Electrochemical Cell Flashcards
Describe the set up of an electrochemical cell
- 2 diff. metal ions connected by wire (external circuit) - e- flow from more reactive metal to less reactive
- Dipped in salt solution of own ions connected by salt bridge (filter paper dipped in KNO<strong>3</strong> )
- Voltmeter show v btw 2 half cells - EMF
- Solutions of aqueous ions uses Pt electrode
Explain the function of the salt bridge
- Allows ions to flow
- Maintain charge balance
State a requirement of the soluble ionic compound used to make a salt bridge (1)
Must not react w/ electrolyte/ions in sol
Why is platinum used?
Inert- won’t react w/ ions
Which two reactions occur w/i an electrochemical cell?
- Oxidation
- Reduction
Are reactions that occur at each electrodes irreversible or reversible?
Reversible
How are electrochemical half-equations always written like?
Reduction reaction going in forward reaction
What happens when 2 half-equations are joined?
- More -ve electrode potential (easily oxidised) is flipped
- More +ve electrode potential (reduced) stays the same
What are electrode potentials measured against?
Standard hydrogen electrodes
Define standard electrode potential of a half cell
Voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode
What are the standard conditions?
- Conc: 1.00 mol dm-3
- Temp: 298k (25oC)
- Pressure: 100kPa
Best oxidising agent
Most +ve (left w/ charge)
Worst oxidising agent
Most -ve (left w/ charge)
Best reducing agent
Most -ve (right)
Worst reducing agent
Most +ve (right)
EMF
EMF = R - L
Outline how to draw electrochemical cells
R | O || O | R
- Double lines: salt bridge
- Lines: state change
- More -ve on left exp. w/ standard H
- O = ions
Most -ve
- Anode
- Most reactive
- Oxidised
- Loses e-
Most +ve
- Cathode
- Least reactive
- Reduced
- Gains e-
Sulfate ions: +0.17v Bromide ions: +1.07v
Justify why sulfate ions should not be capable of oxidising bromide ions (1)
E(Br2/Br-) > E(SO4-2/SO2)
Outline the uses of lithium batteries
- Rechargeable
- Mobile phones, laptops + cars
What are lithium batteries made up of?
- Lithium cobalt oxide (LiCoO2) electrode + graphite electrode
- Electrolyte: lithium salt in organic solvent
What are the half equations for a lithium battery?
- -ve electrode : Li → Li+ + e-
- +ve electrode : Li+ + CoO2 + e- → Li+ [CoO2]-
What happens when a battery is recharged?
Current force e- to flow in opposite direction round the circuit + reverse the reactions
Why can’t non-rechargable batteries be recharged?
Reaction can’t be reversed
Give a use of an alkaline hydrogen-oxygen fuel cell
Power electric vehicles
Outline the structure of an alkaline hydrogen-oxygen fuel cell
- H2 + O2 fed to 2 seperate Pt containing electrodes
- Electrodes seperated by anion-exchange membrane that allows anions (OH-) + water to pass through it but not H + O gas
- Electrolyte: aqueous alkaline solution (KOH)
- Anions flow towards -ve electrode
In an alkaline hydrogen-oxygen fuel cell which electrode is hydrogen fed to?
-ve electrode
In an alkaline hydrogen-oxygen fuel cell which electrode is oxygen fed to?
+ve electrode
What is the half equation for the reaction that occurs at the positive electrode for a hydrogen-oxygen fuel cell that operates in alkaline conditions?
O2 + 2H2O + 4e- → 4OH-
What is the half equation for the reaction that occurs at the negative electrode for a hydrogen-oxygen fuel cell that operates in alkaline conditions?
2H2 + 4OH- → 4H2O + 4e-
What is the overall equation for reaction that occurs at a hydrogen-oxygen fuel cell that operates in alkaline conditions?
2H2 + O2 → 2H2O
What is the half equation for the reaction that occurs at the negative electrode for a hydrogen-oxygen fuel cell that operates in acidic conditions?
H2 → 2H+ + 2e-
What is the half equation for the reaction that occurs at the positive electrode for a hydrogen-oxygen fuel cell that operates in acidic conditions?
O2 + 4H+ + 4e- → 2H2O
How is a current generated in a hydrogen-oxygen fuel cell?
- Hydrogen produces e-
- Oxygen accepts e-
What are the advantages of using fuel cells in cars than the internal combustion engine?
- More efficient - greater proportion of energy available from hydrogen-oxygen reaction is converted into kinetic energy
- Waste product is H2O - no toxic chem
- Don’t need to be recharged
What are the disadvantages of using a fuel cell?
- Need energy to supply hydrogen + oxygen (electorlysis - uses electricity)
- H is highly flammable/explosive
Explain why a fuel cell doesn’t need to be recharged (1)
Hydrogen/fuel is continuously supplied
