Topic 14: Redox II (incomplete) Flashcards
Standard Electrode Potential
also called electromotive force
The driving force which allows electrons to travel from the oxidation half-cell to the reduction half-cell, under standard conditions.
What are standard conditions
Standard: 298K, 1M, 100kPa
Why is a reference electrode necessary?
The standard hydrogen electrode is used as the reference electrode for finding the standard emf of a half-cell. This is because you cannot measure the emf of a standalone cell but you can measure it with respect to a standard, allowing comparison.
What are the features of a standard hydrogen electrode?
- Pt electrode
- H2 gas
- 1 M HCl
Half cell: Metal + metal ion
Solid metal electrode submerged in 1M solution of metal ions
Half cell: Non-metals with their ions in solution
Gas in test tube overturned
Pt electrode
1 M solution of non metal ions
Half cell: Ions of same element in different oxidation states
Pt electrode
1 M of ion 1
1 M of ion 2
Total emf?
emf = E(ox)+E(red)
This formula assumes you changed the sign of the oxidation reaction
WITHOUT SIGN CHANGE
emf = E(red) - E(ox)
What does a more negative emf(red) mean? a
species on LHS is weaker oxidising agent and the species on the right is a stronger reducing agent (i.e prefers being oxidised)
What is the cell diagram for Cu/Cu2+ and H2/H+
Cu | Cu2+ || H+ | H2 | Pt
What is the cell diagram for Cu+/Cu2+ and Cl2/Cl-
Pt |Cu+ , Cu2+ || Cl2 | Cl- | Pt
What does a positive E(cell) indicate about the reaction?
The reaction is feasible/ spontaneous ( ΔG < 0)
What does a positive E(cell) indicate about ΔS(tot)
ΔS (tot) > 0
Since ΔG = -T ΔS, if ΔG < 0 then ΔS(tot) > 0
What is the relationship between E(cell) and ΔS(tot)
proportional
What is the relationship between E(cell) and lnK
K = equilibrium constant
Proportional
What happens if different conditions are used when measuring emf?
Changes in concentration, pressure, and temperature all change the value of E [the tendency of electrons to flow] because they affect the position of equilibrium of the half-reactions.
What is an electrochemical series
A list which describes the arrangement of elements in the order of their increasing electrode potentials ( with reference to standard H electrode )
Explain the limitations of predictions made using standard electrode potentials
- Thermodynamic feasibilty can be deduced but does not provide information about the rate of reaction
- Need to measure le Chatelier’s effect on concentrations using non-standard conditions
- Reaction kinetics also affect prediction (slow rate = no noticeable reaction, high Ea)
How can standard electrode potentials be used to determine if disproportionation reactions are feasible
The E(cell) which is more negative will be the oxidation reaction. Add together E(ox) and E(red). If E(tot) > 0 then the disportionation reaction is feasible.
What is a fuel cell?
An electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode.
How does the voltage in a fuel cell occur?
As the fuel enters the cell it becomes oxidised which sets up a potential difference or voltage within the cell.
What is the half reaction at the anode in a hydrogen-oxygen fuel cell?
Alkaline: H2(g) + 2OH- -> 2H2O + 2e-
Acidic: H2(g) -> 2H+ + 2e-
What is the half reaction at the cathode in a hydrogen-oxygen fuel cell?
Alkaline: O2(g) + 2H2O + 4e- -> 4OH-
Acidic: O2(g) + 4H+ + 4e- -> 2H2O
What fuels can be used to supply H in fuel cells?
Methanol and other hydrogen-rich fuels
what happens at the anode?
oxidation
what happens at the cathode
reduction
