20.1 - 20.3 Electrode potentials and electrochemical cells Flashcards
Define standard electrode potential
The potential difference, in volts, of a substance in an aqueous solution of its ions relative to the standard hydrogen electrode. Under standard conditions; 298k, 100kPa and 1 mol dm-3 solutions
What physical changes can be noted if electrons are allowed to flow between two half cells?
One electrode dissolves:
- Increases the conc of the equivilant ion in solution
Build up on the other electrode:
- Electrons combine with the ion to form more solid
- Decreases the conc of the equivilant ion in solution
What is a salt bridge and why is it used instead of a wire?
Filter paper soaked in a solution of salt (usually saturated potassium nitrate)
It is used rather than a piece of wire to avoid introducing further metal ions to the circuit and altering the potential
Define standard hydrogen electrode
The standard reference electrode which other half cells are connected to in order to measure e.m.f
The e.m.f of the standard hydrogen electrode is 0.00V
Place on the left
Describe the standard hydrogen electrode
- Hydrogen gas is bubbled into a solution of H+(aq) ions
- H doesn’t conduct so electrical contact is made via a piece of unreactive platinum metal
- The platinum is coated with finely divided platinum to increase the surface area and allow any reaction to proceed rapidly
- The electrode is used under standard conditions
How to calculate e.m.f?
Enote (RHS) - Enote (LHS)
What is necessary for a reaction to be feasible?
The e.m.f value must be positive
Describe the correlation between no. of electrons and Enote value
Higher no e- = Lower Enote value
Describe how to write the standard hydrogen electrode
Pt(s)/H2(g)/H+(aq)// …
Describe non-rechargeable cells
- Chemicals are used up overtime and emf drops
- Once one or more of the chemicals have been completely used up, the cell is flat and the emf is 0V
- These cells cannot be recharged and have to be disposed of after single use
Describe zinc-carbon non-rechargeable cells
- Cheaper but has a shorter life
- Useful for short intermittent current
- Zinc walls of the battery get used up during its life so as they become thinner, the battery is prone to leakage
- e.g doorbell
Describe non-rechargeable alkaline cells
- More expensive but has a longer life
- Useful for continuous high current
- e.g torches, radios
Describe rechargeable cells
- Reactions are reversible
- Can be reversed by applying an external current to regenerate the chemicals
Describe Li ion rechargeable cells
- Used in phones, tablets, cameras, laptops e.t.c
Li⁺(aq) + CoO₂(s) + e⁻ ⇌ LiCoO₂(s)
Li(s) ⇌ Li⁺(aq) + e⁻
Describe fuel cells
- Have continuous supply of the chemicals into the cell
- Neither run out or need recharging
- e.g hydrogen-oxygen fuel cell
Define battery
A number of cells connected together
Describe a nickel cadmium rechargeable cell
- Expensive
- Can be recharged up to 500 times
Describe a lead-acid rechargeable battery
- Made of 6 cells
- Used in cars
- When engine is running, battery is recharged by the cars generator
Give the half-equations in an alkaline hydrogen-oxygen fuel cell
H₂(g) + 2OH⁻(aq) ⇌ 2H₂O(l) + 2e⁻
O₂(g) + 2H₂O(l) + 4e⁻ ⇌ 4OH⁻(aq)
Give the advantages of non-rechargeable cells
- Cheaper
- Small/portable
Give the disadvantages of non-rechargeable cells
- Cost more long term
- Short life span
- More waste produced than with rechargeables
- Landfill issues
- Leakage
Give the advantages of a rechargeable cell
- Less cost long term
- Environmentally friendly
- Long life-span
- Portable
Give the disadvantages of a rechargeable cell
- More expensive
- Limited capacity
- Need to be recharged
- Landfill waste
Give the advantages of a fuel cell
- Renewable
- Energy efficient
- Minimal carbon residue
- Minimal sound pollution
- Environmentally friendly (only byproduct is water)
Give the disadvantages of a fuel cell
- Lack of infrastructure to distribute hydrogen
- Hydrogen is expensive to produce and not widely available
- Explosive
- Platinum is expensive
Define electrochemical series
List of electrode potentials in numerical order
Why might the actual emf of a cell be different from the emf calculated?
Non-standard conditions
What alteration do you make to a cell after measuring its emf to allow the cell to go to completion?
Remove the voltmeter
