Chapter 23 Redox and Electrode Potentials Flashcards
How do you write an overall equation from half equations?
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
How do you write half equations for unknown species?
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
How do you balance equations using oxidation numbers?
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
What is the chemistry behind manganate titrations- including colour changes?
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
How do you read the meniscus in manganate titrations and why?
Read from the top of the meniscus as the deep purple colour makes it hard to read the bottom
What are the general steps of a redox titration calculation with Manganate?
n=Cv manganate (titre)
Ratio, from redox, for moles in the flask
Scale up to moles of the original solution
mr or conc
How do you determine the number of waters in the formula of a compound using titrations?
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
What are the steps in the titration for manganate reactions?
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
What is the chemistry behind Iodine-Thiosulphate titrations?
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
What are the main steps taken with Iodine-thiosulphate titrations, including colour changes?
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
What are the general calculation steps of an iodine-thiosulphate titration?
n=cV, moles of thiosulphate
Ratio to I2, then ratio of I2 to oxidising agent
Scale up if necessary, then mr or conc
What is the equation for Cu2+ reacting with iodide?
2Cu2+ + 4I- —-/ 2CuI + I2
What is a voltaic cell?
Something that converts chemical energy into electrical energy
This arises from the movement of electrons in redox reactions
What is a half cell? What is a cell?
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
What is a ion/ion half cell and what is needed?
A cell with the same element but in different oxidation states
Uses an inert metal electrode e.g pt
E.g Fe2+ and Fe3+
What electrode is the anode?
The side that’ll undergo reduction, Effectively removing the electrons
The less reactive metals/ more positive electrode potential
Which electrode is the cathode?
The one that undergoes oxidation, essentially giving electrons
More reactive metal, more negative electrode potential
What is the standard electrode potential?
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
Practically, how do you measure the standard electrode potential of a cell?
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
How do you use electrode potentials to determine the tendency of oxidation and reduction?
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
What is the equation for the cell potential?
Positive electrode - Negative electrode
How do you write in words a Zn/Cu cell?
Zn| Zn2+ || Cu2+ | Cu
Write oxidation/reduction in order Zn is oxidised, Cu2+ is reduced
What happens if you decrease the concentration of HCOOH in this cell equilibrium?
CO2 + 2H+ + 2e- —/ HCOOH. -0.11v
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
What do you need to be careful of when looking at multiple redox systems?
Can further oxidation occur in another step?
Do you need to mention equilibrium?
How do you make predictions from electrode potentials?
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
What are limitations of predictions from electrode potentials?
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
What are primary cells?
Non-rechargeable, voltage falls when the reactants are used up
What are secondary cells?
Rechargeable cells, the reaction reverses when the battery is being charged, remaking the reactants to allow discharge at a later time
How do you write discharging/charging redox equations?
For discharge- same rules as normal
Charging- reverse discharge equation, so products from 1 react to reform reactants
What is a fuel cell?
Use energy from reactions of a fuel with oxygen
E.g hydrogen either acid or alkali
What does it mean when a questions asks you to refer to equilibrium with half cells?
Whether it reduction or oxidation is happening
So which way the equilibrium position moves
What is a reducing agent in terms of electrons?
The species which is oxidised, losing electrons, and giving them to another species so reduces that species
How do you find out a half equation when given one half equation and the overall equation?
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
What is the difference between a modern storage cell and a fuel cell?
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
What are some advantages and disadvantages of hydrogen fuel cells?
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
What do you need to include when drawing an ion half cell?
BOTH IONS not just one
1 mol dm3
How does catalysis with electrode potentials?
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
What are advantages of fuel cells?
Less CO2 produced than fossil fuel alternatives
Renewable sources of fuels e.g biology for Methanol
More efficient than fossil fuels
What are the disadvantages of hydrogen fuel cells?
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
Describe what the hydrogen fuel cell looks like and works?
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
