Redox & electrode potentials Flashcards
What is an oxidising agent?
A substance that tales electrons from the species being oxidised - it contains the species that is being reduced
What is a reducing agent?
A substance that adds electrons to the species being reduced - it contains the species that is oxidised
In manganate (VII) titrations, whay is done to Mn04 - ions?
They are reduced
How is an MnO4 - titration carried out?
1) A standard solution of potassium manganate (VII), KMnO4, is added to the burette
2) Using a pipette, add a measured volume of the solution being analysed to a conical flask. An excess of dilute sulfuric acid is also added to provide the H+ (aq) ions required for the reduction of MnO4-. An indicator is not required - this titration is self indicating
3) During the titration, the manganate (VII) solution reacts and is decolourised as it is being added. The end point of the titration is judged by the first permanent pink colour, indicating there is an excess of MnO4 - ions present.
4) Repeat the titration until concordant titres are obtained - the titres agree within 0.10cm^3
How do you read the meniscus for a KMnO4 titration?
KMnO4 is a deep purple colour so it is difficult to read the bottom of the meniscus through the intense colour. Burette readings are instead read from the top; make sure to read both from the top so the titre is correct
What can manganate (VII) titrations be used for the analysis of?
- Iron (II) ions
- Ethandioic acid
In iodine-thiosulfate titrations, whats oxidised and whats reduced?
Thiosulfate ions, S2O3 2- are oxidised and iodine, I2, is reduced
What are the reactions in an iodine thiosulfate titration?
What is the procedure of an iodine-thiosulfate titration?
1) Add a standard solution of Na2S2O3 to the burette
2) Prepare a solution of the oxidising agent to be analysed. Using a pipette, add this solution to a conical flask. Then add an excess of potassium iodide. The oxidising agent reacts with iodine ions to produce iodine, which turns the solution a yellow-brown colour
3) Titrate this solution with the Na2S2O3. During the titration, the iodine is reduced back to I- ions and the brown colour fades quite gradually, making it difficult to decide on an end point. This problem is solved by using a starch indicator. When the end point is being approached, the iodine colour has faded enough to become a pale straw colour. A small amount of starch indicator is added. A deep-blue black colour forms to assist with the identification of the end point. As more sodium thiosulfate is added, the blue-black colour fades. At the end point, all the iodine will have just reacted and the blue-black colour disappears
What can iodine/thiosulfate titrations be used to determine?
- The Clo- content in household bleach
- The Cu 2+ content in copper (II) compounds
- The Cu content in copper alloys
Copper with an iodine-thiosulfate titration!
What does a half-cell contain?
The chemical species present in a redox half-equation
How can a voltaic half cell be created?
Connecting 2 different half-cells which then allows electrons to flow. The chemicals in the 2 different half-cells aren’t allowed to mix
What is a metal/ metal ion half-cell?
The simplest half-cell that consists of a metal rod dipped into a solution of its metal aqueous ion. At the phase boundary where the metal is in contact with its ions, an equilibrium will be set up
How is a reaction written for a metal/ metal ion half-cell?
So that the forward reaction shows reduction and the reverse shows oxidation
What is an ion/ ion half-cell?
This contains different ions of the same element in different oxidation states. In this type of half-cell there is no metal to transport electrons either into or out of the half-cell, so an inert metal electrode made out of platinum is used
Which is the positive electrode in an operating cell?
The electrode with the less reactive metal - it gains electrons and is reduced
Which is the negative electrode in an
operating cell?
The electrode with the more reactive metal - it loses electrons and is oxidised
What is the standard electrode potential?
The emf of a half-cell connected to a standard hydrogen half-cell under standard conditions of 298K, solution concentrations of 1 mol dm^-3 and a pressure of 100kPa
What does the standard electrode potential look like?
By definition, what is the standard electrode potential of a standard hydrogen electrode?
0 V
How do you measure a standard electrode potential?
Connect it to a standard hydrogen half-cell
- The 2 electrodes are connected by a wire to allow a controlled flow of electrons
- The 2 solutions are connected with a salt bridge which allows ions to flow. The salt bridge typically contains a concentrated solution of an electrolyte that does not react with either solution. An example, is a strip of filter paper soaked in aqueous potassium nitrate
What does an electrode between 2 metal half cells look like?
What is the cell reaction?
The sum of the oxidation and and reduction half cells taking place in each half
What is emf of an electrochemical cell, Ecell?
The difference between the standard electrode potential of the 2 half cells
What is the Ecell equation?
Ecell = EΘ (positive electrode) - EΘ (negative electrode)
Positive electrode - the least negative half cell
In a reaction, where will electrons flow from and to?
From the negative cell and to the more positive one
What does a lower EΘ value generally show?
The greater the tendency it has to release electrons and shift left for a reaction : Fe 2+ (aq) + 2e- <=> Fe(s)
Which electrode does oxidation happen at?
The more negative one as electrons are produced
Which electrode does reduction happen at?
The more positive one as electrons are accepted
If Cu 2+(aq) + 2e- <=> Cu (s) were to be the more positive half cell, what would the equation be?
Cu 2+(aq) + 2e- -> Cu (s)
Reduction is occurring as electrons are being accepted
If Cu 2+ + 2e- <=> Cu (s) were the more negative half cell, what would the equation be?
Cu (s) -> Cu 2+(aq) + 2e-
Oxidation is occurring as electrons are being released
What is the rule for an electrode equation to be feasible?
Combining the 2 half equations at each electrode the right way round - the reverse of this would not be feasible
What are the limitations of using EΘ values to predict feasibility?
- The electrode potentials may indicate the thermodynamic feasibility of a reaction, but they do not account for the rate of reaction (this could be very slow)
- Standard electrode potentials are measured using standard concentrations of 1 mol dm^-3. Many reactions taking place use concentrated or dilute solutions; therefore the value of the electrode potential would be different from the standard value
- The actual conditions used for the reaction may be different from the standard conditions used to record the EΘ values
- Standard electrode values apply to aqueous equilibria - many reactions take place that are not at equilibria
What happens to the electrode potential if the concentration is greater than 1mol dm^-3?
The equilibria will shift to the right, removing electrons from the system and making the electrode value less negative/more positive
What happens to the electrode potential if the concentration is less than 1mol dm^-3?
The equilibrium will shift to the left increasing electrons in the system and making the electrode potential more negative/ less positive
Whats used in batteries and fuel cells?
Redox reactions where there flow of electrons can be used to generate electrical current
What are the 3 main types of cell?
- Primary
- Secondary
- Fuel
What is a primary cell?
Non-rechargable cells that are designed only to be used once
How do primary cells work?
When in use, electrical energy is produced by oxidation and reduction at the electrodes. However, the reaction cannot be reversed. Eventually, the chemicals will be used up, voltage will fall, the battery will go flat, and the cell will be discarded or recycled
What is secondary cell?
Cells that are rechargeable and can be used more than once
How does a secondary cell work?
Unlike primary cells, the cell reaction producing electrical energy can be reversed during recharging. The chemicals in the cell are generated and then can be used again
What are some common examples of secondary cells include?
- Lead-acid batteries used in car batteries
- Nickel-cadium and nickel-metal hydride ; the cylindrical batteries used in radios, torches etc
- Lithium-ion and lithium-ion polymer cells used our modern appliances - laptops, cameras, mobile phone phones - and also being developed for cars
What is a fuel cell?
A cell that uses the energy from the reaction of a fuel with oxygen to create a voltage
How does a fuel cell work?
- The fuel and oxygen flow into the fuel cell and the products flow out. The electrolyte remains in the cell
- They operate continuously providing the fuel and oxygen and supplied into the cell
- They do not need to be recharged
What are the most common types of fuel cells?
Hydrogen
What are the products from a hydrogen cell?
Water only
What are the 2 types of electrolytes a hydrogen cell can have?
Acid or alkali
