Redox II Flashcards

1
Q

What is oxidation and reduction?

A

OXIDATION : loss of electrons
Addition of oxygen
Loss of hydrogen

REDUCTION: gain of electrons
Loss of oxygen
Addition of hydrogen

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2
Q

What are standard conditions for electrode potentials?

A

Ion conc 1.00 Mol/dm
298K temp
100kPa pressure

  • use high resistance voltmeter
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3
Q

What is standard electrode potential?

A

Potential difference produced when standard half cell is connected to a standard hydrogen cell under standard conditions

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4
Q

What is the standard hydrogen electrode ?

A

Half cell in which a 1.00 mol/dm solution of hydrogen ions is in equilibrium with hydrogen gas at 100kPa pressure on the surface of a platinum electrode coated with platinum black at 298K

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5
Q

What is a reference electrode? Why are they needed?

A

Used to measure electrode potentials in place of the standard hydrogen electrode

  • equilibrium position can change electrode potential so need to be able to compare electrode potentials of different species by measuring against common reference/standard
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6
Q

Where is reduction and oxidation more likely to occur?

A

Reduction - at the more + electrode
Oxidation - at the more - electrode

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7
Q

When is a platinum electrode used to find SHE?

A

When the half cells are
NON METALS/ non metal ions or IONS (with different oxidation states)
- platinum is inert/doesn’t take part in reaction /redox equilibrium established on platinum surface

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8
Q

What is the salt bridge for? What chemicals are used and why?

A
  • has mobile ions that complete the circuit
    Potassium chloride /potassium nitrate used too make salt bridge as CHLORIDES/NITRATES are soluble

Should ensure no precipitates form which could affect equilibrium position of half cells

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9
Q

How to calculate emf/potential difference of electrochemical cell?

A

E right - E left = Ecell

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10
Q

How do the E values infdicate how easily the species can be oxidised or reduced? When is reaction feasible?

A

More + the value - easier it is to REDUCE the species on the left of half equation
E.g Cl2 + 2e- —> 2Cl- E= +1.36V
- reaction will proceed in forward reaction

Less + the value , easier it is to OXIDISE the species on right of half equation
E.g Cu2+ + 2e- —> Cu. E = +0.34V
- reaction will proceed in backward reaction

Reaction is FEASIBLE when Ecell is +

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11
Q

How is cell potential liked to entropy,S/equilibrium constant,K?

A

Larger cell potential = BIGGER CHANGE IN ENTROPY
Cell potential directly proportional to entropy change

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12
Q

Limitations of standard electrode potential predictions?

A
  • thermodynamic feasibility of reaction can be deduced but no info about the rate of reaction
  • have to consider the le Chatelier’s effect on conc using non-standard conditions , when measuring electrode potentials
  • rate of reaction may be too slow- looks like no reaction is occurring when it is
  • reaction has high Ea which inhibits reaction
  • not all redox reactions are aqueous
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13
Q

What is oxidised and reduced in s,p,d block?

A

S block elements - oxidised to form +1/+2 ions
P block elements
metal ions: oxidation to form + ions that are consistent/not with group of periodic table
E.g Sn —> SN2+ + 2e-
Al —> Al3+ + 3e-
Non metal ions: reduced to form - ions
Charge : group no. MINUS 8
D block elements - form various ions due to variable oxidation states
- usually oxidised to form + ions (cu2+ , Cr3+, V5+)

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14
Q

What is a fuel cell?

A

Electrochemical cell in which a fuel donates electrons to 1 electrode and oxygen gains electrons at the other electrode

  • methanol/hydrogen rich fuels can be used in fuel cells as well as different electrolytes
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15
Q

Half equations for alkaline hydrogen-oxygen fuel cells?

A

NEGATIVE ELECTRODE:
H2 (g) + 2OH– (aq) → 2H2O (l) + 2e– Eθ=-0.83V

POSITIVE ELECTRODE:
O2 (g) + 2H2O + 4e– → 4OH– (aq) Eθ =+0.40V

Full equation :
2H2 (g) + O2 (g) → 2H2O (l) Eθ = +1.23 V

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16
Q

Half equations for acidic hydrogen-oxygen fuel cell?

A

NEGATIVE ELECTRODE:
H2 (g) → 2H+ (aq) + 2e– Eθ = 0.00 V

POSITIVE ELECTRODE:
O2 (g) + 4H+ (aq) + 4e– → 2H2O (l) Eθ = +1.23 V

Full :
2H2 (g) + O2 (g) → 2H2O (l) Eθ = +1.23 V

17
Q

Benefits of fuel cells?

A
  • water is only by product - good for environment
  • Greater efficiency
  • no harmful oxides of nitrogen produced
18
Q

Limitations of fuel cells?

A
  • hydrogen is flammable - storage/production of this is safety hazard
  • thick walled cylinders/pipes need to store hydrogen (economic impacts)
  • hydrogen is by-product of crude oil industry so relies on non renewable/finite resource
  • expensive to produce
19
Q

Potassium manganate titrations - what is used? When is end point reached?

A

Manganate (VII) = OXIDISING AGENT /reduced to Mn2+ (burette)
Iron = REDUCING AGENT /oxidised to Fe3+
Dilute sulfuric Acid added to iron before reaction starts (doesn’t oxidise under the conditions/doesn’t react with Mn(VII) IONS )

  • purple —> pale pink colour - remains as you swirl
  • pale pink due to excess of manganate (VII)ions after all analyte ions reacted with manganate (VII) ions
20
Q

Iodine thiosulfate titrations ?

A

2S2O32– (aq) + I2 (aq) → 2I–(aq) + S4O62– (aq)

pale yellow colour of the iodine turns paler as it is converted to colourless iodide ions
When the solution is a straw colour, starch is added to clarify the end point -
Blue/black —> colourless

  • titration can be used to determine conc of oxidising agent, which oxidises iodine ions to iodine molecules
21
Q

Why should starch not be added until nearly all iodine has reacted in iodine thiosulfate titration?

A

Blue complex formed with high concs of iodine is INSOLUBLE

22
Q

What are lead acid batteries ?

A

Consists of 6 cells joined tgt in series - powers starter motor in car engines
- use lead metal at negative electrode + lead oxide at positive electrode
- sulfuric acid electrolyte

Pb (s) + SO42- (aq) → PbSO4 (s) + 2e- Eθ = -0.36 V

PbO2 (s) + 4H+ (aq) + SO42- (aq) + 2e- → PbSO4 (s) + 2H2O (l) Eθ = +1.70 V

overall reaction is
PbO2 (s) + 4H+ (aq) + 2SO42- (aq) + Pb (s) → 2PbSO4 (s) + 2H2O (l) Eθcell = +2.06V

23
Q

How do lead acid batteries regenerate lead+lead oxide?

A

Car is in motion - generator provides a push of electrons that REVERSES reaction/regenerate lead and lead oxide

24
Q

Disadvantage of lead acid batteries?

A

Heavy
Contain toxin materials
Sulfuric acid electrolyte is corrosive

25
Q

why are lithium cells used/what do they consist of?

A
  • lithium ion cells power laptops/mobiles
  • used due to low density/high electrode potential

Consists of:
Positive lithium cobalt oxide electrode
Negative carbon electrode
Porous polymer membrane electrolyte

26
Q

Half cell reactions when lithium cell discharges?

A

Li (s) → Li+ (s) + e– Eθ = -3 V

Li+ (s) + CoO2 (s) + e– → Li + (CoO2) – (s) Eθ = +1 V

Overall:
Li (s) + CoO2 (s) → Li + (CoO2) – (s) Eθcell ~ +3.5 to 4

27
Q

Problems with lithium cells?

A
  • global shortage of lithium - unsustainable
  • cells thrown away in landfills
  • lithium on cell fires - raise concerns for safety
28
Q

What are primary and secondary cells?

A

Primary - non rechargeable
Secondary - rechargeable

29
Q

What is the acid/alakli purpose in fuel cells?

A

Electrolyte - allow movement of ions

30
Q

2 advantages of using ethanol fuel cell rather than hydrogen fuel cell?

A
  • easier to store
  • renewable