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OCR Chemistry A > 5.2.3 - Redox & electrode potentials > Flashcards

5.2.3 - Redox & electrode potentials Flashcards

1
Q

Constructing half equations when the oxidation no. INCREASES

A

Delocalised e- goes on the products side

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2
Q

Constructing half equations when the oxidation no. DECREASES

A

Delocalised e- goes on the reactants side

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3
Q

Half eqns. in acidic conditions

A

Add H2Os to balance oxygens

H+ to balance H’s

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4
Q

Half eqns. in alkaline conditions

A

Add H2Os to balance oxygens

OH- to balance H’s

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5
Q

Combining half eqns to get the overall eqn

A

Balance delocalised e- then combine the two

Delocalised e- should cancel

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6
Q

Redox titrations

A

Involves transfer of e- from one species to another

Titrations of an oxidising agent against a reducing agent

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7
Q

Use of acidified KMnO4

A

Purple potassium manganate is in the burette
Sample analysed is in the flask with an excess of dilute sulfuric acid
As the MnO4- ions react, they form Mn 2+ ions which are colourless

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8
Q

Why can’t HCl be used w/ MnO4-

A

MnO4- would oxidise Cl- to Cl2 and then affect the vol of KMnO4 required in the titration

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9
Q

Why can’t conc. H2SO4/HNO3 be used w/ MnO4-

A

They are oxidising agents themselves so affect the vol of KMnO4 required

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10
Q

Why can’t ethanoic acid be used w/ MnO4-

A

It’s a weak acid and would not provide enough H+ ions

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11
Q

Reacting ratio of Fe 2+ : MnO4-

A

5:1

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12
Q

How is Fe (0) used in the redox titration

A

Oxidised w/ H2SO4 to Fe (2+) ready for analysis

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13
Q

How is Fe(+3) used in the redox titration

A

Reacted w/ Zn to reduce it to Fe (2+)

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14
Q

Reacting ratio of C2O4 2-: MnO4-

A

2.5 : 1

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15
Q

C2O4 2-

A

Ethanedioate ion

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16
Q

Self indicating

A

Titration that does not need an indicator

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17
Q

Autocatalysis

A

A reaction where one of the products acts as a catalyst

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18
Q

Reaction w/ MnO4- and C2O4 2-

A

As both ions are -ve they repel each other and the reaction is slow and needs warming at the start
However Mn2+ acts as a catalyst and speeds up reaction

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19
Q

Cu^2+/ I-

A

2 Cu2+ + 4I- —-> 2CuI + I2

CuI - white ppt

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20
Q

ClO-/I-

A

2 I- + ClO- + 2H+ —> Cl- + I2 + H2O

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21
Q

I2/S2O3 2-

A

2 S2O3 + I2 —> 2 I- + S4O6 2-

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22
Q

Reacting ratio Cu2+:I2

A

2:1

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23
Q

Reacting ratio of ClO- : I2

A

1:1

24
Q

Reacting ratio of S2O3 2- : I2

A

2:1

25
Q

Electrochemistry

A

Control of transfer of e- to produce electrical energy

26
Q

Standard electrode potential

A

The emf of a half cell compared to the standard hydrogen electrode, measured at 298 K w/ sol. conc. of 1 moldm3 and gas pressure of 101 kPa

27
Q

Strong reducing agents have …

A

more -ve electrode potentials and so get oxidised

28
Q

Strong oxidising agents have …

A

More +ve electrode potential and so get reduced

29
Q

Calculating cell potential

A

More +ve electrode potential - more -ve electrode potential

30
Q

Where does oxidation occur

A

Anode (+ve)

31
Q

Where des reduction occur

A

Cathode (-ve)

32
Q

How are half cells eqns written

A

Showing the reduction so e- are on the lhs

Species on the lhs are oxidising agents and species on the rhs are reducng agents

33
Q

When is a cell reaction feasible

A

When the cell potential is +ve

34
Q

Why may a feasible reaction not occur

A

HIgh Ea or conditions aren’t standard/ diff conc

35
Q

Explaining changes in a cell

A

….. affects which electrode
The eqm shifts to the … in order to ..
… accepts/ donates more e-
The electrode potential is more +ve/-ve and the cell potential is more/less +ve

36
Q

What is the salt bridge soaked in and why

A

KNO3
Highly soluble
Easily enters solutions to balance charges
Unlikely to form ppts

37
Q

How do electrons travel in a cell

A

In the wire

38
Q

How do ions travel in a cell

A

Through the salt bridge

39
Q

Metal used in half cells

A

Platinum (s)

40
Q

Features of primary cells

A

Non-rechargebale
Can only be used once as the redox reactions arent reversible
Chemicals get used up –> pd falls –> battery goes flat —> discarded
Used in low-current, long-storage devices e.g. wall clocks

41
Q

Redox system in most primary cells

A

Zn/MnO2 (alkaline)
Both involve H2O and OH- (cancel out)
MnO2/ Mn2O3 - more +ve, OA, cathode
Zn/ZnO - more -ve, RA, anode

42
Q

Feautures of secondary cells

A

Rechargable - each reaction is reversed during recharging, regenerating chemicals
Li + in modern devices e.g. phones, tablets
Nickel/cadmium and nickel metal hydride in torches

43
Q

Fuel cells

A

Uses energy from reactions w/ O2 to create a pd (combustion)
Fuel and O2 flow into fuel cells and produts out. Electrolyte remains in cell
Can operate continously and dont have to be recharged

44
Q

Hydrogen fuel cells

A

Can be acid or alkali (generate same potential difference - same overall reaction)
Form no CO2 during combustion, only H2O

45
Q

Acid hydrogen fuel cells

A

H2 enters at anode
H+ moves to cathode
O2 enters at cathode
H2O leaves at cathode

46
Q

Alkali hydrogen fuel cells

A

H2 enters at anode
H2O leaves at anode
OH- moves to cathode
O2 enters at cathode

47
Q

Negative of hydrogen fuel cell

A

H2 is a gas and difficult to store

48
Q

What’s the purpose of a salt bridge

A

Separates the solution
Complete the electrochemical circuit
Balance charges by releasing K+ and NO3- into the different solutions

49
Q

Identifying the +ve electrode using voltmeter readings

A

If +ve, electrode connected to +ve terminal (reduction eqn)

If -ve, electrode connected to -ve terminal (oxidation eqn)

50
Q

Why does the emf of a cell change once connected to electrodes w/ a flowing current

A

Conc of ions change

51
Q

Function of the platinum electrode

A

Allow transfer of electrons

Reaction surface

52
Q

Why may a redox reaction not occur in the absence of light

A

High Ea

Light breaks bonds

53
Q

Properties of platinum that makes it suitable as an electrode

A

Inert

Conductor

54
Q

Reduction of H2O2

A

2H+ + H2O2 +2e- —-> 2H2O

55
Q

pH of SHE

A

0

56
Q

Difference between fuel cell and modern storage cell

A

Fuel reacts w oxygen to give electrical energy

57
Q

Risks with Li based cells

A

Toxicity

Fire

OCR Chemistry A (16 decks)

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