Chapter 23 - Redox And Electrode Potentials Flashcards
Define redox
A reaction in which both reduction and oxidation take place.
Define oxidation
The loss of electrons or an increase in oxidation number.
Define reduction
The gain of electrons or a decrease in oxidation number.
What is the Manganate half reaction?
MnO(4)- + 8H+ + 5e- -> Mn2+ + 4H(2)O
What is the iodine sodium thiosulphate reaction?
2S(2)O(3)2- + I(2) -> S(4)O(6)2- + 2I-
Why is there no clear colour change at the end point of a Vanadium and Potassium Manganate titration?
KMnO(4) is purple/pink
V2+ is violet
What property does the voltmeter have in a cell?
High resistance
Which way does the current flow in the cell?
Negative electrode to the positive electrode
How do the ions flow between cells?
Salt bridge
What is the salt bridge generally made of?
Potassium nitrate
Ammonium nitrate
What is the electrode made of in a standard hydrogen cell?
Platinum
What is in the half cell of Fe3+ and Fe2+?
Inert Pt electrode
Equimolar Fe2+ and Fe3+ (1 mol dm^-3)
When two half cells are connected, which is oxidised and which is reduced?
More reactive metal is oxidised
Less reactive metal is reduced
Define standard electrode potential (E°)
The emf of a half cell compared with a standard hydrogen half cell, measured at 298K with solution concentrations of 1 mol/dm^3 and a gas pressure of 101kPa.
What is the electrode potential of a hydrogen half cell?
0.00V
What is the electrode potential of the cell?
Electrode potential(E°) of the more positive terminal minus the electrode potential(E°) of the more negative terminal
How do we know if a cell is feasible?
If the electrode potential of the cell is positive, the reaction is feasible.
Which electrode goes on the right when asked about a labelled diagram?
- The half-cell with the more +ve E° (electrode potential)
- The metal ions that are more easily reduced
Which electrode goes on the left when asked about a labelled diagram?
- The half-cell with the more -ve E°
- The metal ions that are more easily oxidised
What does the more positive value for E° mean for the position of the equilibrium?
- Equilibrium lies further to the RIGHT than the other half-cell
- So the metal ions are more easily REDUCED than the other half-cell.
- So the metal is an OXIDISING agent
What does the more negative value for E° mean for the position of the equilibrium?
- Equilibrium lies further to the LEFT than the other half-cell
- So the metal ions are more easily OXIDISED than the other half-cell.
- So the metal is an REDUCING agent
Does it go Bigger or smaller, Metal ion –> Metal Solid
E.G. The copper
2Cr + 3Cu2+ –> 2Cr3+ + 3Cu
-gets Bigger, metal ion to metal solid
-Less concentrated
-Copper is REDUCED
2Cr + 3Cu2+ –> 2Cr3+ + 3Cu
Does it go Bigger or smaller, Metal solid –> Metal ion?
E.G. the chromium
2Cr + 3Cu2+ –> 2Cr3+ + 3Cu
-gets Smaller, metal solid to metal ion,
-More Concentrated
-Chromium is OXIDISED
2Cr + 3Cu2+ –> 2Cr3+ + 3Cu
State 2 reasons why the actual cell potential may be different from the value calculated?
- Non- standard conditions
- High Ea, so very slow rate of reaction
What’s in a Metal/Metal ion half cell?
E.g. Cu2+ | Cu half cell
- Metal rod (Cu)
- dipped into aq solution of metal ion (Cu2+)
What’s in a Ion/ ion half-cell?
E.g. Fe3+ | Fe2+
- Contains ions of the same element in different oxidation states ( Equimolar of Fe3+ & Fe2+ (1 moldm-3
- Pt electrode
Why does Fe2+ | Fe half cell sometimes form the metal ion Fe3+?
- Fe2+ oxidises in air to form Fe3+ which has a partially filled d-subshell so gives greater stability.
- So Fe3+ is a more likely product
What’s in a Standard Hydrogen half cell?
H2 | H+
- inert Pt electrode
- Glass tube with holes in to allow bubbles of H2(g) to escape
- H2(g) coming in through the top at 298K,1 atm
- Acid solution at bottom containing 1.0 moldm-3 of H+(aq)
What is the overall cell eqn and both half eqns for the zinc-copper cell?
Zn2+ +2e- –> Zn , -0.76 V
Cu2+ + 2e- –> Cu , +0.34 V
Red: Cu2+ + 2e- –> Cu (MP)
Oxi: Zn –> Zn2+ + 2e- (MN)
Ovll: Zn + Cu2+ –> Zn2+ + Cu
State the key difference between a primary cell and a secondary cell.
- A primary cell is only used one and is non-rechargeable
- A secondary cell is rechargeable
State the key characteristics of a fuel cell.
- A fuel cell uses the energy from the reaction of a fuel with oxygen to create a voltage.
- A fuel cell can be run continuously provided that the fuel and oxygen are continually being supplied to the cell.
