(1) Electrode Potentials Flashcards
What is an electrode/half cell?
A strip of metal dipped into a solution of its own ions.
An equilibrium is set up
How can 2 half cells be connected?
- two metal rods are connected with wires and a high resistance voltmeter
- the two beakers of electrolyte are connected with a salt bridge to complete the circuit.
- salt bridge is soaked in potassium nitrate solution
Why is KNO3 a suitable solution for a salt bridge?
What is the purpose of a salt bridge?
KNO3 is unreactive with the electrodes AND the ions are free to move
To complete the circuit
What features must a solution have to be used as a salt bridge?
- must not react with chemicals in either solution
- ions free to move so conducts electricity
What does the voltmeter do?
Prevents electrons flowing -so enables the voltage to be measured.
-otherwise electrons flow from left electrode to right (most reactive metal to the least reactive metal)
How can you measure the current in a cell?
Voltmeter is replaced with an ammeter = electrons can flow and a current is produced
Why might the current produced by a cell fall to zero after some time?
All the reactants are used up.
What will happen to a cell once the reactants are used up?
stops working OR starts to leak
Draw a diagram to represent a cell with copper and zinc electrodes
Write half equations for each electrode
Zn(s) –> Zn2+(aq) + 2e-
Cu2+(aq) + 2e- –> Cu(s)
Overall:
Zn + Cu2+ –> Zn2+ + Cu
Zn + Cu2+ –> Zn2+ + Cu
This reaction would continue generating a current until what?
- The solid Zinc rod completely reacted.
- All the Cu2+ ions in solution were used up
Key facts about electrodes?
~ left electrode is always the -ve electrode as e-s are produced there
~ oxidation always occurs at the -ve electrode
~ right electrode is always the +ve electrode as e-s are used up there
~ reduction always occurs at the +ve electrode
~ e-s flow from the -ve to +ve electrode
When do we need to use platinum electrodes?
When there is no solid metal in the reaction, such as when there are metal ions of two different charges in the same solution.
-If this is the case a metal rod made from another unreactive metal is needed to connect the circuit (Platinum).
Why is platinum a suitable electrode?
Pt is unreactive AND conducts electricity
Why do we use the Standard Hydrogen Electrode?
To compare single electrodes with one another, this half cell is used as the standard electrode.
Key Features of the Standard Hydrogen Electrode?
- H2 gas is pumped in at 100kPa
- Electrolyte contains [H+] ions of 1 moldm-3 (usually in HCl)
- Platinum electrode
- Temp of 298K
What is the voltage of the Standard Hydrogen Electrode?
ZERO.
All the equations shown in the electrochemical series are…
Reduction equations
The standard electrode potential of Cu2+ is 0.37V.
Why might the electrode potential of a copper cell not be 0.37V?
If the concentration of the CuSO4 solution is not 1 mol dm-3.
Write the conventional cell representation of:
Zn(s) + Cu2+ (aq) —> Cu(s) + Zn2+ (aq)
ROOR
Reduced Oxidised Oxidised Reduced
- Vertical solid lines indicate a phase boundary (i.e. between the solid and aqueous phases).
- A double vertical line in the middle represents the salt bridge.
- Order of electrodes is important too, start from left
Zn(s) │ Zn2+(aq) ‖ Cu2+(aq) │ Cu(s)
Conventional cell representation of the Standard Hydrogen Electrode?
Pt(s)│H2(g) │H+(aq) ‖
What is the conventional cell representation of a solution containing Fe2+ and Fe3+ ions?
The equilibrium taking place between the Pt and the solution is:
Fe2+ —> Fe3+ + e-
‖ Fe3+(aq), Fe2+(aq) │Pt(s)
How can you work out the strongest oxidising agent and weakest reducing agent?
SOWR
Strongest oxidising agent + Weakest reducing agent = Most positive electrode potential
Most negative = weakest oxidising agent + strongest reducing agent
How can you work out the E.M.F?
Work it out for:
Li+ = -3.03
Ag+ = +0.80
Most positive - Least positive
Ecell = (+0.80) - (-3.03)
Ecell = + 3.83V
What is Cell Discharge?
Cell discharge
When the Ecell value is positive, this means the reaction is feasible and the cell discharges – this means it produces a current.
e.g.
Ag+(aq) + Li(s) <=> Ag(s) + Li+(s)
What is Cell Recharge?
Cell recharge
If a reaction is reversible, the cell can be recharged by plugging it into the mains. The reverse reaction will occur when the cell is recharged.
e.g.
Ag(s) + Li+(s) <=> Ag+(aq) + Li(s)
Give an environmental advantage and disadvantage of using rechargeable cells.
Advantage: Metals are reused
Disadvantage: Mains electricity is used to recharge, which may come from combusting fossil fuels, which releases CO2(g).
What happens to electrode potential values if equilibria shifts?
If more electrons : electrode potential becomes more negative
If less electrons : electrode potential becomes more positive
+ has a knock on effect on the e.m.f. value - may become larger or smaller
How can the direction of electron flow be worked out?
Look at diagram.
e. g.
- From right to left
- The Cu2+ is more concentrated on the left, so reduction on Cu2+ is more likely to happen on the left (Cu2+ + 2e- <=> Cu)
- The left electrode is the positive electrode, so the right electrode is the negative electrode
