Redox and electrode potentials Flashcards
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.
Define oxidation
Loss of electrons
Increase in oxidation number
Define reduction
Gain of electrons
Decrease in oxidation number
What happens in a redox reaction
Electrons are transferred from one species to another.
One element is oxidised whilst the other is 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 e.m.f of a half cell compared with a standard hydrogen half cell measured at 298K with solution of concentration of 1.00moldm-3 and 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 it’s aqueous ions.
Write a half equation for zinc (s) to zinc (II)
Zn (s) <—–> Zn2+ + 2e-
Write a half equation for copper (II) to copper (III)
Cu2+ (aq) <——> Cu3+(aq) + e-
What is a standard hydrogen half cell made of (3)
Hydrochloric acid - 1moldm-3
Hydrogen gas - 100KPa
Inert platinum electrode
Why is a hydrogen half cell used as a standard half cell
Easy to control its purity and reproducibility
How to 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 the electrodes
Why might you use other standard electrodes occasionally
Cheaper as platinum is expensive
If the E0 value is more negative, what does it mean in terms of oxidising/reducing power
Better reducing agent (easier to oxidise)
If the E0 value is more positive, what does it mean in terms of oxidising/reducing power
Better oxidising agent (easier to reduce)
How do you calculate the emf of a cell from E0 value
Positive - negative
When would you use a platinum electrode
When both the oxidised and reduced forms of the metal are in aqueous solutions
Why is platinum chosen
Inert and a good conductor to complete circuit
What are the three kinds 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
Chemical in the cell provides electrical energy. When recharging the reactions of the cells can be reversed.
Give an example of rechargeable cells
Lithium ion batteries
Explain why Lithium is used in computer batteries
Low density so the electrode is light and it is very reactive,
What are the drawbacks of using Lithium batteries
Toxic if ingested
Rapid discharge of current can cause fire
Describe how fuel cells work
The cell 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 eg.methanol
What are the two reactions that take place at the two electrons in alkaline hydrogen fuel cell
2H2 + 4OH- ——> 4H2O + 4e-
O2 + 2H2O +4e- ——-> 4OH-
What are the disadvantages of fuel cells
Hydrogen is a flammable gas with a low b.p so dangerous to store
Fuel cells have a limited lifetime and use toxic chemicals in their manufacture
What is the reason some cells cant be recharged
Reaction of the cell is not reversible
Why might the emf of a cell change after a period of time
Concentrations of the ions change- the ions are used up
How can the emf of a cell be kept constant
Reagents are supplied constantly, so the concentrations of the ions remain constant, E0 remains constant
