Redox II Flashcards
What is oxidation and reduction?
OXIDATION : loss of electrons
Addition of oxygen
Loss of hydrogen
REDUCTION: gain of electrons
Loss of oxygen
Addition of hydrogen
What are standard conditions for electrode potentials?
Ion conc 1.00 Mol/dm
298K temp
100kPa pressure
- use high resistance voltmeter
What is standard electrode potential?
Potential difference produced when standard half cell is connected to a standard hydrogen cell under standard conditions
What is the standard hydrogen electrode ?
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
What is a reference electrode? Why are they needed?
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
Where is reduction and oxidation more likely to occur?
Reduction - at the more + electrode
Oxidation - at the more - electrode
When is a platinum electrode used to find SHE?
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
What is the salt bridge for? What chemicals are used and why?
- 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
How to calculate emf/potential difference of electrochemical cell?
E right - E left = Ecell
How do the E values infdicate how easily the species can be oxidised or reduced? When is reaction feasible?
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 +
How is cell potential liked to entropy,S/equilibrium constant,K?
Larger cell potential = BIGGER CHANGE IN ENTROPY
Cell potential directly proportional to entropy change
Limitations of standard electrode potential predictions?
- 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
What is oxidised and reduced in s,p,d block?
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+)
What is a fuel cell?
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
Half equations for alkaline hydrogen-oxygen fuel cells?
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
Half equations for acidic hydrogen-oxygen fuel cell?
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
Benefits of fuel cells?
- water is only by product - good for environment
- Greater efficiency
- no harmful oxides of nitrogen produced
Limitations of fuel cells?
- 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
Potassium manganate titrations - what is used? When is end point reached?
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
Iodine thiosulfate titrations ?
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
Why should starch not be added until nearly all iodine has reacted in iodine thiosulfate titration?
Blue complex formed with high concs of iodine is INSOLUBLE
What are lead acid batteries ?
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
How do lead acid batteries regenerate lead+lead oxide?
Car is in motion - generator provides a push of electrons that REVERSES reaction/regenerate lead and lead oxide
Disadvantage of lead acid batteries?
Heavy
Contain toxin materials
Sulfuric acid electrolyte is corrosive
why are lithium cells used/what do they consist of?
- 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
Half cell reactions when lithium cell discharges?
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
Problems with lithium cells?
- global shortage of lithium - unsustainable
- cells thrown away in landfills
- lithium on cell fires - raise concerns for safety
What are primary and secondary cells?
Primary - non rechargeable
Secondary - rechargeable
What is the acid/alakli purpose in fuel cells?
Electrolyte - allow movement of ions
2 advantages of using ethanol fuel cell rather than hydrogen fuel cell?
- easier to store
- renewable
What is the anode?
Always oxidation occurs
Why is only DILUTE SULFURIC ACID used for manganate titrations?
SUPPLIES 8H+ IONS
Weak acid (cannot supply large amount of H+ ions) /low vols of sulfuric acid mean solution is NOT ACIDIC enough —> MnO2 forms, not Mn2+
MnO2 (brown) mask colour change/lead to greater vol of manganate used in titration
Conc HCl not used : Cl- will be oxidised to Cl2 by MnO4- so greater vol of manganate used/poisonous Cl2 produced
Nitric acid not used: oxidising agent so oxidises Fe2+ to Fe3+
Leads to smaller volume of manganate used
Molar ratio between ethanedioate ions and manganate (VII) ions?
3 MnO4- : 5 FeC2O4
How to reduce uncertainties in titrations and in measuring mass?
Inn titration: replace measuring cylinder with pippettes/burettes with lower apparatus uncertainty
Make titre volume greater by increasing VOL/CONC of substance in conical flask/DECREASE CONC of substance in burette —> reduce uncertainty of burette reading
In measuring mass: use balance that measures to more decimal places
Use a larger mass if weighing a solid
Weighing sample before/after then calculate difference —> more accurate measurement of mass added
How to calculate % difference and how is it used?
Difference (between obtained value and real value)/real value x 100
If % uncertainty due to apparatus < % difference —> discrepancy in result is due to other errors
If % uncertainty due to apparatus > % difference —> discrepancy in result can be explained by sensitivity of equipment
