Topic 14 - Redox 2
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What is standard electrode potential?
voltage measured under standard conditions when the half cell is connected to a standard hydrogen electrode
What are the standard condition for standard electron potential?
temp 298K pressure 100kPa conc 1 moldm-3 of ion
Why is it important to use standard conditions when measuring electrode potentials?
electrode potentials are affected by temp, pressure and conc so if they are kept the same, the value for the half cell’s electrode potential will be the same every time
What does an electrochemical cell contain?
2 solutions 2 electrodes salt bridge (+high resistance voltmeter)
What is the purpose of a salt bridge?
completes the circuit • salt ions can flow through salt bridge • charges of electrodes are balanced out
What is the salt bridge (aka filter paper) soaked in?
potassium nitrate KNO3
What is used as a reference electrode?
standard hydrogen electrode always on left
What is used in the standard hydrogen electrode?
hydrogen gas passed over platinum wire -eqm set up between H2 gas and H+ ions in acidic soln
Why is a reference electrode necessary?
• allows all half cells to be compared to the same value • hydrogen half cell has a value of 0V so the voltage reading will always be equivalent to the electrode potential of the other half cell
E⦵ =
E⦵red - E⦵ox
Is a positive electrode potential feasible or not feasible?
feasible thermodynamically unstable
Is a negative electrode potential feasible or not feasible?
not feasible thermodynamically stable
What is E⦵cell directly proportional to?
ΔStot lnK
Why could the predictions of a reaction’s feasibility (based on electrode potentials) be wrong?
• conditions aren’t standard (diff temp, conc, pressure) as eqm would be affected • reaction is kinetically stable (high Ea)
What reaction happens at the anode in an acidic hydrogen-oxygen fuel cell?
H2 → 2H+ + 2e-
What reaction happens at the cathode in an acidic hydrogen-oxygen fuel cell?
2H+ + 1/2O2 + 2e- → H2O
What reaction happens at the anode in an alkaline hydrogen-oxygen fuel cell?
H2 + OH- → 2H2O + 2e-
What reaction happens at the cathode in an alkaline hydrogen-oxygen fuel cell?
1/2O2 + 2e- → OH-
What is the overall reaction in a hydrogen-oxygen fuel cell?
H2 + 1/2 O2 → H2O
What are the advantages of the hydrogen-oxygen fuel cell?
• renewable energy - no direct carbon emissions or impurities
What are the disadvantages of the hydrogen-oxygen fuel cell?
• hydrogen gas is very flammable • (hydrogen) gas is hard to store • requires energy (eg. from fossil fuels) to obtain hydrogen gas
What are the advantages of a hydrogen-rich fuel with oxygen fuel cell?
• easier to store fuel like methanol (compared to gases) - fuel can come from plant matter
What are the disadvantages of a hydrogen-rich fuel with oxygen fuel cell?
• carbon dioxide produced • not as efficient (as hydrogen-oxygen fuel cell) • water vapour produced