1.9 electrode potentials and cells

Question 1

required practical

Question 2

what is an electrochemical cell made of?

Answer 2

2 different metals dipped in salt solutions of their own ions and connected by a wire (external circuit)
salt bridge
2 reactions- redox process

Question 3

whats a salt bridge?

Answer 3

eg strip of filter paper soaked in KNO3(aq).
allows ions to flow between half cells and balance out charges- completes the circuit

Question 4

what is the cell potential/ cell EMF

Answer 4

voltage between the 2 half-cells (electrons flow from most reactive metal to least.

Question 5

what is electrode potential?

Answer 5

how easily a metal is oxidised
the direction each reaction goes in depends on how easily a metal is oxidised

Question 6

what does a negative cell potential mean?

Answer 6

a metal thats easily oxidised

Question 7

what does a positive cell potential mean?

Answer 7

a metal thats harder to oxidise

Question 8

how do you write the conventional representation of cells (cell notation)

Answer 8

R/O//O/R
- more negative electrode potential on the left
- phase boundary if diff states, comma if same state
- // is the salt bridge
- pt electrode goes on the end if r and o are both aq

Question 9

how do you write half equations for electrodes?

Answer 9

look at conventional representation and write equation from that.
reduction happens at positive electrode and oxidation happens at negative electrode

Question 10

how do you calculate cell potential?

Answer 10

Ecell= E right- E left
should always be positive value as more negative value is being taken away from more positive

Question 11

what factors affect the electrode potential?

Answer 11

reversible so equilibrium position is affected by temp, pressure, conc
changing eq changes cell pot

Question 12

why do you use standard conditions when measuring electrode potential?

Answer 12

so you get the same value for electrode potentials and you can compare values for diff cells

Question 13

what is the standard hydrogen electrode?

Answer 13

way to measure electrode potential
hydrogen gas is bubbled through solution of aq H+ ions. Pt electrode used as a platform for redox reactions
Must use standard conditions

Question 14

what is the standard electrode potential of a half-cell?

Answer 14

voltage measured under standard conditions when half cell is connected to standard hydrogen electrode

Question 15

how do you measure standard electrode potential?

Answer 15

SHE is always on left despite other being more + or more -
SHE has an electrode pot of 0.00V so the voltage reading for the whole cells will be the same as the E right hand side

Question 16

what are standard conditions?

Answer 16

solutions of ions must have a conc of 1.00mol dm-3
298K/ 25c
100kPa

Question 17

what is the electrochemical series?

Answer 17

• list of electrode potentials for diff electrochemical half cells
• the half equations are written as reduction

Question 18

when 2 half-equations are put together in an electrochemical cell, which direction do they go in?

Answer 18

the one with the more negative electrode pot goes in the oxidation direction (backwards) and the one with the more positive electrode pot goes in the reduction direction (forwards)

Question 19

how can you use the electrochemical series to calculate standard cell potential when 2 half-cells are connected?

Answer 19

E cell= E reduced- E oxidised
(reduced has more + electrode pot, oxidised has more - electrode pot)

Question 20

how can you use electrode potentials to predict which direction a redox reaction will go in?

Answer 20

1) find the 2 half equations and write them both as reduction
2) use electrochemical series to see which has more - electrode pot- write this as oxidation
3) combine the 2 equations
4) will give + overall E value. rection wont happen other way round

Question 21

how to work out if a redox reaction is feasible or not?

Answer 21

work out the cell emf
if its +, it is feasible
if its -, it is not feasible

Question 22

what is a non-rechargeable cell?

Answer 22

cell that uses irreversible reactions eg dry cell alkaline battery

Question 23

you can make reactions in non-rechargeable cells run backwards in the right conditions, so why not?

Answer 23

it could leak/ explode because a zinc electrode forms the casing of the battery and this becomes thinner as zinc is oxidised

Question 24

what are rechargeable cells?

Answer 24

use reversible reactions
eg nickel-cadmium and lead-acid

Question 25

how do you recharge batteries?
how is this possible?

Answer 25

current supplied to force electrons to flow in opposite direction around the circuit and reverse the reactions

possible because none of the substances in a rechargeable battery escape/ are used up

Question 26

what is different about a fuel cell compared to most other cells?

Answer 26

in fuel cells, the chemicals are stored separately outside the cell and fed in when required
in most other cells, the chemicals that generate electricity are contained in the electrodes and electrolyte that form the cell

Question 27

how does a hydrogen-oxygen fuel cell work?

Answer 27

(make/see poster with drawing)
- hydrogen and oxgyen gases are fed into 2 separate pt electrodes (porous ceramic with thin layer of pt- cheaper and larger sa)
- electrodes are separated by anion- exchange membrane that allows anions (OH-) and h2o to pass but stops hydrogen and oxygen passing
- electrolyte is an aqueous alkaline (KOH) solution
- hydrogen is fed into - electrode
- electrons flow from - through external circuit to +
- OH- pass through anion-exchange membrane towards - electrode
- oxygen fed to + electrode

Question 28

what are the equations for a hydrogen-oxygen fuel cell?

Answer 28

negative electrode:
H2(g) + 2OH-(aq) -> 2H20(l) + 2e-
positive electrode:
O2(g) + 2H2O(l) + 4e- -> 4OH-(aq)
overall:
2H2(g) + O2(g) -> 2H2O(l)

Question 29

pros and cons of fuel cells

Answer 29

+
- more efficient than internal combustion engine for cars (convert more available energy into kinetic)
- only waste product is water so no toxic chemicals to dispose of/ CO2 emissions
dont need to be recharged

-
- need energy to produce a supply of hydrogen (from electrolysis of water- reuses waste product but requires electricity from burning fossil fuels so not carbon neutral)
- hydrogen is very flammable so needs to be handled carefully when stored/ transported
- infrastructure to provide hydrogen fuel for cars doesnt exist on large scales so refuelling stations are rare