5.2.3- Redox and Electrode Potentials Flashcards
(35 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
Define oxidation
- Loss of electrons
- An increase in oxidation number
Define reduction
- Gain 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
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 half equations and ionic equation for:
SnO + Zn -> ZnO + Sn
Half equations:
- Sn^2+ + 2e- -> Sn
- Zn -> Zn^2+ + 2e-
Ionic equation:
Sn^2+ + Zn -> Sn + Zn^2+
Define standard electrode potential
The e.m.f of a half cell compared with a standard hydrogen half cell measured at 298K with solution concentrations of 1 moldm^-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) = Zn^2+ (aq) + 2e-
Write a half equation for copper (II) to copper (III)
Cu^2+ (aq) = Cu^3+ (aq) + e-
What is a standard hydrogen half cell made of?
- Hydrochloric acid 1 mol dm^-3
- Hydrogen gas at 100kPa
- 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 E^0 value is more negative, what does it mean in terms of oxidising/ reducing power?
Better reducing agent (easier to oxidise)
If an E^0 value is more positive, what does it mean in terms of oxidising/ reducing power?
Better oxidising agent (easier to reduce)
How do you calculate emf of a cell from E^0 values?
E^0 cell = E^0 positive - E^0 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 2 half equations
- Find species that is being reduced
- Calculate its E^0 value minus E^0 value of the species that is being oxidised
- If E^0 overall >0.4V, reaction will occur
What are the 3 main types of electrochemical cell?
- Non rechargeable cells
- Rechargeable cells
- Fuel cells
Describe how non rechargeable cells work
They provide electrical energy until all chemicals have reacted
