Chapter 23 Redox and Electrode Potentials

Question 1

How do you write an overall equation from half equations?

Answer 1

Balance each equation so that the number of electrons in each is the same
Add them together and cancel the electrons/any things on both sides

Question 2

How do you write half equations for unknown species?

Answer 2

First write the oxidation numbers and work out the change
This must be the total change, including balancing numbers

so if 2Fe2+ –/ 2 Fe3+, the next change is 2 x + 1

The change corresponds to the number of electrons that need to be removed / added

Predict any other species, using protons, hydroxides, water… and balance

Question 3

How do you balance equations using oxidation numbers?

Answer 3

See which species change oxidation number, these species are where electrons are transferred
Balance only these species, by thinking about how many electrons are need this oxidation number change
Then balance the rest of the equation

Question 4

What is the chemistry behind manganate titrations- including colour changes?

Answer 4

Manganate is reduced to Mn2+, so acts as an oxidising agent, helps determine the concentration of the agent

When you add the purple manganate, it reduces, forming the colourless Mn2+. However, when all the reducing agent is used up, Manganate remains in the solution, staying purple, and making the solution a baby pink overall

Self indicating

Question 5

How do you read the meniscus in manganate titrations and why?

Answer 5

Read from the top of the meniscus as the deep purple colour makes it hard to read the bottom

Question 6

What are the general steps of a redox titration calculation with Manganate?

Answer 6

n=Cv manganate (titre)
Ratio, from redox, for moles in the flask
Scale up to moles of the original solution
mr or conc

Question 7

How do you determine the number of waters in the formula of a compound using titrations?

Answer 7

Calculate n of moles of unknown
then calculate mr
subtract known salt or metal
Remain is just water, so divide by 18 for the number

Question 8

What are the steps in the titration for manganate reactions?

Answer 8

In a burette, use a filter to add an excess of manganate, and record the starting volume.
Using a pipette and pipette filler, add a known volume of reducing agent to a conical flask. Place on a white tile under the burette.
Open the tap slowly and swirl the flask as you go along. When the colour appears but fades, add dropwise. When the baby pink colour remains, record the volume of manganate added, from the top of the meniscus.
Repeat until concordant results are reached

Question 9

What is the chemistry behind Iodine-Thiosulphate titrations?

Answer 9

Iodide ions are oxidised into Iodine by oxidising agents
Iodine reacts with thiosulphate ions to form iodide and peroxydisulphate. We titrate the thiosulphate to determine how many moles of Iodine and therefore oxidising agent there are

Question 10

What are the main steps taken with Iodine-thiosulphate titrations, including colour changes?

Answer 10

Mix the oxidising agent with an excess of iodide ions- this ensures all of the oxidising agents has reacted. This forms iodine, and an orange-brown solution

Titrate with thiosulphate. As it is added, the brown begins to fade. When pale straw coloured, add starch, which is blue-black in the presence of iodine

Stop titrating when the blue-black colour fades

Question 11

What are the general calculation steps of an iodine-thiosulphate titration?

Answer 11

n=cV, moles of thiosulphate
Ratio to I2, then ratio of I2 to oxidising agent
Scale up if necessary, then mr or conc

Question 12

What is the equation for Cu2+ reacting with iodide?

Answer 12

2Cu2+ + 4I- —-/ 2CuI + I2

Question 13

What is a voltaic cell?

Answer 13

Something that converts chemical energy into electrical energy
This arises from the movement of electrons in redox reactions

Question 14

What is a half cell? What is a cell?

Answer 14

The species in a redox half equation
A cell is made up of 2 half cells, kept separate to allow electrons to flow, with redox reactions on each side

Question 15

What is a ion/ion half cell and what is needed?

Answer 15

A cell with the same element but in different oxidation states
Uses an inert metal electrode e.g pt

E.g Fe2+ and Fe3+

Question 16

What electrode is the anode?

Answer 16

The side that’ll undergo reduction, Effectively removing the electrons
The less reactive metals/ more positive electrode potential

Question 17

Which electrode is the cathode?

Answer 17

The one that undergoes oxidation, essentially giving electrons
More reactive metal, more negative electrode potential

Question 18

What is the standard electrode potential?

Answer 18

The emf/voltage of a half cell when compared to a standard hydrogen half cell under standard conditions, so all solutions 1mol/dm3, 298K and 100KPa

Question 19

Practically, how do you measure the standard electrode potential of a cell?

Answer 19

Connect two cells with a salt bridge, and electrodes from each connected by a wire to a voltmeter
One must be a hydrogen cell, with H+ acid and a section to allow hydrogen to escape

Standard conditions- 1mol/dm3, 100Kpa, 298K

Question 20

How do you use electrode potentials to determine the tendency of oxidation and reduction?

Answer 20

The more negative the electrode potential, the greater the tendency to be oxidised
The more positive the electrode potential, the greater the tendency to be reduced

Question 21

What is the equation for the cell potential?

Answer 21

Positive electrode - Negative electrode

Question 22

How do you write in words a Zn/Cu cell?

Answer 22

Zn| Zn2+ || Cu2+ | Cu

Write oxidation/reduction in order Zn is oxidised, Cu2+ is reduced

Question 23

What happens if you decrease the concentration of HCOOH in this cell equilibrium?

CO2 + 2H+ + 2e- —/ HCOOH. -0.11v

Answer 23

CO2 + 2H+ + 2e- —/ HCOOH. -0.11v

If the concentration of HCOOH decreases, the system will respond by pushing the position of equilibrium to the right
This means the electrode potential is less negative, more positive, as there are fewer electrons
Look at cell equation to see whether the cell itself will have a higher or lower voltage, whether the difference in electrode potential between the two cells has increased or decreased

Question 24

What do you need to be careful of when looking at multiple redox systems?

Answer 24

Can further oxidation occur in another step?
Do you need to mention equilibrium?

Question 25

How do you make predictions from electrode potentials?

Answer 25

Compare the two equations
The one with the more negative electrode potential will be oxidised, the species on the right will react to be oxidised
The one with the more positive electrode potential with reduce, the species on the left will be reduced

Diagonal from the right to the left below

Question 26

What are limitations of predictions from electrode potentials?

Answer 26

Reaction rate- feasibility but no indication of kinetics, Ea too high for a large rate
Concentrations- Assumes 1moldm3 but may not be used
Differing conditions- not always aqueous

Question 27

What are primary cells?

Answer 27

Non-rechargeable, voltage falls when the reactants are used up

Question 28

What are secondary cells?

Answer 28

Rechargeable cells, the reaction reverses when the battery is being charged, remaking the reactants to allow discharge at a later time

Question 29

How do you write discharging/charging redox equations?

Answer 29

For discharge- same rules as normal
Charging- reverse discharge equation, so products from 1 react to reform reactants

Question 30

What is a fuel cell?

Answer 30

Use energy from reactions of a fuel with oxygen
E.g hydrogen either acid or alkali

Question 31

What does it mean when a questions asks you to refer to equilibrium with half cells?

Answer 31

Whether it reduction or oxidation is happening
So which way the equilibrium position moves

Question 32

What is a reducing agent in terms of electrons?

Answer 32

The species which is oxidised, losing electrons, and giving them to another species so reduces that species

Question 33

How do you find out a half equation when given one half equation and the overall equation?

Answer 33

Overall - Half equation
Some will cancel, some species will be left with a minus sign
Move to the other side to get rid of the minus
This is the half equation

Question 34

What is the difference between a modern storage cell and a fuel cell?

Answer 34

Primary/secondary contain a finite amount of each redox system, that will be continually reversed if recharged

Hydrogen fuel cells will only run when supplied with a fuel and oxygen, and the electrolyte (H+ or OH-) is effectively a catalyst

Question 35

What are some advantages and disadvantages of hydrogen fuel cells?

Answer 35

Hydrogen volatile so can be dangerous
Limited infrastructure to supply hydrogen
Toxic catalysts

But:
Only water produced, no toxic chemicals
Easy storage of hydrogen e.g liquidised, surface or within solid
Compact and lightweight

Question 36

What do you need to include when drawing an ion half cell?

Answer 36

BOTH IONS not just one
1 mol dm3

Question 37

How does catalysis with electrode potentials?

Answer 37

If reaction between 2 species A and B
A= 1.98
B=0.5

A catalyst will be any redox system in between these two values
Reaction with B, then the intermediate will react with A
Or vice versa, so a catalyst - see AQA Q 2018 P3 MCQ

Question 38

What are advantages of fuel cells?

Answer 38

Less CO2 produced than fossil fuel alternatives
Renewable sources of fuels e.g biology for Methanol
More efficient than fossil fuels

Question 39

What are the disadvantages of hydrogen fuel cells?

Answer 39

Hydrogen difficult to transport, as it is difficult to store
Must be stored as liquid under pressure and very cool- but difficult to occur
Or adsorbed or absorbed by a solid, but these must be replaced and these are environmental issues
Use toxic chemicals in production
No hydrogen infrastructure
Hydrogen production uses energy e.g electrolysis of water or methane, but use energy

Question 40

Describe what the hydrogen fuel cell looks like and works?

Answer 40

Fuel attracted to the positive electrode
Oxygen attracted to the negative electrode

Separated by a semi-permeable membrane, with electrolyte of either protons or hydroxyl
In acid, hydrogen oxidised, electrons move through the wire and protons through the membrane
Combine with oxygen on the other side to produce water

In hydroxyl, hydroxyl combines with hydrogen to form hydroxyl
Moves through and combines with oxygen to form water