Electrode Potentials and Electrochemical Cells Flashcards
What happens when a rod of a metal is dipped into a
solution of its own ions?
An equilibrium is set up between the solid metal and the aqueous metal ions
Write a half-equation for zinc (s) to zinc (II).
Zn (s) ⇌ Zn2+(aq) + 2e-
Write a half-equation for copper (II) to copper (III).
Cu2+(aq) ⇌ Cu3+(aq) + e-
What is the simplest salt bridge made of?
Filter paper soaked in saturated solution of KNO3
potassium nitrate
Why are salt bridges necessary?
Complete the circuit, but avoid further metal/ion potentials as does not perform electrochemistry.
Allows ion movement to balance the charge. Do not react with electrodes
What symbol is used to represent a salt bridge in
standard notation?
||
What type of species goes on the outside (furthest from the salt bridge) in standard cell notation?
The most reduced species
What does | indicate?
Phase boundary (s/l/g)
How would an Aluminium/ Copper cell be represented?
Al(s) | Al3+(aq) || Cu2+(aq) | Cu(s)
What happens at the left-hand electrode?
Left hand electrode is where oxidation occurs.
Left hand electrode is the half cell with the most negative Eo value
What happens at the right hand electrode?
right hand electrode is where reduction occurs.
Right hand electrode is the half cell with the most
positive Eo value
Which side of the cell has the most negative Eo value? what happens to the metal with the most negative Eo value?
Oxidation - left hand electrode
What conditions is the standard hydrogen electrode used in?
Temperature = 298 K Pressure = 100 kPa [H+] = 1.00 mol dm-3
What is the standard hydrogen electrode used for?
Comparing other cells against. EO of SHE is defined as 0, so all other Eo values are compared against it.
Why might you use other standard electrodes
occasionally?
They are cheaper/ easier/ quicker to use and can
provide just as good a reference.
Platinum is expensive
If an Eo value is more negative, what does it mean in terms of oxidising/reducing power?
Better reducing agent (easier to oxidise)
If an Eo value is more positive, what does it mean in terms of oxidising/reducing power?
Better oxidising agent (easier to reduce)
What factors will change Eo values?
Concentration of ions
Temperature
What happens if you reduce the concentration of the ions in the left hand half cell?
- Equilibrium moves to the left to oppose the change of removing ions
- this releases more electrons, the Eo of the left hand cell becomes more negative
- the e.m.f. Of the cell increases.
How do you calculate the emf of a cell from Eo values?
Eo cell = Eo right - Eo left
When would you use a Platinum electrode?
When both the oxidised and reduced forms of the metal are in aqueous solution
Why is Platinum chosen?
Inert so does not take part in the electrochemistry
Good conductor to complete circuit
How would you predict if a reaction would occur?
- Take the 2 half equations.
- Find the species that is being reduced
- Calculate its Eo value minus the Eo value of the species that is being oxidised
- If Eo overall > 0, reaction will occur.
What was the first commercial cell made from
(Daniell cell)?
Zinc/copper (II)
What are zinc/carbon cells more commonly known as?
Disposable batteries
What are the two reactions that take place in
zinc/carbon cells?
Zn oxidised to Zn2+
NH4+ reduced to NH3 at carbon electrode
What are the reactions that occur in a lead/acid battery (car batteries)?
Pb + SO42- → PbSO4 (s) + 2e-
PbO2 + 4H+ + SO42- + 2e- → PbSO4 + 2H2O
How are cells recharged (if they are rechargeable)?
Reactions are reversible and are reversed by running a higher voltage through the cell than the
cell’s Eo
Nickel/cadmium cells are rechargeable AA batteries
etc. What reactions occur at the electrodes?
Cd(OH)2 (s) + 2e- → Cd(s) + 2OH-
NiO(OH) (s) + H2O + e- → Ni(OH)2 (s) + OH-
Where are lithium-ion cells used?
Mobile phones
Laptops
What reactions occur on discharge in lithium-ion cells?
Li+ + CoO2 + e- → Li+[CoO2]-
Li → Li+ +e-
+ e
What is a fuel cell?
A cell that is used to generate electric current;
does not require electrical recharging
What are the reactions that take place at the two electrons in an alkaline hydrogen fuel cell?
2H2 + 4OH- → 4H2O + 4e-
O2 + 2H2O + 4e- → 4OH-
Why is it better to use a fuel cell than to burn H2 in
air, even though the same overall reaction occurs?
In combustion, sulfur containing compounds (SO2 , SO3) and nitrogen containing compounds (NO2, NOx) are produced due to the high temperatures and the S and N in air.
These are bad for the environment.
This does not occur in a fuel cell; the only product is water.
More efficient
Disadvantages of fuel cells
Hydrogen is a flammable gas with a low b.p. → hard and dangerous to store and transport → expensive to buy
Fuel cells have a limited lifetime and use toxic chemicals in their manufacture
How do you find the weakest reducing agent from a table of electrode potential data?
Most positive Eθ value.
Then it is the PRODUCT of the reduction equation i.e. imagine equation going from right to left
What is the reason that some cells cannot be recharged?
Reaction of the cell is not reversible - a product is produced that either dissipates or cannot be converted back into the reactants
Why might the e.m.f. Of a cell change after a period of time?
Concentrations of the ions change - the reagents are used up
How can the e.m.f. Of a cell be kept constant?
Reagents are supplied constantly, so the
concentrations of the ions are constant; Eo remains constant
