20. Electrode potentials and electrochemical cells Flashcards
What occurs when you place two different metals in a salt solution and connect them together
An electric current flows so that electrons pass from the more reactive metal to the less reactive
Give the equation for the half cell of zinc
Zn(s) —> Zn2+ (aq) + 2e-
What is oxidation
Loss of electrons
What is reduction
Gain of electrons
What is an oxidising agent
An electron acceptor
What is an reducing agent
An electron donator
Give an example of an ionic solution used for a salt bridge
KNO3
What is a salt bridge
In its simplest form, a piece of paper socked in an ionic solution which allows the movement ions between half cells
Why is a salt bridge used rather than a wire
Wire only allows the transfer of electrons whereas a salt bridge transfers ions
What does a more negative potential difference mean
- a more negative electrode loses electrons more readily
- it is more easily oxidised
- it is a better reducing agent
What can be used to measure the potential difference of a half cell
SHE - standard hydrogen electrode
What is in a SHE electrode
Hydrogen gas is bubbled into a solution of H+ (aq) ions (HCl) with a Pt electrode
What are the standard conditions in cells
100kPa
1 mol dm-3
298K
What is the potential of a SHE
Defined as 0
What is emf
- electromotive force E
- the maximum potential difference between two half cells
Which side does a SHE always go
Left
What does EMF=
EMF= E(R)- E(L)
The species with the highest what is written next to the salt bridge
oxidation state
What is an oxidation state
Represents how many electrons a species has lost
Why is the reaction Cu2+ (aq) + Zn (s) —> Cu (s) + Zn2+ (aq) feasible given:
Zn2+ (aq) + 2e- Zn(s) E= -0.76V
Cu2+ (aq) + 2e- Cu(s) E= +0.34
Zn2+/Zn E < Cu2+/Cu therefore electrons are flowing from the more negative to the more positive (Zn giving Cu2+ electrons)
EMF= +ve therefore feasible as electrons moving from left to right (-ve to +ve)
In a redox system that only involves metal ions but not metal e.g. Fe3+/Fe2+ what is used as an electrode
Platinum electrode
What is a battery
A number of cells connected together
Name 2 non-rechargeable cells
- zinc/copper
- zinc/carbon
What does zinc/carbon cell consist of
- carbon electrode coated in manganese (IV) oxide
- zinc canister
- paste of ammonium chloride (NH4Cl)
- water
Why is the carbon coated in manganese (IV) oxide (zinc/carbon cell)
H2 gas produced in the redox reaction is oxidised to water by the manganese (IV) oxide to prevent build up of pressure
Why is there water in the zinc/carbon cell
Ammonia gas dissolves in it to prevent build up of pressure
Why are zinc/carbon cells prone to leakage
- as zinc is used up the walls become thin and prone to leakage
- NH4Cl is corrosive and acidic
- old batteries should therefore be removed from equipment
Name a rechargeable battery
Lead-acid battery
How are rechargeable batteries recharged
By reversing the cell reactions by applying external voltage greater than the voltage of the cell to drive the electrons in the opposite directions
What are lead acid batteries used in
Used to operate the starter motors of cars.
What do acid lead batteries consist of
Six 2V cells connected in series (giving 12V)
What do acid lead cells consist of
- two plates dipped into a solution of sulfuric acid
- positive plate is made of lead coated with lead (IV) oxide
- the negative plate is made of lead
Name 3 portable batteries
Nickel/cadmium
Lithium ion
Alkaline hydrogen-oxygen fuel cell
Why is the nickel/cadmium battery replacing the traditional zinc/carbon
They are rechargeable so more cost effective
What does the lithium ion cell consist of
- positive electrode is made of lithium cobalt oxide LiCoO2
- negative electrode is carbon
- these are layered with a sandwich of solid electrolyte in between
What are the reactions that take place in the lithium ion cell
negative electrode: Li+ +e- —> Li
positive electrode: Li+(CoO2)- —-> Li+ +CoO2 + e-
What are the reactions that take place in an alkaline hydrogen-oxygen fuel cell
negative electrode: 2H2 (g) + 4OH- (aq) ---->4H2O (l) +4e- positive electrode: O2 (g) + 2H2O (l) + 4e- --->4OH- (aq) overall: 2H2 (g) + O2 (g) ---->2H2O (l)
What happens in an alkaline hydrogen-oxygen fuel cell
-hydrogen enters at the negative electrode:
2H2 (g) + 4OH- (aq) —->4H2O (l) +4e-
-this releases electrons which flows through the circuit to the other electrode
-this reaction then takes place at the positive electrode O2 (g) + 2H2O (l) + 4e- —>4OH- (aq)
-this accepts electrons from the other electrode and releases OH- ions
What are the electrodes made of in an alkaline hydrogen-oxygen fuel cell
porous platinum based material
Why are alkaline hydrogen-oxygen fuel cells used in spaceships
The only bi-product is water which can be used by the astronauts
Why are alkaline hydrogen-oxygen fuel cells not as green as they appear
Most hydrogen is made from crude-oil, the non renewable
Why is using an alkaline hydrogen-oxygen fuel cell in cars not safe/ good
It requires the storage of hydrogen gas which can explode/ is extremely flammable.
It is also hard to store hydrogen as it takes up alot of space but dangerous to pressurise.
