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?
