Topic 3 - Electrolytic Processes Flashcards by Adam Harward

What is electrolysis?

Decomposing a compound using electricity

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During electrolysis, what is electricity applied to?

An electrolyte (ionic substance either molten or dissolved in water)

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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

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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

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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

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What charge does a cation have?

Positive

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What charge does a cathode have?

Negative

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What charge does an anode have?

Positive

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What charge does an anion have?

Negative

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What always happens in electrolysis?

Oxidation + reduction reaction

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What is oxidation?

Loss of electrons

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What is reduction?

Gaining electrons

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In the electrolysis of molten lead bromide, what forms at the anode and cathode?

Anode: bromine (g)

- Cathode: lead (l)

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What is he half equation for the reaction at the cathode during the electrolysis of molten lead bromide?

Pb[2+] + 2e[-] –> Pb

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What is he half equation for the reaction at the anode during the electrolysis of molten lead bromide?

2Br[-] –> Br2 + 2e[-]

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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)