance
How can the current be increased by changing the distance between the electrodes?
Hall-Heroult cell, graphite
Example of an active electrode
bauxite
Example of a use of Hall-Heroult cell
oxidises itself to CO2 and CO
Why is graphite in some cases an active electrode?
high temperature, presence of O2
When is graphite an active electrode?
Briefly outline the process of extraction of Al from bauxite
NaOH -> heated to anhydrous form -> add cryolite -> electrolysis in molten state
wiring
Use of copper
Na3AlF6
Chemical formula of cryolite
lowers working temperature
What is the function of cryolite?
Why is Al2O3 heated?
remove water, called anhydrous form
99.9%
How pure must copper be for wiring?
CuFeS2, Cu2S
Ores of copper
copper pyrite
Name of CuFeS2
copper glance
Name of Cu2S
Equation for copper pyrite
CuFeS2 + 4O2 -> Cu2S + 2FeO + 3SO2
FeO + SiO2 -> FeSiO3
What is done with the FeO from copper pyrite? (equation)
slag
Name of FeSiO3
roads
Use of slag
Equation for copper glance
2Cu2O + Cu2S -> 6Cu + SO2
no O2
Conditions for separation of copper glance
What is blister copper?
copper with many bubbles of SO2 and other impurities
Zn, Au, Ag, Sn
What other metals are present in blister copper?
Ag, Au
What metals make up the anode sludge in electrolysis of blister copper?
Zn, Sn
What metals get oxidised in electrolysis of blister copper?
What happens to the cations from the impurities of blister copper?
move to cathode, compete with Cu2+ in electrolyte to get reduced, not reduced
Why should the electrolyte be refreshed in purification of copper?
Cu2+ ion concentration decreases, Zn2+/Sn4+/2+ concentration increases. [is Sn 2+ or 4+?]
not necessarily copper - could be Sn or Zn
Where do the electrons come from when copper gets reduced?
object to be plated
In electroplating, what is the negative electrode?
metal used for electroplating
In electroplating, what is the positive electrode?
Uses of electroplating
silver spoons, chromium car bumpers
AgNO3
silver nitrate
liquid, fused
Other names for molten ionic compounds
yellowish green
Colour of Cl2
reddish brown
Colour of Br2
greenish yellow
Colour of F2
the voltage supplied must be more than Ecell
What does the Ecell value of an electrolytic cell show?