Chemistry A2 - REDOX and eletrode potentials (23) Flashcards
What are the steps for balancing half equations?
1) Write down the species before and after a reaction
2) Balance any atoms apart from oxygen and hydrogen
3) Balance any oxygens with water
4) Balance any hydrogens with H+
5) Balance charges with electrons
What is the half equation which shows that (MnO4)- has been reduced?
(MnO4)- + 8H+ + 5e- —> Mn2+ + 4H2O
Why add excess dilute sulfuric acid into the conical flask containing the reducing agent when carrying out a redox titration?
To ensure there are sufficient H+ ions to allow the reduction of the oxidising agent
What is a half cell?
Half of an electrochemical cell
When do we use platinum in a half cell?
When we have a half cell with 2 aqueous ions
Why do we use platinum in half cells?
It is inert and so doesn’t react with any of the chemicals or ions in the beaker
How is an electrochemical cell formed?
By joining 2 different half cells together
What are the two half cells in an electrochemical cell joined by?
A wire, voltmeter, and a salt bridge
Where do the electrons flow in an electrochemical cell?
From the more reactive metal to the less reactive one
What is the salt bridge made from and what is its purpose?
It is filter paper with saturated KNO3 solution. Ions flow through it which balances the charges
What is the electrode potential measured in and what does it tell us?
It is measured in volts and tells us how easily the half cell gives up electrons (oxidised)
How are the equations in electrochemical cells always written?
In the reduced form, meaning we always show the equations with reduction in the forward direction
How do we know which element is undergoing oxidation/reduction by using the electrode potentials?
Using the NO PRoblem rule:
The most Negative half cell will undergo Oxidation
The most Positive half cell will undergo Reduction
How to we measure electrode potentials?
By using the standard hydrogen electrode. We compare them to this which has an electrode potential value equal to 0.00V
What are the conditions for standard electrode potentials?
- Temperature at 298K
- Pressure at 100kPa
- Concentrations of ions are [1]
How do we calculate the standard cell potential?
Reduced - oxidised
What are the four steps for carrying out a manganate(VII) titration?
- A standard solution of potassium manganate(VII) is added to the burette
- Using a pipette, add a measured volume of the solution being analysed into the conical flask. An excess of dilute sulfuric acid is also added to provide the H+ for the reduction of (MnO4)-
- During the titration, the manganate(VII) solution reacts and is decolourised as it is added. The end point is judged by the first permanent pink colour, indicating an excess of (MnO4)- ions
- Repeat until you obtain two concordant titres
Why do we read the meniscus from the top when the solution is potassium manganate(VII)?
It is a very deep purple colour and so it is difficult to see the bottom
What is the equation for the oxidation of thiosulfate ions?
(2S2O3)2- –> (S4O6)2- + 2e-
What is the equation for the reduction of iodine?
I2 + 2e- –> 2I-
What colour is CuI (s)?
White
What are primary cells?
They are non-rechargeable and designed to be used once only
Why are primary cells only used once?
When in use, electrical energy is produced by oxidation and reduction at electrodes. However, the reactions cannot be reversed. Eventually the chemicals will get used up, voltage will fall, the battery will go flat, and the cell will be discarded or recycled
What are secondary cells?
They are rechargeable
