topic 14: redox II

standard electrode potential

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

298K, 1M, 100kPa

standard electrode potential

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

298K, 1M, 100kPa

why is a reference electrode necessary

• the standard hydrogen electrode is used as the reference for finding the standard emf of a half-cell

• this is because you cant 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

• 1M of ion 1

• 1M of ion 2

total emf

emf = E(red) - E(ox)

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 Cell indicate about the reaction

the reaction is feasible

what is the relationship between Ecell and ΔS(tot)

proportional

what does a positive E cell indicate about ΔS(tot)

ΔS(tot)>0

what is the relationship between Ecell and ln`k

proportional

what happens if different conditions are used when measuring emf

changes in conc., pressure, and temp. all change the value of E 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

explain the limitations of predictions made using standard electrode potentials

• thermodynamic feasibility can be deduced but doesn’t proceed information about the rate of reaction

• need to measure le chateliers effect on concs. using non-standard conditions

• reaction kinetics also affect predictions

how can standard electron potentials be used to determine if disproportionation reactions are feasible

• the E(cell) which is smaller will be the oxidation reaction

• add together E(ox) and E(red)

• if E(tot)>0 then disproportionation reaction is feasible

what is a fuel cell

an electrochemical cell in which a file 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 cell reaction at the anode in a hydrogen-oxygen fuel cell (oxidation)

• 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