5.2.3: Redox and electrode potentials Flashcards
Define oxidising agent
A species that is reduced in a reaction
and causes another species to be
reduced
Define reducing agent
A species that is oxidised in a reaction
and causes another species to be
reduced
Define oxidation
Loss of electrons
An increase in the oxidation number
Define reduction
Gain of electrons
Decrease in the 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
Why is 2CrO4 2- + 2H+ → Cr2O7 2- + H2O not a redox reaction?
Chromium is oxidised whereas hydrogen remains the same oxidation state (no element 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
298 K 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
Write a half-equation for zinc (s) to zinc (II).
Zn (s) ⇌ Zn2+(aq) + 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 marks)
● Hydrochloric acid 1 mol dm-3
● Hydrogen gas at 100 kPa
● Inert platinum electrode
Why is 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?
They are cheaper/easier/quicker to use
and can provide just as good a
reference.
Platinum is expensive
If an Eo value is more negative, what does it mean in
terms of oxidising/reducing power?
Better reducing agent (easier to oxidise)
If an Eo 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 Eo
values?
Eo cell = Eo positive - Eo negative
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 would occur?
Take the 2 half equations.
Find the species that is being reduced
Calculate its Eo value minus the Eo value of the species that is being oxidised
If Eo 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 it is 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 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 e.g methanol
What are the reactions that take place at the two
electrons in an 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. → hard and
dangerous to store and transport → expensive to buy
Fuel cells have a limited lifetime and use toxic chemicals in their manufacture
What is the reason that some cells cannot be
recharged?
Reaction of the cell is not reversible - a product is produced that either dissipates or cannot be converted back into the reactants
Why might the e.m.f. Of a cell change after a period
of time?
Concentrations of the ions change - the reagents are used up
How can the e.m.f. Of a cell be kept constant?
Reagents are supplied constantly, so the concentrations of the ions are constant;
Eo remains constant
