Physical Unit 1.11: Electrode Potentials & Cells (Redox Equilibria) Flashcards

1
Q

describe the +ve & -ve potential of an electrode in terms of equilibria

A

electrode w -ve potential:
when equlibrium lies to the left, the metal becomes -vely charged due to e-s being released & building up on the metal

electrode w +ve potential:
when equilibrium lies to the right, the metal becomes +vely charged due to e-s being used up to form metal from metal ions

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2
Q

what does the position of equilibrium depend on?

A

the metal
reactive metals tend to form Mn+ ions so -ve charge builds up on the metals
more reactive metals have -ve potentials
more unreactive metals have +ve potentials

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3
Q

define electrode/half-cell

A

a metal dipping into a solution of its ions

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4
Q

what electrode is used when there is no solid metal involved in the half-equation & why?

A

pure platinum electrode
inert
a solid metal is needed to allow the flow of e-s

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5
Q

what are the 3 types of electrodes?

A

metal electrodes
gas electrodes
redox electrodes

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6
Q

describe metal electrodes

A

metal solution surrounded by a solution of its ions

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7
Q

describe gas electrodes

A

for a gas & a solution of its ions
inert metal (usually Pt) is the electrode to allow the flow of e-s

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8
Q

describe redox electrodes

A

for 2 different ions of the same element
2 types of ion are present in solution with an inert metal electrode (usually Pt) to allow the flow of e-s
e.g. Pt(s)|Fe2+(aq)|Fe3+(aq)

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9
Q

how do you measure the potential of an electrode?

A

connect half-cell to another half-cell of known potential & measure the potential difference b/w the 2 half-cells
the standard hydrogen electrode (SHE) is assigned the potential 0 & is the primary standard = the potential to which all others are compared/measured
so unknown half-cell connected to SHE with SHE on the left

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10
Q

what is an electrochemical cell?

A

2 half-cells connected

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11
Q

what does primary standard mean?

A

the potential to which all others are compared/measured

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12
Q

describe how 2 half-cells are joined together to give a complete circuit

A

2 metal electrodes are joined with a wire - to allow the transfer of e-s
the metals are dipped into a solution of their ions
the 2 solutions are joined with a salt bridge - to allow the flow of ions
a voltmeter (high resistance so low current) is often included to measure the potential difference (emf)

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13
Q

what can a salt bridge be made of?

A

a piece of filter paper soaked with a solution of unreactive ions
a tube containing unreactive ions in agar gel
e.g. KNO3 is often used in a salt bridge as K+ & NO3- are quite unreactive

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14
Q

what are the standard conditions?

A

1.0moldm-3 solution of the ions (NB for redox electrode, 1.0moldm-3 for each ion)
298K
100kPa if half-cell includes gases
pure metal electrode

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15
Q

why are standard conditions required?

A

the position of the redox equilibrium changes when conditions change

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16
Q

how will the emf change if [a condition that affect position of equilibrium] is increased/decreased?

A

either:
emf increases/more +ve
as equilibrium shifts right so e-s used up
so more reduction happens
or
emf decreases/more -ve
as equilibrium shifts left so e-s released
so more oxidation happens

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17
Q

draw standard hydrogen electrode
what is the half equation?
what is the cell notation?

A

see booklet
2H+(aq) + 2e- <–> H2(g) convention to write as reduction
Pt(s)|H2(g)|H+(aq)

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18
Q

how is the conventional representation of cells written?

A

RO||OR
oxidation||reduction
|| indicates salt bridge
| indicates phase boundary - if no phase change, use a comma
solid metal electrode on the outside

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19
Q

emf =

A

Eθright - Eθleft
when measuring Eθ vs SHE, SHE always on the left

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20
Q

define secondary standard & why are they used?

A

= another standard electrode that has been calibrated against the SHE

SHE is difficult to use bc uses a gas, H2 is flammable & hard to keep at 100kPa

21
Q

describe the redox process

A

metal atoms lose e-s at the -ve electrode/anode = oxidation
these e-s travel through the wire to the +ve electrode/cathode & are gained by ions to produce metal atoms = reduction
+ve electrode normally RHS

22
Q

emf is not affected by the # of e-s in equation

23
Q

what must be true for a reaction to be feasible?

A

emf must be +ve
Eθ ox. agent > Eθ red. agent
[…] is a better oxidising agent thatn […]

24
Q

if emf is -ve?

A

feasible in reverse direction

25
Q

what happens when a cell is discharged vs recharged?

A

discharged: feasible reaction occurs, emf is +ve
recharged: opposite, non-feasible reaction occurs, emf is -ve

26
Q

why does the emf of a cell change when electrodes are connected & current flows?

A

conditions are no longer standard
the conc. of ions changes

27
Q

what is the electrochemical series?
describe

A

list of electrode potentials in order of decreasing or increasing potential
written as reductions
+ve potentials = good at attracting e-s so good oxidising agents
-ve potentials = good at releasing e-s so good reducing agents

28
Q

flip & add

A

flip more -ve potential if have the choice

29
Q

can […] oxidise […]?

A

emf must be +ve

30
Q

when to use flip & add vs when to use R-L to calculate emf

A

if given cell notation, do R-L
if given equations, do flip & add

31
Q

what are the different types of cell?

A

non-rechargeable
rechargeable
fuel cells

32
Q

define battery

A

more than one cell joined together

33
Q

describe non-rechargeable cells

A

chemicals are used up over time & emf decreases
when 1 or more of the chemicals have been used up, the cell is flat & emf = 0
cells cannot be recharged so are single use & must be disposed off

34
Q

(graph of emf vs time for non-rechargeable cells)

A

see booklet

35
Q

describe rechargeable cells

A

reactions are reversible - reversed by applying an external current & regenerate the chemicals

36
Q

what are the equations for lithium ion (rechargeable) cell?

A

used in phones, laptops etc.
Li+ + CoO2 + e- <–> LiCoO2 +0.60V
Li+ + e- <–> Li -3.00V
emf = +3.60V

overall reaction during discharge: Li + CoO2 –> LiCoO2
Eθ is +ve

overall reaction during recharge:
LiCoO2 –> Li + CoO2
Eθ is -ve - not feasible so more than 3.60V needs to be applied

37
Q

(graph of emf vs time for rechargeable cells)

A

see booklet

38
Q

describe fuel cells

A

have continuous supply of chemicals into the cell so do not run out of chemicals & do not need recharging

39
Q

what is the most common fuel cell?

A

hydrogen-oxygen fuel cell

40
Q

diagram for hydrogen-oxygen fuel cell

A

see booklet

41
Q

what is the overall equation for hydrogen-oxygen fuel cells?

A

2H2 + O2 –> 2H2O
emf +1.23V
same equation & emf in alkaline & acidic conditions

42
Q

what are the equations for the reactions at the -ve & +ve electrodes of hydrogen-oxygen fuel cells in alkaline conditions?

A

at -ve electrode: ox.
H2 + 2OH- –> 2H2O + 2e-
at +ve electrode: red.
O2 + 2H2O + 4e- –> 4OH-

43
Q

what are the equations for the reactions at the -ve & +ve electrodes of hydrogen-oxygen fuel cells in acidic conditions?

A

at -ve electrode: ox.
H2 –> 2H+ + 2e-
at +ve electrode: red.
O2 + 4H+ + 4e- –> 2H2O

44
Q

what is the cell notation for the hydrogen-oxygen fuel cell in alkaline conditions?

A

Pt(s)|H2(g)|OH-(aq),H2O(l)||O2(g)|H2O(l),OH- (aq)|Pt(s)

45
Q

what is the cell notation for the hydrogen-oxygen fuel cell in acidic conditions?

A

Pt(s)|H2(g)|H+(aq)||O2(g)|H+(aq),H2O(l)|Pt(s)

46
Q

are hydrogen-oxygen fuel cells efficient & what is the waste product?

A

yes
water is the only waste product

47
Q

what is the formula for efficiency?

A

useful energy output/total energy input

48
Q

table of benefits & risks of using different types of cells

A

see booklet