Describe a standard hydrogen electrode
- Hydrogen gas bubbled through
- 1.0 mol dm–3 HCl to make H+
- At 298K and 100kPa
- Pt electrode
Why is the electrode potential 0 for H?
By definition
Why do we use porous ceramic materials?
- Increases the surface area
- so faster rate of reaction
Advantage of fuel cell over re-chargeable cell
- Can operate continuously without stopping to recharge
- Hydrogen and oxygen supplied continuously in fuel cell
Why fuel cells aren’t really carbon neutral
- Hydrogen may need to be made using an energy source that is not ‘carbon neutral’
Why does the cell emf change when a current flows
- The concentration of the ions change
- measurements no longer standard
- the e.m.f is determined when no current flows
What is the purpose of a salt bridge and what is it made from?
- Allows ions to move between electrode, carrying charge
- Made from KNO3
Features of Pt
- Conducts electricity
- Unreactive
- Does not create a potential difference
Why does the voltage in a ‘lead-acid’ changes over time?
- Reagents are being used up so the concentration decreases
What is the type of cell that has a constant voltage and why is it constant?
- Fuel cell
- Reagents are constantly supplied
- Concentration of reagents remain constant
Features of salt bridge (KNO3)
- Inert, so it does not react with either electrode
- Allow electrical connection between electrodes
- The ions in the ionic substance in the salt bridge move through the salt bridge
Features of porous separator?
Allows ions to pass through it
Function of C(s) in the cell
Allows electrons to flow (conductor)
Why does leaking occur with non-rechargeable cells?
- Metal (Zn) is ‘used up’
Advantage of using rechargeable batteries over non-rec
- Does not leak
- No landfill problems as less waste
- less energy needed for mining
What happens to the emf of this reation when [Mg2+] is decreased?
Mg + Fe2+ ⇔ Mg2+ + Fe
- Emf increases
- Equilibrium shifts to the right to oppose reduction in [Mg2+]
- More Mg2+ is formed
- E(Mg2+/Mg) decreases
How is a current generated in a fuel cell?
- Hydrogen electrode produces electrons
- Oxygen electrode accepts electrons
Why can’t a cell be recharged?
- Reaction is not reversible
