3.1 Redox and standard electrode potential Flashcards
(20 cards)
what are the rules for assigning oxidation numbers to elements within a compound?
All elements have an oxidation state of 0, even those found as diatomic molecules such as O2.
The oxidation states of all atoms in a neutral molecule must add up to 0.
The oxidation states of all atoms in an ion must add up to the overall charge of the ion.
The more electronegative atoms have negative oxidation states.
Certain elements have characteristic oxidation states:
Group 1 metals always have an oxidation state of +1.
Group 2 metals always have an oxidation state of +2.
Aluminium always has an oxidation state of +3.
Hydrogen always has an oxidation state of +1 (except as part of a metal hydride, where it has an oxidation state of -1).
Fluorine always has an oxidation state of -1.
Oxygen always has an oxidation state of -2 (except in a peroxide where it has an oxidation state of -1, or in a compound with fluorine, where the oxidation state is positive).
Chlorine always has an oxidation state of -1 (except in a compound with oxygen or fluorine, where the oxidation state is positive).
What is the first step in creating a half equation?
you must identify the species which are oxidised and reduced and separate them into separate half-equations.
If the charges are not balanced in a half equation, what can be done to balance them?
adding electrons where needed
In all half cell reactions that take place in a lab everything is standardised, what are values?
Always 1 mol dm-3 concentration and when gas are used its always 1 atm of pressure. Also with the gas reactions an inert platinum electrode is used to allow the electrons to flow in and out.
What is used to join two half cells together (apparatus)?
- A wire that allows the electrons to flow from the half cell where oxidation occurs to the half cell where reduction occurs. If the potential difference is being measured then a high resistance voltmeter is used aswell.
- A salt bridge completes the circuit and allows ions to flow without the solutions mixing. Current is a flow of charge and in the salt bridge the charge is a flow of ions.
What half cell set up do you use when you have a solid metal and a solution of metal ions?
The metal / metal ion half-cell. It consists of an electrode made of the metal dipped into a 1 moldm −3 solution of the metal ions.
What half cell set up do you use when an element in an aqueous ion is oxidised or reduced to form a different aqueous ion
It consists of a solution made up of both the oxidised and reduced form, both of concentration 1 moldm−3, and an electrode made of platinum and coated with very fine platinum powder (known as platinum black).
What half cell set up is used when either the reduced or oxidised species in a half equation is a gas and the other species is an aqueous ion?
It is made up of a 1 moldm−3
solution of the aqueous ions associated with the gas, a platinum black electrode dipped into the solution, and glass cover over the platinum electrode that surrounds the electrode in a steady stream of the gas kept at a pressure of 1 atmosphere (1 atm)
What are the rules for representing half cells?
The metal electrodes are placed at the far left and far right of the cell and should include any platinum electrodes if used in the half-cells.
The two half-cells are separated by a double vertical line, which represents the salt bridge.
If there is a change in state between two species in a half-cell (e.g. solid zinc and aqueous zinc ions), this is shown as a single vertical line known as a phase boundary.
If two species in a half-cell are in the same state (i.e. both aqueous), this is shown as a comma.
The reduced form of each half-cell is either the conducting metal or the species next to platinum electrode.
The oxidised form of each half-cell is closest to the salt bridge.
How would you measure the standard electrode potential of a half cell?
The half-cell is connected to a standard hydrogen electrode. This is simply a gas electrode that uses hydrogen gas at 1 atm, a solution of H+(aq) ions of concentration 1 moldm
−3
(usually using 0.5 mol dm-3 sulfuric acid), and a platinum black electrode.
The standard hydrogen electrode (SHE) is connected to the positive terminal of the voltmeter and is placed on the left hand side of the circuit.
The standard hydrogen electrode involves the following equilibrium:
2H
+
(aq)+2e
−
⇌H
2
(g)
The half-cell being tested would be connected to the negative terminal of the voltmeter and be placed on the right.
The two half-cells would be connected by a salt bridge.
The temperature is maintained at 298K.
The standard hydrogen electrode must be connected to the positive or negative terminal of the voltmeter?
positive
The standard hydrogen electrode is always positioned on the left or right hand side by convention?
left
When the voltmeter gives a negative reading, the hydrogen half -cell undergoes oxidation or reduction?
reduction
When the voltmeter shows a negative reading, the electrons flow towards or away from the hydrogen half-cell?
towards
If the species within the half-cell connected to standard hydrogen electrode is a stronger reducing agent than hydrogen, the voltmeter would give a positive or negative reading?
negative
If two standard hydrogen electrodes were connected together, the voltmeter reading would be?
0
Any half-cell that has a positive value when connected to the SHE undergoes ? and so contains a species which is an ? agent towards H+ ions .
Any half-cell that has a positive value when connected to the SHE undergoes reduction and so contains a species which is an oxidising agent towards H+ ions
Any half-cell that has a negative value when connected to the SHE undergoes ? and so contains a species which is a ? agent towards H2 molecules.
Any half-cell that has a negative value when connected to the SHE undergoes oxidation and so contains a species which is a reducing agent towards H2 molecules.
What is the equation of calculating the EMF when two different half cells are linked up?
EMF = positive - (negative)
