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