Redox II

Question 1

What is oxidation and reduction?

Answer 1

OXIDATION : loss of electrons
Addition of oxygen
Loss of hydrogen

REDUCTION: gain of electrons
Loss of oxygen
Addition of hydrogen

Question 2

What are standard conditions for electrode potentials?

Answer 2

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

• use high resistance voltmeter

Question 3

What is standard electrode potential?

Answer 3

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

Question 4

What is the standard hydrogen electrode ?

Answer 4

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

Question 5

What is a reference electrode? Why are they needed?

Answer 5

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

Question 6

Where is reduction and oxidation more likely to occur?

Answer 6

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

Question 7

When is a platinum electrode used to find SHE?

Answer 7

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

Question 8

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

Answer 8

• 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

Question 9

How to calculate emf/potential difference of electrochemical cell?

Answer 9

E right - E left = Ecell

Question 10

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

Answer 10

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 +

Question 11

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

Answer 11

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

Question 12

Limitations of standard electrode potential predictions?

Answer 12

• 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

Question 13

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

Answer 13

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+)

Question 14

What is a fuel cell?

Answer 14

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

Question 15

Half equations for alkaline hydrogen-oxygen fuel cells?

Answer 15

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

Question 16

Half equations for acidic hydrogen-oxygen fuel cell?

Answer 16

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

Question 17

Benefits of fuel cells?

Answer 17

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

Question 18

Limitations of fuel cells?

Answer 18

• 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

Question 19

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

Answer 19

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

Question 20

Iodine thiosulfate titrations ?

Answer 20

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

Question 21

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

Answer 21

Blue complex formed with high concs of iodine is INSOLUBLE

Question 22

What are lead acid batteries ?

Answer 22

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

Question 23

How do lead acid batteries regenerate lead+lead oxide?

Answer 23

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

Question 24

Disadvantage of lead acid batteries?

Answer 24

Heavy
Contain toxin materials
Sulfuric acid electrolyte is corrosive

Question 25

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

Answer 25

• 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

Question 26

Half cell reactions when lithium cell discharges?

Answer 26

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

Question 27

Problems with lithium cells?

Answer 27

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

Question 28

What are primary and secondary cells?

Answer 28

Primary - non rechargeable
Secondary - rechargeable

Question 29

What is the acid/alakli purpose in fuel cells?

Answer 29

Electrolyte - allow movement of ions

Question 30

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

Answer 30

• easier to store
• renewable