14 - Redox II Flashcards
What is oxidation?
Oxidation is the loss of electrons.
Oxidation number becomes more positive.
What is reduction?
Reduction is the gain of electrons.
Oxidation number becomes more negative.
Standard electrode potential?
Standard electrode potential is the potential difference / emf measured when a half-cell is connected to a standard hydrogen electrode, under standard conditions.
What are the standard conditions when measuring standard electrode potential?
298K temperature
100kPa pressure of gases
1.00 moldm^-3 concentration of ions.
Why is a reference electrode necessary?
- It is impossible to measure the absolute potential of a half-cell on its own. Only possible to measure potential difference between two electrodes.
- By convention, standard electrode potential of a half-cell is measured by connecting to a standard hydrogen electrode, which is given a potential of 0 volts. Measured under standard conditions.
Features of a standard hydrogen electrode
Hydrogen gas (in) at 100kPa.
Platinum wire.
Platinum foil covered in porous platinum.
Surrounding solution = HCl acid [H+(aq)] = 1moldm^-3
What is the salt bridge for?
- Needed to complete the electrical circuit.
- usually contains concentrated solution of potassium nitrate KNO3 (liquid/gel).
- Salt should be unreactive with the half-cells and their solutions.
Why use a high resistance voltmeter?
- To avoid any flow of current through the circuit.
- In this state, it is possible to measure the maximum possible potential difference.
- The reactions will not be occuring as the very high resistance voltmeter stops the current from flowing.
electromotive force?
emf
The standard electrode potential of a half-cell connected to a standard hydrogen electrode, measured under standard conditions.
E cell
How is a half cell for chlorine set up?
- Very similar to SHE
- Cl2 gas 100kPa
- bubbled over platinium foil covered in porous platinium.
- platinum wire
- immersed in a solution of [Cl-(aq)] = moldm^-3, e.g sodium chloride solution.
How do you a half cell?
substance losing electrons (most easily oxidised) written closest to the electrode (not salt bridge).
How to write Cl2 half cell?
Pt(s) I 2Cl-(aq), Cl2(g) II
notice how Cl- is closest to the platinum electrode, since it is the thing that loses electrons (oxidised).
Chlorine and its E value
Value is postive and moderately high. Means that position of chlorine equilibrium lies further to the right than the hydrogen equilibrium.
Chlorine is more likely to gain electrons than hydrogen ions. (Notice not chlorine ions. Chlorine is the substance that gains electrons to form the ions).
Chlorine is therefore good at removing electrons from other substances and gaining them itself. Therefore it is a good oxidising agent (it is reduced itself).
How to measure redox potential for Fe2+ / Fe3+ system (ions of the same element with different oxidation numbers).
- insert a platinum electrode (platinum foil covered in porous platinum and platinum wire) into a beaker containing solution of Fe2+ and Fe3+ ions, [Fe2+(aq)] = 1.00moldm^-3 and [Fe3+(aq)] = 1.00moldm^-3.
- connect it to a standard hydrogen electrode.
How is the electrochemical series arranged?
- The most negative E values are placed at the top of the series; most positive at the bottom.
- The more negative the E value, the more powerful the reducing agent.
- The more positive the E value, the more powerful the oxidising agent.
Useful rule of thumb in electrochemical cells
- The equilibrium with the more negative (less positive) E value will shift to the left.
- The equilibrium with the more positive (less negative) E value will shift to the right.
How to calculate E cell of an electrochemical cell?
E (more positive) - E (more negative)
E (right) - E (left)
e.g zinc = -0.76V, copper = +0.34 V,
E cell = 0.34 - (-0.76) = +1.10V
Although a reaction may be thermodynamically feasible (when comparing E values), why might the reaction not take place?
- Reaction has a very large activation energy.
- Reaction may not be taking place under standard conditions.
A reaction is deemed not thermodynamically feasible under standard conditions.
How can you make the reaction take place?
- increase the concentration of the two reactants.
- causes shift in equilibrium to desired side. Causes E values to become more positive and more negative.
Therefore, a reaction that is initially deemed not thermodynamically feasible under standard conditions, may become feasible (and hence the reaction occurs) under non-standard conditions.
Gibbs free energy and emf equation
∆G = -nFEcell
What does positive E cell mean?
If E cell is +ve, the reaction as written from left to right in the cell diagram is thermodynamically feasible as ∆S total will be positive.
If you have done E (right) - E (left) and you have E cell which is -ve, it means that reaction from left to right is not thermodynamically feasible. Reaction will occur from right to left instead.
E cell can be used to predict the direction of a cell reaction.
Relationship between equilibrium constant (k) and E cell?
lnK ∝ E cell
Relationship between total entropy change and E cell?
∆S total ∝ E cell
If a question asks you to write the equation of a storage cell being recharged, what do you write?
write the reverse of the discahrge equation.
