Electrode potential & fuel cells

What is this?

Electrochemical cell

What do electrochemical cells involve:

two different metals dipped in salt solutions of their own ions (2 half cells), connected by a wire and salt bridge

What is a salt bridge?

made from filter paper soaked in ionic substance such as KNO3 and allows ions to flow through and balance our charges in order to complete circuit

How do you determine which metal is oxidised?

The metal that has the most NEGATIVE electrode potential as it will lose electrons more easily and donate them to the hydrogen half-cell (MORE REACTIVE METAL)

What does a positive electrode potential mean?

The metal accepts electrons from the hydrogen half cell so is more likely to be reduced

What causes a large voltage to be produced?

Large difference in reactivity

What is the direction of the flow of electrons?

electrons flow through the wire from the MORE reactive metal to the LESS reactive metal

What is the name of the voltage between the 2 half cells?

cell potential (Ecell or emf - electromotorforce)

If a reaction is reversible what determines the direction of the reaction?

depends on how easily metal loses electron

What is the standard electrode potential of a half cell?

emf of a half cell compared with a standard hydrogen half cell measured at 298K with solution concentrations of 1 mol dm^-3 and gas pressures of 100KPa

Why must standard conditions be used?

Equilibium is reversible so is altered by concentration, temperature, and pressure so therefore needs to remain the same

What is the standard electrode potential of a hydrogen cell?

0V

What is this?

Standard hydrogen electrode

What does a standard hydrogen electrode consist of?

1. Concentrated HCl (1 mol dm^-3 which is the source of H+ ions

2. H₂ gas at 100KPa and 298K bubbled through H+ solution

   1. Platinum electrode which is inert but allows electrons to pass into or out of the half cell by a connecting wire

What is a standard hydrogen half cell used for?

Used as a reference to measure all other half cells to find their standard electrode potentials

What do you do if half cell contains aqueous ions of the SAME element with diff oxidation numbers?

Include a solid platinum electrode to allow electrons to pass in and out and solution needs to be EQUIMOLAR (same conc of ions in solution)

What is the overall cell potential dependant on?

Dependant on the electrode potentials of the half cells involved

What is this?

Electrochemical series

What does the electrochemical series show?

a species will react with the species below it, this species is oxidised so favours the backwards reaction

What does a more reactive metal have?

More NEGATIVE electrode potential as it wants to lose electrons so is negative to favour oxidation and donate electrons to the hydrogen half cell

What does a more reactive non-metal have?

more POSITIVE electrode potential as it wants to gain electrons to is positive to favour reduction and accept electrons from the hydrogen half cell

What is the equation for electrode potential?

REDUCED electrode potential - OXIDISED electrode potential

What does a positive overall electrode potential mean?

The reaction can happen

What do non-rechargeable cells do?

provide electrical energy until chemicals have reacted so much the voltage falls and the cell is flat so is discarded

What happens in rechargable cells?

chemicals react to provide electrical energy and the direction of the reaction is reversed when charging so chemicals in cell are regenerated and can be used again

What do fuel cells do?

use energy from the reaction of a fuel with oxygen to create a voltage

Give an example of a fuel cell:

Hydrogen-oxygen fuel cell

Describethe features of a fuel cell:

reactants flow in and products flow out while electrolyte stays in, don’t need to be recharged, can be operated continuously as long as the fuel and oxygen continue to flow in

Redox equilibrium for hydrogen fuel cell:

2OH^-(aq) + H₂(g) ⇌ 2H₂O (l) + 2e-

½ O₂(g) + H₂O + 2e- ⇌ 2OH^-

What are the two limitations for predicting reactions?

non-standard conditions and unfavourable kinetics

How does non standard conditions limit prediction of reactions?

an increase in concentration of reactants will shift the equilibrium right so increases ease of losing / gaining electrons so the electrode potential of cell will be decreased / increased

How do unfavourable kinetics limit prediction of reactions?

Very slow rate of reaction due to high activation energy will affect the overall reaction

How is unfavourable kinetics determined?

If the electrode potential difference is less than 0.4V the reaction is unlikely