Chapter 13 - Electrochemistry Flashcards
(46 cards)
Define electrolysis
Process of passing an electric current though a compound to chemically separate its components
What is an electrolytic cell?
Device that converts electrical energy into chemical energy
What are the parts of an electrolytic cell?
1) Battery
2) Electrodes - Anode, cathode
3) Electrolyte
Describe the battery
- “electron pump”
- electrons enter from the anode and exit to the cathode
- positive charge on anode and negative charge on cathode
Describe the electrolyte
- electrically conductive substance in molten/aqueous state
- contain mobile ions that act as mobile charge carriers to conduct electricity
- e.g. NaCl(aq), MgBr(aq)
Describe the anode
- positive electrode connected to positive terminal
- contains delocalised electrons to conduct electricity
- e.g. metal plates and carbon (e.g. graphite) rods
Describe the cathode
- negative electrode connected to negative terminal
- contains delocalised electrons to conduct electricity
- e.g. metal plates and carbon (e.g. graphite) rods
What is the direction of electron flow in the external circuit?
Electrons move from the negative terminal to the positive terminal of the power supply
What happens at the anode?
- Anions are attracted to the positively charged anode
- Anions can lose electrons to the anode and become oxidised
- X^n- → X + ne-
What happens at the cathode?
- Cations are attracted to the negatively charged cathode
- Cations can gain electrons from the cathode and become reduced
- Y^n+ + ne- → Y
What are the steps to writing an overall equation? (e.g. molten NaCl)
1) Write half equations:
Cathode: Na⁺(l) + e⁻ → Na(l)
Anode: 2Cl⁻(l) → Cl₂(g) + 2e⁻
2) Balance:
2Na⁺(l) + 2e⁻ → 2Na(l)
3) Combine and cancel:
2Na⁺(l) + 2Cl⁻(l) → 2Na(l) + Cl₂(g)
4) Write the final overall equation:
2NaCl(l) → 2Na(l) + Cl₂(g)
Why are electrodes inert?
- Electrolysis is carried out to decompose compounds. - If the electrodes take part in the process, contamination of products occurs.
- Hence inert electrodes are used as they are unreactive
What are the 2 commonly used materials for electrodes?
Graphite and platinum
What are the advantages and disadvantages of using graphite as an electrode?
Adv:
1) High mp –> Won’t melt during electrolysis of MBIC
Disadv:
1) Reacts to O2 under high temp to produce CO2
2) Needs to be replaced periodically
What are the advantages and disadvantages of using platinum as an electrode?
Adv:
1) Does not take part in electrolysis
Disadv:
1) Low mp –> Might melt during electrolysis of MBIC
2) Mainly used for aqueous electrolytes
What is considered aqueous?
Solutions are formed when a solute dissolves in a solvent. Aqueous solutions are formed when water is the solvent
What is the equation for water?
H2O (l) ⇌ H+ (aq) + OH- (aq)
What are the additional cations and anions in aqueous solutions?
OH- (hydroxide) and H+ (hydrogen) ions
List the metals in order of increasing ease of discharge of cation
Potassium
Sodium
Calcium
Magnesium
Aluminium
Carbon
Zinc
Iron
Tin
Lead
Hydrogen
Carbon
Silver
Gold
Platinum
Why is it that a more reactive metal is harder to convert back?
The more reactive a metal, the more stable its ion - harder to convert ion back to metal
Less reactive a metal, the less stable its ion - easier to convert ion back to metal
If an aqueous solution contained Cu2+ and Ag+ ions, which would be selectively discharged?
Ag+ ions are selectively discharged at the cathode. Cu2+ only starts getting discharged after all the Ag+ ions are discharged from the solution
Which metals can be discharged before hydrogen in an aqueous solution?
Copper, silver, gold, platinum. Those above hydrogen will not be discharged first
How do anions discharge from dilute aqueous solutions?
They discharge based on their position in the electrochemical series
The electro chemical series ranks ions in terms of _____
electrochemical reactivity
