Electrolysis Flashcards
Why can’t covalent compounds conduct electricity?
- in covalent bonds, all the electrons on the outer shell are used to form it
- this means that there are no mobile electrons that can carry a current as they are all used up
- no mobile charge carriers means thta no current can flow
- so covalent compounds cannot conduct electricity
Why can graphite conduct electricity despite having a giant covalent lattice structure?
- in graphite, each carbon atoms forms 3 bonds to other carbons instead of 4
- this means that layers are produced
- this leaves one free delocalised electron, which become mobile charge carriers and are free to move around the layers
- this allows them to conducts electricity as a current can flow
Why can ionic compounds only conduct electricity when they are molten or in an aqueous solution?
- when solid, ionic compounds cannot conduct electricity as their ions are vibrating in fixed positions within the lattice
- however, when molten or in aqueous solutions, the ions dissociate from each other and become free to move
- this makes them mobile charge carriers as they are no longer in fixed positions so can carry a current
- this means that ionic compounds can conduct electricity in molten or aqueous form
What is the positive ion called?
Cation
What is the negative ion called?
Anion
What is electrolysis?
The chemical change caused by passing an electric current through a compound that is either molten or in an aqueous solution
What is an electrode?
- The conductor through which electricity is passed into and out of an electrolyte
- often made out of carbon/graphite
What is an electrolyte?
The liquid that undergoes electrolysis; this is either molten of an aqueous solution containing mobile ions
What state do insoluble ionic compounds have to be in for electrolysis?
Insoluble Ionic compounds have to be melted into a molten state in order to undergo electrolysis
What do some industrial companies do to save money when doing electrolysis on molten ionic compound?
as lots of energy is needs to melt an insoluble ionic compounds (as they have high melting points), substances such as cryolite can be added to the ionic compound to reduce its melting point. This means less energy is required to melt it = cheaper
OIL RIG
Oxidation
Is
Loss of electrons
Reduction
Is
Gain of electrons
What two reactions occur simultaneously in electrolysis?
Reduction and oxidation (so they are often called redox reactions)
What is the positive electrode called?
Anode
What is the negative electrode called?
Cathode
Describe what will be observed in the electrolysis of lead(II)bromide
- once molten, if a lightlbulb is in circuit, it will light up as electrons start flowing
- bubbling will occur around the anode
- no obvious change will be seen near cathode, but a lump of silver metal will have formed under once heating stops
What are the ions present in lead(II) bromide?
- anion = bromide Br- (goes to anode)
- cation = lead(II) Pb2+ (goes to cathode)
Which electrode do each ion go to?
- cation = cathode
- anion = anode
Explain what happens when lead(II) bromide undergoes electrolysis
- molten state = ions move around freely
- negative bromide ions attracted to anode (as opposite charges attract)
- bromide ions become discharged and lose an electron to become bromine
- they form bromine diatomic molecules to be stable
- given off as bromine gas
- electrons from bromide ions travel to cathode and are given to lead(II) ions
- lead(II) ions become lead atoms by gaining 2 electrons
- lead metal will deposit underneath cathode
Lead half equation
Pb2+ (aq) + 2e- => Pb (s)
Bromine half equation
2Br- (aq) => Br2 (g) + 2e-
What happens to water during electrolysis?
The ions in water (H+ and OH-) naturally dissociate from each other
What ions are present during the electrolysis of sodium chloride (NaCl) solution?
- Na+ (from ionic compound)
- Cl- (from ionic compound)
- H+ (from water)
- OH- (from water)
Describe what would be observed in the electrolysis of sodium chloride solution
a gas would collect in the test tube over the cathode and the same would happen for the anode
Explain what would happen during the electrolysis of sodium chloride solution
- hydrogen is less reactive than sodium, so it is easier to add an electron onto it for it to be discharged
- this means that hydrogen ions are attracted to the cathode and are given an electron
- hydrogen ions become hydrogen molecules and hydrogen gas is given off
- gas is collected in the inverted test tube above the cathode
- halide ions (in this case Cl-) are preferentially attracted to anode instead of OH- ions
- an electron is removed from chloride ion and become chlorine molecules + chlorine gas is given off and collected in the inverted test tube above the anode
- the remaining ions (Na + and OH-) together form sodium hydroxide solution
What ions are present during the electrolysis of dilute sulfuric acid?
- H+ (from the sulfuric acid)
- SO42- (from the sulfuric acid)
- H+ (from the water)
- OH- (from the water)
Describe what would be observed during the electrolysis of sulfuric acid
Twice as much gas would be collected in the test/measuring tube inverted above the cathode than the anode
Explain what would happen during the electrolysis of dilute sulfuric acid
- as the only positive ion present is H+ from both the water and dilute acid, it is attracted to the cathode and is discharged by being given an electron onto it
- hydrogen ions become hydrogen molecules (H2 molecules) to be stable and gas is given off (seen by effervescence) at the cathode and gas is collected
- OH- is preferentially discharged at any given time so OH- become oxygen molecules and water molecules
- effervescence occurs by the anode and gas is collected
- remaining ions (H+ and SO42-) become slightly more concentrated sulfuric acid
Half equation of hydrogen
2H+ (aq) + 2e- => H2 (g)
Half equation of chlorine
2Cl- (aq) => Cl2 (g) + 2e-
Half equation of hydroxide ions
4OH- (aq) => 2H2O (l) + O2 + 4e-
Why is twice as much hydrogen produced from the electrolysis of dilute sulfuric acid than oxygen?
- for every four electrons that leave the hydroxide ions on the anode, 2 hydrogen molecules are produced (each molecule only needs 2 electrons each as 1electron per atom is needed)
- but this only makes 1 oxygen molecule (each molecule needs 4 electrons as each atom needs 2 to be stable)
- therefore, twice as many hydrogen molecules are made than oxygen so the final volume of hydrogen is X2 greater than that of oxygen
What ions are present during the electrolysis of copper(II) sulfate?
- Cu2+ (from the ionic compound)
- SO42- (from the ionic compound)
- H+ (from the water)
- OH- (from the water)
Describe what happens during the electrolysis of copper(II) sulfate
- the electrolyte changes colour from blue to colourless
- bubbling occurs at the anode
- shiny pink metal will form around the cathode
Explain what would happen during the electrolysis of copper(II) sulfate
- as copper is less reactive than hydrogen, it would be attracted to the cathode and would gain two electrons to become copper atoms
- copper will buildup on the cathode
- OH- ions are preferentially attracted to the anode and lose an electron to produce oxygen and water molecules
- effervescence occurs by oxygen production at the anode
- remaining ions (H+ and SO4) become sulfuric acid
Why does the electrolyte change colour from blue to colourless during the electrolysis of copper(II) sulfate?
- copper ions are discharged to the cathode electrode to form copper metal as less reactive ions are discharged first
- copper(II) ions, when removed from the solution, cause discolouring of copper(II) sulfate solution
What does sodium chloride solution produce at each electrode during electrolysis? What remains?
- cathode = hydrogen gas
- anode = chlorine gas
- remains = sodium hydroxide solution
What does dilute sulfuric acid produce at each electrode during electrolysis? What remains?
- cathode = hydrogen
- anode = oxygen
- remains = more concentrated sulfuric acid
What does copper(II) sulfate produce at each electrode during electrolysis? What remains?
- cathode = copper metal
- anode = oxygen
- remains = sulfuric acid
What reaction always happens where in electrolysis?
- reduction always occurs at the cathode
- oxidation always occurs at the anode
What are the four rules when main rules to compare the reactivity of ions during electrolysis?
- if the metal is high in the reactivity series, hydrogen is produced at the cathode instead of the metal
- if the metal is below hydrogen in the reactivity series, the metal is produced at the cathode
- solutions containing halides always obtain the halogen at the anode
- if solutions contain common negative ions (sulfates, nitrates, hydroxides), oxygen is always obtained at the cathode as hydroxide ions are preferentially attracted to the anode to produce oxygen
