Unit 1: Section 8 - Electrode potentials and cells CDS * Flashcards
electrode potentials the electrochemical series batteries and fuel cells
how can an electrochemical cell be made?
a cell can be made from 2 different metals dipped in salt solutions of their own ions and connected by a wire (external circuit)
the solutions are connected by a salt bridge (internal circuit)
what happens in an electrochemical cell?
there are always two reactions within an electrochemical cell - one’s an oxidation and one’s a reduction - it’s a redox process
what happens in the zinc/ copper electrochemical cell?
zinc loses electrons more easily than copper (more -ve). so in the zinc electrode zinc is oxidised to form Zn 2+ (aq) ions. this releases electrons into the external circuit
in the other half-cell, the same number of electrons are taken from the external circuit, reducing the Cu 2+ ions to copper atoms
how do electrons move in an electrochemical circuit?
electrons flow through the wire from the more reactive metal to the less reactive one
what does a voltmeter do in electrochemical cells?
you can put a voltmeter in the external circuit to measure the voltage between the 2 half-cells. this is the cell potential or EMF
what do you do if the half-cells contain 2 aqueous ions of the same element?
a platinum electrode is needed to conduct electricity, it is an inert metal so it won’t react with the ions
how do you combine half equations in electrochemical cells?
both the half equations are written with the reduction reaction in the forwards direction.
the metal that has the most positive electrode potential is the reduction reaction so the other half equation is flipped to become an oxidation
what is the standard electrode potential?
the standard electrode potential, E°, of a half-cell is the voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode
what are standard conditions for a standard hydrogen electrode?
any solutions must have a concentration of 1 mol/dm^3
temperature must be 298K (25*C)
pressure must be 100kPa
current = 0 - use high resistance voltmeter
how to find the electrode potential of a half equation using the standard hydrogen electrode?
under standard conditions, the reading on the voltmeter when a half-cell is connected to the standard hydrogen electrode will be the standard electrode potential of that half-cell
what is an electrochemical series?
a list of standard electrode potentials for different electrochemical half-cells
how do you calculate standard cell potentials or EMF when 2 half-cells are joined together?
you can take the standard electrode potential value of the reduction reaction from the value of the oxidation value.
(most positive) - (most negative)
what is the shorthand for drawing electrochemical cells?
the half cell with the more positive potential (reduced) goes on the right
the oxidising agents (on same side as electrons) (more +ve) go in the centre of the cell diagram
a double line shows the salt bridge between the 2 half cells
a single line separates substances in different phases (states)
what are batteries?
types of electrochemical cell which provide the electricity we use to power things like watches and mobile phones. some types of cell are rechargeable while others can only be used until they run out
compare rechargeable and non-rechargeable battery prices:
non-rechargeable batteries are cheaper
you can recharge and re-use rechargeable batteries so they last longer and work out cheaper in the long run
what are lithium cells?
made up of a lithium cobalt oxide electrode and a graphite electrode. the electrolyte is a lithium salt in an organic solvent. Li --> Li+ + e- Li+ +CoO2 + e- --> Li+[CoO2]- EMF = +3.6 V used in mobile phones and laptops
how are rechargeable batteries recharged?
a current is supplied to force electrons to flow in the opposite direction around the circuit and reverse the reactions. this is difficult or impossible in non-rechargeable batteries
what happens in fuel cells?
the chemicals are stored separately outside the cell and fed in when electricity is required
what is the alkaline hydrogen-oxygen fuel cell?
hydrogen and oxygen gases are fed into 2 separate platinum-containing electrodes. the electrodes are separated by an anion-exchange membrane that allows anions and water to pass through it, but not hydrogen and oxygen gas.
the electrolyte is an aqueous alkaline
the electrons flow from the negative electrode through an external circuit to the positive electrode
the OH- ions pass through the anion-exchange membrane towards the negative electrode
what is the overall reaction of the alkaline hydrogen-oxygen fuel cell?
2H2(g) + O2(g) –> 2H2O(g)
what can the alkaline hydrogen-oxygen fuel cell be used for?
to power electric vehicles
how are fuel cells more efficient?
they convert more of their available energy into kinetic energy to get the car moving.
internal combustion engines waste a lot of their energy producing heat
what are the advantages of water being the only waste product of fuel cells?
there are no toxic chemicals to dispose of and no CO2 emissions from the cell itself
why don’t fuel cells need to be recharged?
as long as hydrogen and oxygen are supplied, the cell will continue to produce electricity
how are fuel cells not carbon neutral
you need energy to produce a supply of hydrogen and oxygen. they can be produced from the electrolysis of water, but this requires electricity and this electricity is normally generated by burning fossil fuels.
what happens to the electrodes and electrolytes in an electrochemical cell over time?
e.g. Zn(s) + Cu 2+(aq) –> Cu(s) + Zn 2+(aq)
less zinc
less copper in solution (fades to colourless)
more copper
more zinc in solution
what does the standard hydrogen electrode consist of?
platinised platinum electrode covered in platinum black
platinum wire connecting platinum electrode and high resistance voltmeter
hydrogen gas at 100kPa
1M hydrochloric acid solution
what is the best oxidising agent?
F2
the F2/F- half equation has the most positive electrode potential
so F2 is the best at being reduced
what are the different electrodes called?
negative electrode = anode
oxidation happens here - loses electrons
positive electrode = cathode
reduction happens here - gains electrons
What are some examples of non-rechargeable cells?
zinc-carbon
alkaline
What are some examples of rechargeable cells?
Lithium ion
lead-acid
nickel-cadmium
what is the zinc-carbon cell?
Zn(NH3)2 2+ + 2e- ---> Zn + 2NH3 2MnO2 + 2H+ +2e- ----> Mn2O3 +H2O EMF = +1.5 V standard cell short life
what is an alkaline cell?
Zn 2+ + 2e- —> Zn
MnO2 +H2O + Zn —-> MnO(OH) + OH-
EMF = +1.6 V
why do alkaline cells last longer than zinc-carbon cells?
the zinc-carbon cells have a faster rate of reaction so run out quicker than the alkaline cells
what is the reaction for when a lithium cell is in use?
CoO2 + Li —> LiCoO2
also known as discharging
this reaction is slower so the battery lasts longer
what is the reaction for when a lithium cell is charging?
LiCoO2 —> CoO2 + Li
this reaction is faster so it charges quickly
what is a lead-acid cell?
PbO2 + 3H+ + H2SO4 - + 2e- —-> PbSO4 + 2H2O
Pb + H2SO4 - + Pb —–> PbSO4 + H+ + 2e-
In use :
PbO2 + 2H+ + 2H2SO4 - + Pb –> 2PbSO4 + 2H2O
charging:
2PbSO4 + 2H2O –> PbO2 + 2H+ + 2H2SO4 - + Pb
it’s used in sealed car batteries
EMF= +2.04V
what is the nickel - cadmium cell?
NiO(OH) +2H2O + 2e- ---> Ni(OH)2 + 2OH- Cd + 2OH- ----> Cd(OH)2 +2e- In use: NiO(OH) +2H2O +Cd --> Ni(OH)2 + Cd(OH)2 charging: Ni(OH)2 + Cd(OH)2 --> NiO(OH) + 2H2O + Cd EMF = +1.4V
what happens in an acidic hydrogen - oxygen fuel cell?
H2 ===> 2H+ + 2e-
O2 + 4H+ + 4e- ===> 2H2O
H2 enters the cell at the anode end and produces H+ ions and electrons
H+ ions diffuse through the electrolyte down conc. gradient to cathode
electrons move through a wire to the cathode
Oxygen enters at the cathode end and reacts with the H+ ions and uses the electrons to produce water
what happens in an alkaline hydrogen - oxygen fuel cell?
H2 + 2OH- ===> 2H2O + 2e-
O2 + 2H2O +4e- ===> 4OH-
H2 enters the cell at the anode end and reacts with OH- ions to produce H2O and electrons
H2O diffuses through the electrolyte down water potential gradient to cathode
electrons move through a wire to the cathode
Oxygen enters at the cathode end and reacts with the water and uses the electrons to produce OH- ions
OH- ions diffuse down conc. gradient to the anode
What are the benefits of using cells?
portable source of electrical energy
what are the risks of using cells?
waste issues
What are the benefits of using non-rechargeable cells?
cheap
what are the risks of using non-rechargeable cells?
waste issues
What are the benefits of using rechargeable cells?
less waste
cheaper in the long run
lower environmental impact
what are the risks of using rechargeable cells?
some waste issues - at end of useful life
What are the benefits of using hydrogen fuel cells?
only waste product is water
do not need re-charging
very efficient
what are the risks of using hydrogen fuel cells?
need constant supply of fuels
hydrogen is flammable and explosive - it needs to be handled carefully when it is stored or transported
hydrogen usually made using fossil fuels
high cost of fuel cells
