Redox and Electrode Potentials Flashcards
(28 cards)
Define oxidising agent.
A species that is reduced in a reaction and causes another species to be oxidised.
Define reducing agent.
A species that is oxidised in a reaction and causes another species to be reduced.
What are the half equations and the ionic equation for: SnO + Zn —> ZnO + Sn
Half equations:
- Sn2+ + 2e- —> Sn
- Zn —> Zn2+ + 2e-
Ionic equation:
- Sn2+ + Zn —> Sn + Zn2+
Define standard electrode potential.
The emf of a half cell compared with a standard hydrogen half cell measured at 298K with solution concentration of 1 mol dm-3 and a gas pressure of 100kPa
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.
What is a standard hydrogen half cell made of? (3)
- hydrochloric acid 1 mol dm^-3
- hydrogen gas at 100kPa
- Inert platinum electrode
Why is a hydrogen half cell used as a standard half cell?
Easy to control its purity abs reproducibility.
How do you make a simple salt bridge?
Soak a piece of filter paper in an aqueous solution of KNO3 or NH4NO3
Why are salt bridges necessary?
To complete the circuit by connecting the two solutions. This enables charge to be transferred between the half cells. They do not react with electrodes.
Why might you use other standard electrodes occasionally?
They are cheaper/easier/cheaper 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)
How do you calculate the emf of a cell from E values?
E(cell) = E(+) — E (-)
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 and good conductor to complete circuit.
How would you predict if a reaction if a would occur?
Take the 2 half equations.
Find the species that is being reduced.
Calculate its E value minus the E value of the species that is being oxidised.
If E overall > 0.4V, reaction will occur
What are the 3 main types of electrochemical cells
- non-rechargeable cells
- rechargeable cells
- fuel cells
Describe how non-rechargeable cells work.
They provide electrical energy until all the chemicals have reacted.
Describe how rechargeable cells work.
Chemicals in the cell provide electrical energy. When recharging the reactions of the cells can be reversed.
Give some examples of rechargeable cells.
- nickel and cadmium batteries
- lithium ion batteries
- lithium polymer batteries
Explain why lithium is used in laptop batteries.
Lithium has low density so the electrode is light and very reactive.
What are the drawbacks of using lithium batteries?
- they are toxic if ingested
- rapid discharge of current can cause fire or even explosions
Describe how fuel cells work.
The cells uses external supplies of fuel and an oxidant. These external supplies need to be continuously supplied.
Modern fuel cells are based on what type of fuels?
- hydrogen
- hydrogen rich fuels, e.g. methanol
What reactions take place at the two electrodes in an alkaline hydrogen fuel cell?
2H2 + 4OH- —> 4H2O + 4e-
O2 + 2H2O + 4e- —> 4OH-
