9. Redox Processes Flashcards
What is a redox reaction?
The simultaneous reduction and oxidation.
What is an oxidation number?
A theoretical description of the charge that an atom would have if it were to be bonded ionically.
What does an increase in oxidation number mean?
Oxidation has occurred.
List the 8 rules of oxidation numbers.
- Uncombined elements have an O.N of 0.
- The O.N on a monatomic ion= the charge on an ion.
- Group 1= +1
- Group 2= +2
- Fluorine= -1
- Hydrogen= +1
- Oxygen= -2
- Sum of O.N= total charge on the compound.
What is disproportionation?
The simultaneous oxidation and reduction of an element in a single species.
Give an example of a disproportionation reaction.
the decomposition of hydrogen peroxide.
What is a primary cell also known as?
A voltaic cell
Summarise how primary cells work.
They use irreversible chemical reactions to generate an electrical current.
What is a half-cell?
A metal in contact with an aqueous solution of its own ions.
What does a primary voltaic cell look like?
Two different half cells connected by an external wire and a salt bridge.
What is the purpose of a salt bridge in a primary cell?
Allows the free movement of ions.
What are salt bridge made of?
A filter paper/ glass tube with a saturated solution of KNO3.
What does the voltage produced by a primary cell depend on?
The relative difference between the two metals in the activity series.
Where is the half cell undergoing reduction placed?
On the right
In a primary voltaic cell, what is the negative electrode called?
Anode.
In a primary voltaic cell, what is the positive electrode called?
Cathode.
What process occurs at the anode?
Oxidation
What process occurs at the cathode?
Reduction
Give a summary of a secondary cell.
A rechargeable cell that involves redox reactions that can be reversed using electrical energy.
What is a battery?
An electrochemical cell in which chemical energy is converted into electrical energy.
Give an example of a primary cell.
Zinc - copper
Why does an electrochemical cell have internal resistance?
It takes time for the ions to diffuse.
What direction is the electron flow in a primary voltaic cell?
Anode –> Cathode
What are the three advantages of fuel cells that use alcohol biofuels?
- Growing renewable crops to make biofuel absorbs CO2
- Preserves finite fossil fuels.
- H2 production could use a renewable source.
What are the three disadvantages of fuel cells that use alcohol biofuels?
- Fuel cell releases CO2.
- Land is taken which could be used for food.
- CO2 is released from the agriculture refinement.
Summarise fuel cells.
Require a constant supply of fuel, which undergoes a redox reaction.
What can be used as fuel in a fuel cell?
hydrogen and methanol
Give the half equation at the anode for a methanol fuel cell.
CH3OH + H2O –> CO2 + 6H+ + 6e-
Give the half equation at the cathode for a methanol fuel cell.
4H+ + O2 + 4e- –> 2H2O
What is the overall reaction for a methanol fuel cell?
2CH3OH + 3O2 –> 2CO2 + 4H2O
Give the equation for the anode in a hydrogen fuel cell.
H2 –> 2H+ + 2e-
Give the equation for the cathode in a hydrogen fuel cell.
4H+ + 4e- + O2 –> 2H2O
What is the overall equation for a hydrogen fuel cell?
2H2 + O2 –> 2H2O
What carries the charge across the bridge in a fuel cell?
Protons
What are the 6 advantages of a hydrogen fuel cell?
- Only waste product is H2O
- H2 leak has limited impact
- Quieter than conventional engines
- Expelled steam can be used to generate energy.
- Moderate operating range of 150-200C (lower operating cost)
- Jobs created when funding is invested.
What are the 3 disadvantages of a hydrogen fuel cell?
- Pressurised hydrogen is flammable
- H2 produced either by electrolysis or burning fossil fuels.
- No refill stations and no vehicles get converted.
How does charging a battery work?
The external current reverses the chemical reactions at each half cell, raising the cell’s potential difference until it can be discharged again.
How are lead-acid cells used?
In cars to provide the initial power to start the motor.
Give the equation at the anode for a lead-acid cell.
Pb (s) + SO4^2- (aq) –> PbSO4 (s) + 2e-
Give the equation at the cathode for a lead-acid cell.
PbO2 (s) + SO4^2- (aq) + 4H+ + 2e- –> PbSO4 (s) + 2H2O (l)
What is the overall equation for a lead-acid cell?
Pb (s) + PbO2 (s) + 2SO4^2- (aq) + 4H+ –> 2PbSO4 (s) + 2H2O (l)
What temperature does molten electrolysis require?
1000C
