Unit 1: Section 8 - Electrode Potentials and Cells 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 (II)
Zn (s) ⇌ Zn2+(aq) + 2e-
Write a half equation for copper (II) to copper (III)
Cu2+(aq) ⇌ Cu3+(aq) + e-
What is the simplest salt bridge made of?
Filter paper soaked in saturated solution of KNO3 (potassium nitrate)
Why are salt bridges necessary?
- Complete the circuit, but avoid further metal/ion potetnials as does not perform electrochemistry
- Allows ion movement to balance the charge
- Do not react with electrodes
What symbol is used to represent a salt bridge in standard notation?
||
What type of species goes on the outside (furthest from the salt bridge) in standard cell notation?
The most reduced species
What does|indicate?
Phase boundary (solid/liquid/gas)
What happens at the left-hand electrode?
- Left hand electrode is where oxidation occurs
- Left hand electrode is the half cell with the most negative E0 value
What happens at the right-hand electrode?
- Right hand electrode is where reduction occurs
- Right hand electrode is the half cell with the most positive E0 value
Which side of the cell has the most negative E0 value? What happens to the metal with the most negative E0 value?
Oxidation - left hand electrode
What conditions is the standard hydrogen electrode used in?
Temperature - 298K
Pressure - 100kPa
[H+] - 1.00 mol dm^-3
What is the standard hydrogen electrode used for?
- Comparing other cells agaisnt E0
- E0 of SHE is defined as 0, so all other E0 values are compared against it
Why might you use other standard electrodes occassionally?
- They are cheaper/easier/quicker to use and can provide just as good as reference
- Platinum is expensive
If an E0 value is more negative, what does it mean in terms of oxidising/reducing power?
Better reducing agent (easier to oxidise)
If an E0 value is more positive, what does it mean in terms of oxidising/reducing power?
Better oxidising agent (easier to reduce)
What factors will change E0 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 electons
- The E0 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 E0 values?
E0cell = E0right - E0left
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 as the electrode?
- 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 secies that is being reduced (this is effectively the right hand electrode)
- Calculate its E0 value minus the E0 value fo the species that is being oxidised (effectively the left hand cell)
- If E0 overall > 0, reaction will occur
What was the first commercia cell made from (Daniell cell)?
Zinc/copper (II)
What are zinc/carbon cells more commonly known as?
Disposable batteries
What are the 2 reactions that take place in zinc/carbon cells?
Zn oxidised to Zn2+
NH4+ reduced to NH3 at carbon electrode
What are the reaction that occur in a lead/acid battery (car batteries)?
Pb + SO42- -> PbSO4(s) + 2e-
PbO2 + 4H+ + SO42- + 2e- -> PbSO4 + 2H2O
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 E0
Nickel/cadmium cells are rechargeable AA batteries etc. What reactions occur at the electrodes?
Cd(OH)2(s) + 2e- -> Cd(s) + 2OH-
NiO(OH)(s) + H2O + e- -> Ni(OH)2(s) + OH-
Where are lithium-ion cells used?
- Mobile phones
- Laptops
What reactions occur on discharge in lithium-ion cells?
Li+ + CoO2 + e- -> Li+[CoO2]-
Li -> Li+ + e-
What is a fuel cell?
A cell that is used to generate electric current; does not require electrical recharging
What are the reactions that take place at the 2 electrodes in an an alkaline hydrogen fuel cell?
2H2 + 4OH- -> 4H2O + 4e-
O2 + 2H2O + 4e- -> 4OH-
Why is it better to use a fuel cell than to burn H2 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 effecient
Disadvantages of fuel cells?
- Hydrogen is a flammable gas with a low b.p. - hard and dangerous to store and transport and expensive to buy
- Fuel cells have a limited lifetime and use toxic chemicals in their manufacture
How do you find the weakest reducing agent from a table of electrode potential data?
Most positive E0 value - it is the PRODUCT of the reaction equation
What is the reason that some cells cannot be recharged?
Reaction of the cell is not reversible - aproduct is produced that either dissipates or cannot be converted back into the reactants
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; E0 remains constant
