Topic 14 Redox II Flashcards
if equilibrium lies to the left, the electrode will have a ____ potential.
If equilibrium lies to the right, the electrode will have a ____ potential.
Negative
Positive
What are the conditions under which the potential is 0.00V?
cell at:
- 298K
- 100kPa / 1atm
- 1.0 moldm-3 of ions involved in half eq
how to write conventional representations of cells?
AKA cell diagrams
e.g.
Zn(s) | Zn2+(aq) || Cu2+(aq)|Cu(s)
put highest oxidation state (of metal) closest to ||
only | when change of states, use comma if not
include H+ or H2O if involved in half eq (using [X +H+] but exclude e-s
ΔG =
-RTlnK
or
ΔH - TΔS
ΔStot =
RlnK
and
ΔSsyst + ΔSsurr
ΔG is proportional to …
-lnK
And
- E cell
ΔStot is proportional to …
lnK
ΔG is DIRECTLY proportional to …
-lnK and -E cell
ΔStot is DIRECTLY proportional to …
E°cell
How to determine whether at cathode or anode?
Reduction at cathode (more positive one, usually RHS)
Oxidation at anode
How to find electrode potential?
RHS - LHS
What metal is used for Fe3+ / Fe2+ electrode system?
Platinum
bcs no Fe (s) involved
What metal is used for Ti2+/Ti electrode system?
Ti
Titanium
bcs Ti involved
What is the most soluble compound used as an electrolyte of a half cell?
NITRATES is go to
What is the definition of standard electrode potential?
the voltage produced when a standard half-cell is connected to a standard hydrogen cell under standard conditions
what do salt bridges do?
allow flow of ions
What are the 3 types of half cells?
metal
gas
redox
limitations of electrode potential predictions?
- non standard conditions = values different to E standard cells
- RoR is slow, may appear reaction not happening
- Ea way too high, preventing feasible reactions taking place
What is the standard hydrogen electrode (SHE) used for?
primary standard for other potentials to compare with
When 2 half cells are put together, which one goes forwards and which one backwards?
Compare SHE values
more positive one goes on RHS and goes forward
more neg goes backwards and LHS
What happens when you put two half cells with very similar standard electrode potential values together?
- becomes feasible
- more positive one becomes more neg (releases e-s)
- more neg becomes more pos (steals e-s)
- can end up switching places (of originially which one is more +ve and -ve)
How to know what materials are used for electrodes in the cell when only ions are given?
When there is a metal with oxidation number 0, use the metal solid.
If all have charges, meaning Pt used as ions given are reactants
Deduce the oxidation half equation under alkaline conditions
Where [Cr(OH)6]3- oxidises into CrO4 2-
[Cr(OH)6]^3- + 2OH- -> CrO4^2- + 4H2O + 3e-
Deduce the equation for when chromate(VI) ions are acidified into orange dichromate(VI) ions. (1 mark)
2CrO4 2- + 2H+ -> Cr2O7 2- + H2O
State what chemical is submerged in the salt bridge. (1 mark)
KNO3
Why is KNO3 specifically used for salt bridges, and not using unreactive metal wires instead? (1 mark)
KNO3 salt bridge allows flow of ions but latter does not
Wire allows flow of electrons not ions
Which species is the strongest reducing agent on the data booklet?
Na+
Because it has the most negative E cell value
Using electrochemical series, with two equations, which one goes on LHS and RHS?
More negative on LHS
What battery cell has potential difference plotted against time and the line is horizontally straight?
Hydrogen Fuel cell
What is the hydrogen fuel cell (alkaline) equation?
2H2 + 4OH- -> 4H2O + 4e-
E cell = -0.83V
What is the hydrogen fuel cell (acidic) equation?
2H2 -> 4H+ + 4e-
Give one reason why the e.m.f. of the lead-acid cell changes after several hours. (1 mark)
reagents/ions are used up
Explain why rechargable cells are connected to solar cells for road lights that is on 24/7. (2 marks)
- solar cells do not supply electrical energy all the time
- rechargeable cells can store electrical energy to use at night time
Suggest one reason why many waste disposal centres contain a separate section for cells and batteries. (1 mark)
- prevent pollution of environment by toxic substances
- recycle valuable components
does the negative electrode take in or release e-s?
release
hence it is negative
Give a reason why water is not used as a solvent in a lithium cell.
lithium will react with water (to form LiOH)
Suggest a reason why a cell cannot be electrically recharged. (1 mark)
Reaction is not reversible
What is the essential property of the non-reactive porous separator of a non-rechargable cell?
Allow ions to pass through
The zinc electrode acts as a container for the cell and is protected from external damage. Suggest why a cell often leaks after being used for a long time. (1 mark)
Zinc has reacted (oxidised) and is used up
Suggest the function of the carbon rod in the non-rechargeable cell. (1 mark)
allow electrons to flow
A right hand half cell has VO2^+ ions and VO^2+ ions. The electrolyte is prepared using equal volumes of what conc of acidified of both?
2 Moldm-3 each
Because they mix together in the mixture, so total volume will increase
and will react at 1:1 ratio
After reacting they become 1moldm-3 each (which is standard)
How to determine strength of reducing agent of ions from data booklet?
Strongest on RHS on equation
Highest strength lowest down ?
What is the alternative name for standard electrode potential?
Standard reduction potential
(As e-s are gained so they’re being reduced)
2CrO4 ^2- + 2H+ <—> Cr2O7^2- + H2O
Explain any change in position of equilibrium if a few drops of NaOH solution is added to the equilibrium system. (2 marks)
Shifts to LHS
OH- reacts w H+ to make more H2O
So less H+, more H2O
What’s the ratio of Fe2+ : MnO4 ^- ?
5:1
What’s the ratio of Fe2+ : MnO4 ^- ?
5:1
How to find percentage of copper in brass?
+ Conc HNO3
Then neutralise with sodium carbonate Na2CO3
Then add KI (cloudy brown)
Then titrate with sodium thiosulfate
Then when pale yellow, add starch (turns blue black)
End point = one drop to colourless
Brass is an alloy of which two metals?
Copper and zinc
What is best used to neutralise acids?
Sodium carbonate
Neutralises gently
How to identify the end point of a thiosulfate titration?
Add starch as indicator before end point (when pale yellow)
From b,he black to colourless
State one advantage of the hydrogen-oxygen fuel cell over the use of petrol as fuel in a vehicle. (1 mark)
- harmless product
- is renewable energy , less reliant on non renewable resources
- more efficient energy production
Explain how an acidic fuel cell works
- Fuel cell use energy released on the reaction of fuel (hydrogen with oxygen) to generate a voltage
- at anode: H2 (g) ⇌ 2H+ (aq) + 2e-
- at cathode (they combine) O2 + 4H+ + 4e-⇌ 2H2O (l)
Give one advantage and one disagree of fuel cells.
Ad
- only water produced, CO2 is produced in an internal combustion engine
Disad
- production and transport and storage of hydrogen is difficult and expensive
State the direction of electron flow in hydrogen-oxygen fuel cell + reference redox reactions happening.
E-s move from negative to positive electrode
Hydrogen is getting oxidised, oxygen is getting reduced
