Redox I & II Flashcards
What does a half equation show
Shows the gain or loss of electrons and is balanced for atoms and charge
What is oxidation
Loss of electrons
What is reduction
Gain of electrons
What does the oxidation state number represent
represents the number of electrons lost/gained by an atom relative to its elemental/atomic state.
Whats an oxidising agent
a species that oxidises other species (by gaining electrons from them)
Whats a reducing agent
a species that reduces other species (by losing/giving electrons to them)
What does the overall redox equation show
the species or ions changing oxidation states.
What is a half cell / electrode
an equilibrium system consisting of a solid metal in a solution of its own ions.
What is electrode potential?
a measure of the equilibrium position for a half-cell / electrode.
What is the standard electrode potential
the voltage produced by a half-cell when connected to a standard hydrogen electrode, under standard conditions (298K, 100kPa, all solutions 1 mol dm-3)
What is the problem with not having a salt bridge
The circuit of a cell is not complete
What is a half cell / electrode
An equilibrium of a metal and its ions
e.g. Cu(s) ⇌ Cu2+(aq) + 2e-
What are the standard conditions for a standard electrode potential
100kPa, 298K, 1 mol dm-3
Describe a standard hydrogen electrode (4 marks)
Platinum rod allowing the H2 half reaction to take place, the rod being unreactive
H2 gas bubbles form
under standard conditions which are 298K, 100kPa and in this case, 1 M of H+ ions (acid)
What happens if we heat up the below reaction and why
Na+ + e- ⇌ Na
equilibrium shifts left
as the forwards reaction is exothermic as we are making bonds and forming attractions
so the system shifts its equilibrium to counteract the change and so it moves in the endothermic direction
this is the backwards reaction
What happens if we heat up the below reaction and why
Cr3+ + 3e- ⇌ Cr
equilibrium shifts left
as the forwards reaction is exothermic as we are making bonds and forming attractions
so the system shifts its equilibrium to counteract the change and so it moves in the endothermic direction
this is the backwards reaction
How do represent a cell with a magnesium electrode and a silver electrode
Mg(s) I Mg2+(aq) I I Ag+(aq) I Ag(s)
What goes on the right side of the shorthand for representing cells and what does this mean
The most positive thing
this means Ecell values will always positive UNLESS its with a hydrogen cell. Then it could be either positive or negative
If a iron cell has an E value of -0.44 and a lead cell has an E value of -0.13, what is the
Ecell value
+0.31
Why are fuel cells awesome (good)
- only waste product is water
- light weight
- doesn’t require fossil fuels
Why are fuel cells not awesome (bad)
- isn’t widely available
- often made from methane
- hydrogen is explosive when stored badly
why can a titration be self indicating
one of the reactants is coloured, and a tiny excess of one of the reagents signals the end point
When does a redox reaction show feasibility?
when the emf is positive
When is a redox reaction not feasible using emf ?
When the emf is negative
Why might a redox reaction not happen even if the emf is positive?
kinetic factors, such as Ea, temperature, concentrations
Copper reacts with concentrated nitric acid. Why?
2H+ = H2 E = 0.0
Cu2+ + 2e- = Cu E = +0.34
NO3- + 2H+ + e- = NO2 + H2O E = +0.80
NO2- runs right and Cu runs left
Cu is less positive than the NO3-
E cell = +0.80 + -0.34 = +0.46
emf is pos and so the reaction is feasible
How do E cell and Δ S total link?
ΔG = -n F Ecell
ΔG = -T ΔStotal
so…
-n F Ecell = -T ΔStotal
and because n and F are both constant for a GIVEN CELL REACTION and temperature
ΔS total is directly proportional to E cell
what is a salt bridge usually soaked in
KNO3
what is the point if a salt bridge
maintains electrical neutrality within the cell by providing ions to either side
why is a salt bridge made up of soaked paper rather than metal
the metal may form their own metal/ion potentials in the circuit
why are the comparative readings in the data book called the right hand electrode systems
because the left hand electrode will always be the hydrogen electrode when we measure them
if the value of an electrode potential is positive where does equilibrium lie
to the right
if the value of an electrode potential is negative where does equilibrium lie?
to the left
combine the half cells
Al3+ + 3e- ⇌ Al -1.66
Cu2+ + 2e- ⇌ Cu +0.34
- rewrite in the correct directions
combine them like normal redox, by balancing the electrons
so 3Cu2+ + 2Al -> 3Cu + 2Al3+
how do ln k and e cell link
ΔG = - n F Ecell
ΔG = -RT ln K
so - n F E cell = - RT ln K
so at a given T, ln K = E cell due to the removal of constants
You want to find the concentration of a solution of
FeSO4.
You take 25cm3 of the solution, acidify it, then titrate
with 0.01mol dm-3 KMnO4 solution. It requires 14.30cm3
of KMnO4 solution to fully react.
Half equations:
Fe2+ 🡪 Fe3+ + e-
MnO4- + 8 H+ + 5 e- 🡪 Mn2+ + 4H2O
- Ratio will be 5 Fe2+ : 1 MnO4-
- Moles of KMnO4 = 0.01 x 0.0143 = 0.000143 mol
- So mol Fe2+ = 0.000143 x 5 = 0.000715 mol
- So conc of Fe2+ = 0.000715 / 0.025 = 0.0286 mol dm-3
You want to find the conc of another solution of FeSO4.
You take 25cm3 of the solution, acidify it, then titrate
with 0.02mol dm-3 K2Cr2O7 solution. It requires
22.60cm3 of K2Cr2O7 solution to fully react.
Half equations:
Fe2+ 🡪 Fe3+ + e-
Cr2O72- + 14 H+ + 6 e- 🡪 2 Cr3+ + 7H2O
- Ratio will be 6 Fe2+ : 1 Cr2O72-
- Moles of K2Cr2O7 = 0.02 x 0.0226 = 0.000452 mol
- So mol Fe2+ = 0.000452 x 6 = 0.002712 mol
- So conc of Fe2+ = 0.002712 / 0.025 = 0.108 mol dm-3
What does an alkali fuel cell electrode look like
At the negative electrode:
H2 (g) + 2OH- (aq) -> 2H2O (l) + 2e-
At the positive electrode:
½ O2 (g) + H2O (l) + 4e- -> 4OH- (aq)
What does an acid fuel cell electrode look like
At the negative electrode:
H2(g) 🡪 2H+(aq) + 2e-
At the positive electrode:
½ O2(g) + 2H+(aq) + 2e- 🡪 H2O(l)
