1.11 Electrode Potentials And Electrochemical Cells
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What is the definition of a cell?
Redox reactions that generates electrons
Why is a high resistance voltmeter used in measuring EMF?
EMF may be very low so allows it to be measured
Why is a saltbridge used in a cell?
Ions free to move and carry charge To complete the circuit
What is the order of a conventional cell notation?
Most positive on RHS Most oxidised form on inside nearest salt bridge Most reduced form on outside
What are the features of the more reactive metal of a cell?
• More negative e cell value • Therefore is oxidised • Equilibrium lies to left
What a re the features of the least reactive metal in a cell?
• More positive e cell value • Therefore reduced • Equilibrium lies to right
What is the effect of changing conditions of a cell?
• If current is allowed rto flow reaction will occur until e cell falls to 0 • Use le chateliers principles to increase again
What are the features of a standard hydrogen electrode?
• Pt electrode • H+ aqueous solution is a conc of 1 moldm^-3 • H2 gas at 100kpa • 298K • Sits on the LHS
What does it mean if overall E cell = +ve?
Reaction is spontaneous
What can cause a reaction which has an E cell of +ve to not be spontaneous?
• High Ea • Slow rate of reaction • No visible reaction
What is the process for answering questions on why reactions occur?
• Work out reactants and write on arrows • Check that e cell works so that oxidation=more -ve • Check e- are equal and combine half equations • E potential reduced>E potential oxidised • E cell= E RHS- E LHS 6 So E cell reduced oxidises e oxidised
What are the three types of electrochemical cells?
• Primary- None rechargeable cells • Secondary- Rechargeable cells • Fuel cells-Require continual input of reactants
Why can’t none rechargeable batteries be recharged?
• Negative electrode is often in casing so would get thinner • Could potentially leak/explode
What is the relationship between the recharge and discharge equation in Secondary e cells?
• Same equation just in reverse • Same e cell just with opposite sign
What are the two equations in lithium ion cells?
Li+ + CoO2 +e-n ><Li+[CoO2]- Li+ +e- >< Li
What is used instead of solution in a Lithium ion cell?
Powdered graphite as is very conductive
What possible conditions are there in a hydrogen fuel cell?
Acid or Alkaline
What is necessary for discharge and recharge to work?
For product to still be attached to electrode
What are the equations for alkali conditions in a Hydrogen fuel cell?
2H2O + 2e- >< H2 + 2OH- O2+2H2O+4e-><4OH- Overall: 2H2 + O2 —> 2H2O
What are the advantages and disadvantages of H2 fuel in a car?
Adv- No CO2,less Nox,SO2,CO, More efficient (more energy for kinetic rather than thermal) Disadvantages- Expensive: Storing,transporting, electrolysis to make H2, Potentially dangerous if not stored properly
How can alkaline fuel cells be made to have a higher e cell?
• Increasing temperature to increase rate of reaction