Electrode potentials

Question 1

What must be in each half cell?

Answer 1

1M solution
electrode of solid

Question 2

Why does a voltage form in electrochemical cells?

Answer 2

One of the half cells is more easily oxidised and releases electrons
More electrons build up on this electrode
This is the negative electrode, and creates a PD

Question 3

Why use a high resistance voltmeter?

Answer 3

Stops the current from flowing in the circuit, so maximum PD is recorded

Question 4

What is a salt bridge for? What is it made of?

Answer 4

used to connect the circuit and free moving ions balance out the charges

tissue dipped in potassium nitrate solution

Question 5

Requirements for salt bridge

Answer 5

unreactive with electrodes and electrode solutions

Question 6

Why not use a metal wire as a salt bridge?

Answer 6

it would set up its own electrode system with the solutions

Question 7

What happens if the current is allowed to flow (eg short circuit/bulb replaces voltmeter)

Answer 7

Reactions occur separately at each electrode and go to conpletion
Voltage drops to 0
+ve electrode always undergoes reduction
-ve electrode always undergoes oxidation

Question 8

_ | _ || _ |_

Answer 8

R O O R 🐅

Question 9

Iron electrode for iron(ii) and iron(iii) cell diagram

Answer 9

||Fe3+(aq), Fe2+(aq)|| Pt

Question 10

If a cell diagram has many species eg the MnO4- redox reaction, how would it be laid out?

Answer 10

ignoring H2O and H+

Question 11

When to use a solid vertical line in cell diagrams?

Answer 11

Change of state, every time it occurs

Question 12

How to measure electrode potentials?

Answer 12

connect to reference standard hydrogen electrode with known potential of 0v

Question 13

hydrogen electrode equillibrium?

Answer 13

H2g <=> 2H+(aq) + 2e-

Question 14

4 components of standard hydrogen electrode?

Answer 14

H2 gas at 100kPa
HCl 1moldm-3 (any 1M H+ solution)
298K Temperature
Platinum electrode

Question 15

Why use standard conditions for measuring a half cell’s PD?

Answer 15

Position of redox eqm will change with conditions
PD can become more +/-ve

Question 16

4 standard electrode potential conditions

Answer 16

all ion solutions at 1moldm-3
temperature 298K
100kPa pressure
no current flowing

Question 17

Ecell =

Answer 17

Empve - Emnve
Erhs - Elhs

Question 18

Most positive half cell will always ..

Answer 18

REDUCE (go forwards)

Question 19

Most negative product on electrochemical series is

Answer 19

the strongest reducing agent (oxidised)

Question 20

The most positive reactant on electrochemical series is

Answer 20

the strongest oxidising agent (reduced)

Question 21

Ecell = (not Emp -Emn etc)

Answer 21

Ered - Eox

Question 22

Explanation for why A will oxidise B

Answer 22

E° A > E° B, A will oxidise B

Question 23

What is Ecell a measure of?

Answer 23

How far from eqm the cell reaction lies
More +ve = more likely to occur

Question 24

How does increasing reactant concentrations in an electrochemical cell affect Ecell?

Answer 24

Increases Ecell due to chateliers principle

Question 25

Effect of temp increase on Ecell?

Answer 25

most cells are exothermic in the spontaneous direction
Le chatelier says Ecell would decrease as eqm reactions will shift backwards

Question 26

3 types of cells

Answer 26

Non rechargeable
Rechargeable
Fuel cells

Question 27

Lithium ion cell uses

Answer 27

powering cameras and mobile phones

Question 28

Lithium ion cell equations and standard electrode potentials

Answer 28

Li+ + CoO2 + e- -> Li+[CoO2]- E=+0.6V
Li+ + e- -> Li E=-3.0V

Question 29

Overall discharge reaction in lithium ion cells

Answer 29

Li + CoO2 -> LiCoO2

Question 30

Lithium ion cell conventional diagram

Answer 30

Li | Li+ || Li+, CoO2- | LiCoO2 | Pt

Question 31

Why not use water as a solvent for lithium ion cells?

Answer 31

It would react with it vigorously

Question 32

Fuel cell meaning

Answer 32

Cell using energy from reacting a fuel with oxygen to create a voltage

Question 33

Hydrogen fuel cell equations and overall equation under alkaline conditions

Answer 33

4e- + 4H2O -> 2H2 + 4OH-
4e- + 2H2O + O2 -> 4OH-

overall reaction: 2H2 + O2 -> 2H2O

Question 34

How come fuel cells don’t have 0 voltage eventually?

Answer 34

Constant supply of reactants

Question 35

Hydrogen fuel cell conditions

Answer 35

High temps increase rate but reduces Ecell, so high pressure counteracts this

Question 36

Advantages/Disadvantages of fuel cells

Answer 36

+less politution and CO2
+greater efficiency

-storing/transporting it is unsafe due to pressurised liquid
-flammable chemicals
-limited lifetime
-high production costs

Question 37

3 ways hydrogen can be stored in fuel cells

Answer 37

as a liquid under pressure
adsorbed on the surface of a solid material
absorbed into a solid material

Question 38

Why aren’t hydrogen fuel cells green fuels?

Answer 38

electricity used to produce hydrogen via electrolysis of water must be produced from renewable resources

Question 39

State how the EMF of a cell will change if the surface area of the platinum electrode is increased

Answer 39

No change

Question 40

standard electrode potential definition

Answer 40

potential difference
of an electrochemical cell comprising of a half cell connected to a standard hydrogen electrode
under standard conditions
1moldm-3
298K
1atm

Question 41

+ve EMF

Answer 41

Feasible/spontaneous

Question 42

-ve EMF

Answer 42

unfeasible