Unit 1: Section 8 - Electrode potentials and cells CDS *

Question 1

how can an electrochemical cell be made?

Answer 1

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)

Question 2

what happens in an electrochemical cell?

Answer 2

there are always two reactions within an electrochemical cell - one’s an oxidation and one’s a reduction - it’s a redox process

Question 3

what happens in the zinc/ copper electrochemical cell?

Answer 3

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

Question 4

how do electrons move in an electrochemical circuit?

Answer 4

electrons flow through the wire from the more reactive metal to the less reactive one

Question 5

what does a voltmeter do in electrochemical cells?

Answer 5

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

Question 6

what do you do if the half-cells contain 2 aqueous ions of the same element?

Answer 6

a platinum electrode is needed to conduct electricity, it is an inert metal so it won’t react with the ions

Question 7

how do you combine half equations in electrochemical cells?

Answer 7

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

Question 8

what is the standard electrode potential?

Answer 8

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

Question 9

what are standard conditions for a standard hydrogen electrode?

Answer 9

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

Question 10

how to find the electrode potential of a half equation using the standard hydrogen electrode?

Answer 10

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

Question 11

what is an electrochemical series?

Answer 11

a list of standard electrode potentials for different electrochemical half-cells

Question 12

how do you calculate standard cell potentials or EMF when 2 half-cells are joined together?

Answer 12

you can take the standard electrode potential value of the reduction reaction from the value of the oxidation value.
(most positive) - (most negative)

Question 13

what is the shorthand for drawing electrochemical cells?

Answer 13

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)

Question 14

what are batteries?

Answer 14

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

Question 15

compare rechargeable and non-rechargeable battery prices:

Answer 15

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

Question 16

what are lithium cells?

Answer 16

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

Question 17

how are rechargeable batteries recharged?

Answer 17

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

Question 18

what happens in fuel cells?

Answer 18

the chemicals are stored separately outside the cell and fed in when electricity is required

Question 19

what is the alkaline hydrogen-oxygen fuel cell?

Answer 19

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

Question 20

what is the overall reaction of the alkaline hydrogen-oxygen fuel cell?

Answer 20

2H2(g) + O2(g) –> 2H2O(g)

Question 21

what can the alkaline hydrogen-oxygen fuel cell be used for?

Answer 21

to power electric vehicles

Question 22

how are fuel cells more efficient?

Answer 22

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

Question 23

what are the advantages of water being the only waste product of fuel cells?

Answer 23

there are no toxic chemicals to dispose of and no CO2 emissions from the cell itself

Question 24

why don’t fuel cells need to be recharged?

Answer 24

as long as hydrogen and oxygen are supplied, the cell will continue to produce electricity

Question 25

how are fuel cells not carbon neutral

Answer 25

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.

Question 26

what happens to the electrodes and electrolytes in an electrochemical cell over time?

Answer 26

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

Question 27

what does the standard hydrogen electrode consist of?

Answer 27

platinised platinum electrode covered in platinum black platinum wire connecting platinum electrode and high resistance voltmeter hydrogen gas at 100kPa 1M hydrochloric acid solution

Question 28

what is the best oxidising agent?

Answer 28

F2 the F2/F- half equation has the most positive electrode potential so F2 is the best at being reduced

Question 29

what are the different electrodes called?

Answer 29

negative electrode = anode oxidation happens here - loses electrons positive electrode = cathode reduction happens here - gains electrons

Question 30

What are some examples of non-rechargeable cells?

Answer 30

zinc-carbon | alkaline

Question 31

What are some examples of rechargeable cells?

Answer 31

Lithium ion lead-acid nickel-cadmium

Question 32

what is the zinc-carbon cell?

Answer 32

``` Zn(NH3)2 2+ + 2e- ---> Zn + 2NH3 2MnO2 + 2H+ +2e- ----> Mn2O3 +H2O EMF = +1.5 V standard cell short life ```

Question 33

what is an alkaline cell?

Answer 33

Zn 2+ + 2e- ---> Zn MnO2 +H2O + Zn ----> MnO(OH) + OH- EMF = +1.6 V

Question 34

why do alkaline cells last longer than zinc-carbon cells?

Answer 34

the zinc-carbon cells have a faster rate of reaction so run out quicker than the alkaline cells

Question 35

what is the reaction for when a lithium cell is in use?

Answer 35

CoO2 + Li ---> LiCoO2 also known as discharging this reaction is slower so the battery lasts longer

Question 36

what is the reaction for when a lithium cell is charging?

Answer 36

LiCoO2 ---> CoO2 + Li | this reaction is faster so it charges quickly

Question 37

what is a lead-acid cell?

Answer 37

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

Question 38

what is the nickel - cadmium cell?

Answer 38

``` 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 ```

Question 39

what happens in an acidic hydrogen - oxygen fuel cell?

Answer 39

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

Question 40

what happens in an alkaline hydrogen - oxygen fuel cell?

Answer 40

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

Question 41

What are the benefits of using cells?

Answer 41

portable source of electrical energy

Question 42

what are the risks of using cells?

Answer 42

waste issues

Question 43

What are the benefits of using non-rechargeable cells?

Answer 43

cheap

Question 44

what are the risks of using non-rechargeable cells?

Answer 44

waste issues

Question 45

What are the benefits of using rechargeable cells?

Answer 45

less waste cheaper in the long run lower environmental impact

Question 46

what are the risks of using rechargeable cells?

Answer 46

some waste issues - at end of useful life

Question 47

What are the benefits of using hydrogen fuel cells?

Answer 47

only waste product is water do not need re-charging very efficient

Question 48

what are the risks of using hydrogen fuel cells?

Answer 48

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