Electrochemistry Flashcards
(18 cards)
What are the 3 definitions of oxidation
- loss of electrons
- gain in oxygen
- increase in oxidation number
What are the 3 definitions of reduction
- gain of electrons
- loss of oxygen
- decrease in oxidation number
What is an oxidising agent
Electron acceptor , species undergoes reduction
What is a reducing agent
Electron donor , species undergoes oxidation
What is the equation for E •
E red - E oxi
What are the standard conditions for measuring emf
298k
1moldm-3
If gasses involved pressure at 1atm
What is a salt bridge
Contains salt solution to keep the charges balanced , the salt must be one that will not react with either of the half cells
What is cell potential affected by
- temperature
- pressure of any gases
- concentration of solutions
What is emf measures relative to
A standard hydrogen electrode which is assigned a E value of 0.00v
Since the standard electrode potential of a half cell is 0 resulting emf is the standard electrode potential of a given half cell
What is the definition of standard electrode potential
The emf of a half cell compared with a standard hydrogen half cell measured at 298k , with solution concentration of 1moldm-3 and gas pressure of 1 atm
What is the electrode potential a measure of
Its tendency to lose or gain electrons
The more positive the positive the greater the tendency to gain electrons, species act as more powerful oxidising agents
The more negative the potential the greater the tendency to lose electrons
Describe what happens when you link two equilibria together
Electrons flow from one equilibrium to the other
This upsets the equilibrium and le chanteliers principles applies , the positions of the equilibrium move and keep on moving until they turn into 2 one way reactions
Eqm with more negative E will move to the left
Eqm with more positive E will move to the right
What reactions are feasible
Any reaction with a positive E value will work
Limitations of E values
- cell emf only gives information about the feasibility of a redox reaction from an energetic point of view
- it cannot tell how fast a feasible reaction is likely to proceed therefore some feasible redox reactions may not appear to take place just because they are too slow
- only deals with aqueous solutions of ions and reducing tendencies may be quite different in other solvents
What are fuel cells
Generate electricity from an electrochemical redox in which oxygen (from air) and fuel (hydrogen) combine to form water
Why is platinum electrode used
It is un reactive / inert
Provides a surface where the reaction can take place
What happens in a fuel cell
Electricity is generated when hydrogen and oxygen combine
At anode
2H2 -> 2H+ + 2 e-
At cathode
O2 + 4H+ 4e- -> 2H2O
Overall
2H2 + O2 -> 2H2O
What are advantages and disadvantages of a fuel cell
Advantages
-eliminates pollution by burning fossil fuels
- higher efficiency than diesel
Disadvantages
- expensive to produce
- refuelling takes longer
