C5 Flashcards
Define electrolysis
The breakdown of ionic compounds when molten/in aqueous solution by the passage of electricity
Ionic compounds can only conduct electricity in molten or dissolved in water form (when fluid ions and electrons can move freely)
Electricity/current causes the ionic compound to break down to form new substances
Doesn’t apply to covalent compounds (can’t conduct electricity) or solid ionic compounds (electrons or ions can’t move)
Define electrode
A rod of metal or graphite which an electric current flows in or out of an electrolyte.
Electrodes are inert (chemically inactive) so they will not react with electrolyte/product of electrolysis (except in electroplating(plating a metal usually to prevent corrosion))
Are only there to transfer electrons
Define electrolyte
The ionic compound that is molten or in aqueous form (dissolved solution) that can conduct electricity
Define anion and anode
Anode
The positive electrode of the electrolysis cell
Anion
A negatively charged ion which is attracted to the anode
Define cation and cathode
Cathode
Negatively electrode of an electrolysis cell
Cation
Positively charged ion which is attracted to the cathode
Explain electrolysis (including the gain and loss of electrons for ions)
Current needs to flow
Charge must be transferred around a circuit by charge carriers
Power supply provides cathode with supply of electrons (negatively charged)
Cations in electrolyte move to cathode (gain electrons)
Anions in electrolyte move to anode (lose electrons)
Electrons go from anode -> power supply and then back to cathode
Complete circuit:
Electrons are charge carriers in external circuit
Ions are charge carriers in electrolyte
What are the products+obervations made by the (inert) electrodes in electrolysis?
Molten lead (II) bromide
Molten lead (II) bromide
Cathode -> Pb-2 + 2e- => Pb (reduction)
Anode -> 2Br => Br2 + 2e- (oxidation)
Lead bromide -> lead + bromide
PbBr2 -> Pb + Br2
Add lead bromide to beaker (heat -> molten)
Ions can move freely and conduct a charge
Add 2 electrodes and connect to power supply
Turn power on and allow electrolysis to take place
Negative bromide ions move to anode (lose one electron to form bromine) -> bubbling and brown bromine gas given off
Positive lead ions move to cathode (gain electrons to form lead metal) -> deposits onto surface of electrode
What are the products+obervations made by the (inert) electrodes in electrolysis?
Concentrated sodium chloride solution
2NaCl (aq) + 2 H2O (l) => H2 (g) + Cl2 (g) + 2 NaOH (aq)
Inert electrodes -> platinum, carbon/graphite
Chlorine (Cl-) and hydroxide (OH-) ions are pulled to anode
Chlorine ‘wins’ (more easily loses electrons) -> loses electrons (oxidation) and creates pale green chlorine gas
Hydrogen (H+) and sodium (Na+) ions are pulled to cathode
Hydrogen ‘wins’ (they are less reactive (when metal in compounds is more reactive than hydrogen-> produce hydrogen instead of metal)) and gain electrons (reduction) and creates clean hydrogen gas
(H+ and OH- come from water)
How can you test for the products of a concentrated sodium chloride solution?
Industrial uses:
Chlorine -> bleach (test: damp pH paper -> white)
Hydrogen -> margarine (test: pop with little splint)
Sodium hydroxide -> soap/detergent (test: pH alkaline)
What are the products+obervations made by the (inert) electrodes in electrolysis?
Dilute sulfuric acid
Electrolysis can split H2O => H+ and OH-
But water doesn’t conduct electricity well
Conductivity improved by adding dilute sulfuric acid
-> more ions (more current)
-> more H+ and OH- as it dilutes
Cathode
H+ ions attracted and gain electrons -> hydrogen gas (pop lit splint test)
Anode
OH- ions attracted and lose electrons -> oxygen gas (relit glowing splint) and water
What generally forms at anodes? What is an ‘example’ involves halide ions?
Generally, negatively charged OH- and nonmetals (minus hydrogen) ions are attracted to anodes
Ex: Halide ions (Cl-, Br-, etc) present -> discharged (becomes halogen)
If not OH- is discharged (becomes oxygen)
Both present-> always one anion left in solution
Concentration of solution affects -> concentrated (halogen) or dilute (oxygen from OH-)
What generally forms at cathodes?
Generally positive H+ and metal ions are attracted
Either hydrogen gas or metal deposit will be produced
Metal above hydrogen in reactivity series -> hydrogen
Hydrogen above metal in reactivity series -> metal
More reactive stays in the solution
What is the product of copper (II) sulphate using carbon/graphite electrodes?
Aqueous copper sulphate -> Cu2+, SO4 2+, H+, OH-
Product at cathode:
Cu2+ and H+ are both attracted to the cathode
Copper -> less reactive than hydrogen
Copper ions -> discharged at cathode (gain electrons/ reduction) -> copper metal
Cu2+ + 2e- => Cu
Product at anode:
SO4 2- and OH- are both attracted to
OH- loses electrons easily than SO4 2-
OH- loses electrons/oxidation to form oxygen gas
4OH- => O2 + 2 H2O + 4e-
What is the product of copper (II) sulphate using copper electrodes (purifying copper)?
Electrolysis here is used to remove impurities from the copper for electrical wires (metal dissolve in solution of the metal)
Cathode increases in mass while anode decreases
Anode (impure copper):
Copper atoms -> oxidized at anode -> forms ions
Anode gets smaller as copper dissolves (forms ions)
Copper loses electrons which goes through current -> Cu2+
Cathode:
Copper ions -> reduced at cathode -> form copper atoms
Electrode gets bigger as pure copper attaches
Lost electrons rejoin -> Cu
Atoms of impurities becomes free and settle at the bottom
What are the products of these common aqueous solutions?
Concentrated sodium chloride (NaCl)
Diluted sodium chloride (NaCl)
Concentrated aqueous copper (II) sulfate (CuSO4)
Dilute sulfuric acid (H2SO4)
Concentrated sodium chloride (NaCl)
Anode -> chlorine gas
Cathode -> hydrogen gas
Diluted sodium chloride (NaCl)
Anode -> oxygen gas
Cathode -> hydrogen gas
Concentrated aqueous copper (II) sulfate (CuSO4)
Anode -> oxygen gas released
Cathode -> copper deposit
Dilute sulfuric acid (H2SO4)
Anode -> oxygen gas
Cathode -> hydrogen gas
How do oxidation, reduction, anodes and cathodes relate to the gaining and losing of electrons?
Oxidation
Loses electrons
Reduction
Gains electrons
Anode
Negatively charged ions lose electrons
Cathode
positively charged ions gain electrons
What do ionic half equation show?
The oxidation and reduction of ions involved
Charges must always be balanced as the discharged substance is always neutral
Create/recall an example of an ionic half equation
Molten lead bromide
Anode: 2 Br- => Br2 + 2e-
Cathode: Pb2+ + 2 e- => Pb
Concentrated aqueous sodium chloride
Anode: 2 Cl- => Cl2 + 2e-
Cathode: 2 H+ + 2 e- => H2
Dilute sulfuric acid
Anode: 4 OH- => O2 + 2 H2O + 4 e-
Cathode: 2 H+ + 2 e- => H2
Explain electroplating with copper
Electroplating -> surface of one metal is coated with a different metal
Anode ->pure metal you want to coat with
Cathode -> metal being coated
Electrolyte -> aqueous solution of soluble salt of pure metal at anode
Anode
Copper atoms gain electrons to form ions
Cathode
Copper ions are attracted to the cathode and are deposited on the strip of metal, coating the metal in a layer of copper
Describe the process of manufacturing aluminum from pure aluminum oxide in molten cryolite
Electrolysis of aluminium oxide occurs in steel tanks lined with graphite.
Lining = cathode
Graphite blocks that hang over tank = anode
Bauxite = purified → aluminium oxide
Aluminium oxide is dissolved in molten cryolite
Cryolite = lower melting point (less expensive to heat) + doesn’t interfere with reaction
At the cathode (negative electrode):
Aluminium ions (Al3+) gain electrons (reduction) -> Molten aluminium forms
Al3+ + 3e- → Al
At the anode (positive electrode):
Oxide ions (O2-) lose electrons (oxidation)
Oxygen is produced at the anode:
2O2- → O2 + 4e-
The overall equation for the reaction is: 2Al2O2 (l) → 4Al (l) + 3O2 (g)
The carbon in the graphite anodes reacts with the oxygen produced to produce CO2
C (s) + O2 (g) → CO2 (g)
As a result the anode wears away and has to be replaced regularly
