PHYSICAL - electrode potentials and cells Flashcards
what happens when a rod of 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+ –> Cu3+ + e-
what is the salt bridge made of?
filter paper soaked in saturated solution of KNO3 ( potassium nitrate)
why are salt bridges necessary?
to complete the circuit = allows ion movement to balance the charge.
Do not react with electrodes
symbol to represent salt bridge?
||
What type of species goes on the outside (furthest from the salt bridge) in the standard cell notation?
The most reduced species
What does | indicate?
Phase boundary (solid/ liquid/ gas)
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 E value
What happens at the right hand electrode
Right hand electrode is where reduction occurs
Half cell with most positive E value
What conditions is the standard hydrogen electrode used in?
temp - 298K
pressure - 100kPa
[H+] = 1.00 mol dm-3
What is the standard hydrogen electro used for?
Comparing other cells against the E
SHE is defined as 0
So all the other E values are compared against it
Why won’t you use other standard electrodes occasionally?
- cheaper
- quicker and easier to use
- provide just as good reference
- Pt is expensive
If E value is more negative, what does it mean in terms of oxidising/ reducing power?
Better reducing agent (easier to oxidise)
If E value is more positive, what does it mean in terms of oxidising/reducing power?
Oxidising agent (easier to reduce)
What factors will change E values?
Concentration of ions and temperature
what is the effect of reducing conc of ions in lef hand of half cell?
- equilibrium shifts to the left to oppose the change of removing ions
- releases more electrons
- E value becomes more negative in left hand cell
- EMF of cell increases
How do you calculate the EMF of a cell from E values?
Ecell = E right - E left
Why would you use a platinum electrode?
both oxidised and reduced forms of the metals are an aqueous solution
- inert so doesn’t partake in electrochemistry
- good conductor to complete circuit
How would you predict if a reaction would occur?
Take 2 half equations
Find species being reduced
Calculate overall e-value
If overall E value > 0, reaction will occur
What are zinc/carbon cells most commonly known as?
Disposable batteries
How are cells recharged?
Reactions are reversible and are reversed by running a high voltage through the cell and the cells E
Where are lithium-ion cells used?
- Mobile phone
- laptops
