Topic 3 - Electrolytic Processes Flashcards
What is electrolysis?
Decomposing a compound using electricity
During electrolysis, what is electricity applied to?
An electrolyte (ionic substance either molten or dissolved in water)
In electrolysis, why does the electrolyte have to be molten or dissolved in water?
So there are free electrons to conduct electricity, has to be a flow electrons for circuit to be complete
How do the electrons move along the electrolyte in electrolysis?
Taken from ions at +ve anode + given to other ions at -ve cathode. As ions gain/lose electrons they become atoms/molecules
How is molten sodium chloride electrolysed?
- Cathode: sodium (Na+) ions reduced (gain electron) becoming sodium atoms
- Cathode: Na+ + e- –> Na
- Anode: chlorine (Cl-) ions oxidised (lose electron) + pair up forming chlorine molecules
- Anode: 2Cl- –> Cl2 + 2e-
- Sodium collected from cathode for use in street lamps + coolant in some nuclear reactors
What charge does a cation have?
Positive
What charge does a cathode have?
Negative
What charge does an anode have?
Positive
What charge does an anion have?
Negative
What always happens in electrolysis?
Oxidation + reduction reaction
What is oxidation?
Loss of electrons
What is reduction?
Gaining electrons
In the electrolysis of molten lead bromide, what forms at the anode and cathode?
- Anode: bromine (g)
- Cathode: lead (l)
What is he half equation for the reaction at the cathode during the electrolysis of molten lead bromide?
Pb[2+] + 2e[-] –> Pb
What is he half equation for the reaction at the anode during the electrolysis of molten lead bromide?
2Br[-] –> Br2 + 2e[-]
Why do molten ionic compounds electrolyse differently to salts dissolved in water?
- Salts dissolved in water also have H[+] + OH[-] ions
- At anode, simplest ion discharged
- At cathode, metal will be discharged if less reactive than hydrogen. If less reactive H discharged
How is concentrated sodium chloride solution electrolysed?
- Ions of Na[+], Cl[-], H[+], OH[-]
- Cathode: 2 H ions take an electron each forming 1 H molecule
- Anode: 2 Cl ions lose an electron each forming 1 Cl molecule
- NaOH left in the solution
During the electrolysis of copper chloride solution, CuCl2, what are the half equations at the anode and the cathode?
- Cathode: Cu[2+] + 2e[-] –> Cu
- Anode: 2Cl[-] –> Cl2 + 2e[-]
During the electrolysis of copper chloride solution, CuCl2, what is produced at the anode and cathode?
- Cathode: copper (s)
- Anode: chlorine (g)
During the electrolysis of copper sulfate solution, CuSO4, what are the half equations at the anode and the cathode?
- Cathode: Cu[2+] + 2e[-] –> Cu
- Anode: 4OH[-] –> 2H2O + O2 + 4e[-]
During the electrolysis of copper sulfate solution, CuSO4, what is produced at the anode and cathode?
- Cathode: copper (s)
- Anode: oxygen (g)
During the electrolysis of sodium sulfate solution, Na2SO4, what are the half equations at the anode and cathode?
- Cathode: 2H[+] + 2e[-] –> H2
- Anode: 4OH[-] –> 2H2O + O2 + 4e[-]
During the electrolysis of sodium sulfate solution, Na2SO4, what us produced at the anode and cathode?
- Cathode: hydrogen (g)
- Anode: oxygen (g)
During the electrolysis of sodium chloride solution, NaCl, what are the half equations at the anode and cathode?
- Cathode: 2H[+] + 2e[-] –> H2
- Anode: 2Cl[-] –> Cl2 + 2e[-]
During the electrolysis of sodium chloride solution, NaCl, what is produced at the anode and cathode?
- Cathode: hydrogen (g)
- Anode: chlorine (g)
Why does copper need to be purified by electrolysis?
- Unreactive
- Can be obtained easily by reduction with carbon from ore
- Copper produced isn’t pure enough for use in electrical conductors
How is electrolysis used to purify copper?
- Electrolyte is copper (II) sulfate w/ Cl[2+] (aq) ions
- Cathode is pure piece of pure copper
- Anode is big lump of pure copper that’ll dissolve
- Electrical supply pulls electrons off copper atoms at anode so they go into solution as Cu[2+] ions
- Offering electrons at anode to Cu[2+] ions turn them to copper atoms
- Impurities dropped at anode as sludge, whilst pure copper atoms bond to cathode
- Pure copper deposited on cathode: Cu[2+] (aq) + 2e[-] –> Cu (s) (copper ions reduced to copper atoms by gaining electrons)
- Copper dissolved from impure anode: Cu(s) –> Cu[2+] (aq) + 2e[-] (copper ions oxidised to copper ions by losing electrons)
Why do the masses of the anode and the cathode change in the electrolysis of copper sulfate solution?
- Uses copper electrodes
- Copper ions move from anode to cathode
- Only when impure anode + pure cathode/2 pure copper electrodes
How do you demonstrate the change of mass of electrodes in the electrolysis of copper sulfate solution?
-Weigh anode + cathode before electrolysis
-Place electrodes in copper sulfate solution + connect to power pack
-After 5 mins, switch off power pack, remove electrodes + dry
-Weigh electrodes again
(Mass of anode decreases as copper ions lost; mass of cathode increases as copper ions deposited)
What is electroplating?
Coating the surface of a metal with another metal using electricity (through electrolysis)
How does electroplating work?
- Cathode: object being electroplated
- Anode: pure bar of metal used for plating
e. g. - Cathode: brass cup
- Anode: pure bar of silver
- Electrolyte: silver nitrate solution
- Silver ions from electrolyte move towards cathode + metal gets deposited on brass cup
- Anode keeps silver ions in the solution ‘topped up’
What is electroplating used for?
- Decoration: jewellery/decorative items often electroplated w/ metals like gold/silver, improving their appearance
- Prevention of corrosion: household objects e.g. cutlery + cooking utensils electroplated to prevent corrosion. Metals for protections are unreactive + don’t corrode easily (e.g. nickel, chromium)
