Electrode Potential Flashcards
What happens when a rod of a metal is dipped into a solution of its own ions ?
An equilibrium is set up between the solid metal and the aqueous metal ions
Write a half-equation for zinc (s) to zinc (2+)
Zn (s) = Zn2+(aq) + 2e-
Write a half-equation for copper (II) to copper (lI).
Cu2+(aq) = Cu3+(aq) + e-
What is the simplest salt bridge made out of ?
Filter paper soaked in a saturated solution of KNO3 (potassium nitrate)
What is the purpose of a salt bridge ?
- complete the circuit
- allows the ions to move to balance the charge
- do not react with electrodes
What is electrode potential?
Measure of how easily a metal is oxidised
What electrode potential does a metal that is easily oxidised have ?
Negative
What electrode potential does a metal that is hard to oxidise have ?
Positive
Why is there an equilibrium within a cell ?
There is a tendency for metal ions to form + ions and go into solution, but there is also the tendency for for metal ions to gain electrons in solution .
What happens if the equilibrium shifts to the left ?
The metals acquires a negative charge due to the build up of electrons on the metal
What happens when the equilibrium shifts to the right ?
Positive charge will build up on the metal as electrons have been used up to form metal from the metal ions
What happens at the left hand electrode ?
- oxidation
- is the half cell with the most negative Eº value
What happens at the right hand electrode ?
- reduction
- electrode with the most positive Eº value
What conditions is the standard hydrogen electrode used in ?
- 298K
- 100kPa
- [H+] 1.00 mol dm-3
What is the standard hydrogen electrode used for ?
- Comparing other cells against it because the Eº of SHE is 0
Why might you use other standard electrodes ?
They are cheaper/easier/quicker to use and can provide just as good a reference.
Platinum is expensive
If an E° value is more negative, what does it mean in terms of oxidising/reducing power?
Better reducing agent (easier to oxidise)
If an E° value is more positive, what does it mean in terms of oxidising/reducing power?
Better oxidising agent (easier to reduce)
What factors will change E° values?
Concentration of ions
Temperature
What happens if you reduce the concentration of the ions in the left hand half cell?
Equilibrium moves to the left to oppose the change of removing ions; this releases more electrons, the E° of the left hand cell becomes more negative, so the e.m.f. Of the cell increases.
How do you calculate the emf of a cell from E° values?
E°cell = E° right - Eº left
When would you use a Platinum electrode?
When both the oxidised and reduced forms of the metal are in aqueous solution
Why is Platinum chosen?
Inert so does not take part in the electrochemistry
Good conductor to complete circuit
How would you predict if a reaction would occur?
- Take the 2 half equations.
- Find the species that is being reduced (this is effectively the right hand electrode)
- Calculate its E° value minus the E° value of the species that is being oxidised (effectively the left hand cell).
- If E° overall > 0, reaction will occur.
How does Concentration and Pressure Effect the EMF?
- Increasing the concentration of the solutions used in the electrochemical cell makes the cell
EMF more positive as fewer electrons are produced in the reaction. - Increasing the pressure of the cell will make the cell EMF more negative as more electrons
are produced.
What are the two reactions that take place in zinc/carbon cells (disposable batteries)?
Zn oxidised to Zn2+
NH4+ reduced to NH3 at carbon electrode
What are the reactions that occur in a lead/acid
battery (car batteries)?
Pb + SO42- → PbSO4 (S) + 2e-
PbO2 + 4H+ + SO42- + 2e- → PbSO4 + 2H20
How are cells recharged (if they are rechargeable)?
Reactions are reversible and are reversed by running a higher voltage through the cell than the cell’s E°
What is a fuel cell ?
A cell that is used to generate an electrical current and does not require charging as it has a constant supply of the chemicals it needs
What are the reactions that take place at the two electrons in an alkaline hydrogen fuel cell?
2H2 + 40H-→ 4H20 + 4e-
O2 + 2H20 + 4e- → 40H-
Why is it better to use a fuel cell than to burn H, in air, even though the same overall reaction occurs?
In combustion, sulfur containing compounds (SO2, SO3) and nitrogen containing compounds (NO2, NOx) are produced due to the high temperatures and the S and N in air. These are bad for the environment.
This does not occur in a fuel cell; the only product is water.
More efficient
Disadvantages of fuel cells?
Hydrogen is a flammable gas with a low b.p. → hard and dangerous to store and transport → expensive to buy
Fuel cells have a limited lifetime and use toxic chemicals in their manufacture
Why might the e.m.f. Of a cell change after a period of time?
Concentrations of the ions change - the reagents
are used up
How can the e.m.f. Of a cell be kept constant?
Reagents are supplied constantly, so the concentrations of the ions are constant; E° remains constant
