Electrode potentials
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What connects the two half cells
A salt bridge
What do simple half cells consist of
A metal (Acts as an electrode) and a solution of a compound containing that metal (e.g. Cu and CuSO4)
Why is a high resistance voltmeter used
To stop the current from flowing in the circuit
Why does a voltage form in a Zn/Cu electrochemical cell?
• Different metals have different tendencies to lose electrons. • Zinc oxidises more readily than copper: Zn→Zn2+ +2𝑒− • This means electrons build up on the zinc electrode. • A potential difference forms between the two electrodes. Zinc = negative terminal (oxidation) Copper = positive terminal (reduction)
What is the purpose of the slat bridge and what is it made up of and what solution is typically used
• To complete the circuit • Allows ions to move between half cells A salt bridge is usually made from a piece of filter paper (or material) soaked in a salt solution, usually potassium nitrate
What conducts the charge in a salt bridge
The free moving ions
Why would potassium chlroide not be suitable
chloride would not be suitable for copper systems because
What happens if current is allowed to flow? (2)
• The reaction occurs separately at each electrode • Voltage will fall to zero as the reactants are used up
The most positive electrode will always undergo
reduction. Cu2+ (aq) + 2e- –> Cu(s) (positive as electrons are used up)
The most negative electrode will always undergo
oxidation. Zn(s) –> Zn2+ (aq) + 2e- (negative as electrons are given off)
How do you improve electrical contatc between the elctrodes and solutions
• Rubbing electrode with sandpaper to remove any oxide layer on the surface of the electrode. • Wiping the electrodes with propanone to remove any grease on the surface of the metal.
What do the solid verticle lines represent in cell diagrams
The boundaries between phases e.g. solid electrode and solution (Electrolyte)
Where is the more positive half cell written
On the right
What does the double line represent
The salt bridge between the two half cells
What happens if a System does not include metals.
a platinum electrodemust be used
Why is a platinum electrode used
It is unreactive and can conduct electricity
What does the platinum provide
A conducting surface for electron transfer
e.g. for Fe2+ (aq) Fe3+ (aq) + ethere is no solid conducting surface, a platinum electrode must be used. The cell diagram is drawn as:
| Fe3+ (aq), Fe2+ (aq) |Pt
If the system contains several speciese.g. MnO4- + 8H+ + 5e-Mn2+ + 4H2Othen in the cell diagram the balancing numbers, H+ ions and H2O can be left out.
or if on left hand side Pt | Mn2+ (aq), MnO4-(aq) | | | MnO4- (aq), Mn2+ (aq) |Pt
What happens if If a half equation has several physical states
then the solid vertical line should be used between each different state boundary. e.g. 4e- + 2H2O (l) +O2(g) 4OH-(aq) –> | | O2 | H2O, OH-| Pt and Cl2(g) + 2e- –> 2Cl-(aq) | | Cl2 (g)| Cl- (aq) | Pt
How is electrode potential measured?
You cannot measure the absolute potential of a single half-cell. You can only measure the potential difference between two electrodes. The half-cell must be connected to a reference electrode.
By convention what is the reference electrode used
The Standard Hydrogen Electrode (SHE). The SHE is assigned a potential of 0.00 V.
WHat is the hydrogen electrdoe equilibrioum
H2(g) <=> 2H+ (aq) + 2e-
How is a hydrogen electrdoe represented in a cell diagram
Pt |H2 (g) | H+(aq)