Electrochemical cells Flashcards
2 ways of constructing an electrochemical cell
-metal dipped in its ions
-platinum electrode with 2 aqueous ions
why is platinum commonly used?
Why does it need these features
inert
electrically conductive
Electrochemical cell set up
2 half cells joined by wire, high resistance voltmeter and salt bridge
What is EMF
voltage between two cells
purpose of voltmeter?
measure EMF
observation during oxidation
electrode being oxidised gets thinner,as more ions are being dissolved insolution
type of reaction that occurs in electrochemical cells
redox
observation during reduction
electrode being reduced gets thicker,as more ions are receiving electrons
what is used as a salt bridge and why?
KNO3
-the tube has unreactive ions that can move between two half cells to carry the flow of electrons
-maintains charge balance
-doesn’t interfere with the reaction
Define electrochemical series
A list of half cells and their standard electrode potentials in descending numerical order
What are electrode potentials?
measure of how easily the half cell gives up electrons (oxidised)
In the electrochemical series, which form are the equations written in?
reduction (gains electrons)
in equilibrium
Which side of the equation has reducing agents?
Right hand side
They are more easily oxidised
Which side of the equation has oxidising agents?
Left hand side
They are more easily reduced
Which half cell undergoes oxidation?
More negative half cell
Which half cell undergoes reduction?
More positive half cell
What is SHE?
Standard Hydrogen Electrode
Used as a reference to measure standard electrode potentials, because they can’t be measured on their own
Standard conditions for electrochemical cells?
298K
1.00 mol dm-3
100kPa
Which is the strongest reducing agent?
Most negative half cell
Which is the strongest oxidising agent?
Most positive half cell
Formula to calculate standard cell potential
E r - E o
How to draw cell notation?
Reduced | Oxidised || Oxidised | Reduced
Most negative half cell goes on the left
Anticlockwise
Oxidised is on top of reduced
Combine both equations to obtain a feasible reaction
Use the formula
Positive value = feasible
What are the two forms of batteries?
Rechargeable and non-rechargeable
Compare rechargeable and non-rechargeable batteries
Non-rechargeable:
-cheaper
Rechargeable:
-cheaper in the long-term, as they are reversible
-when plugged in a current is supplied
Example of a rechargeable battery
Lithium ion batteries
-phones, electric cars, etc
-one electrode has lithium cobalt oxide (LiCoO2)
-other electrode has graphite (C)
-electrolyte = lithium salt dissolved in organic solvent
(Electrolyte allows ions to flow between electrodes)
Li+ + e- ~ Li -3.04
Li+ + CoO2 + e- ~ Li+[CoO2]- +0.56V
How do fuel cells work?
-electricity is generated by a continuous external supply of chemicals
-e.g. alkaline hydrogen-oxygen fuel cell
-platinum electrode is a good conductor but inert
-hence it is used as a negative electrode here
-OH- ions are carried towards anode
Hydrogen electrode produces electrons
Oxygen electrode accepts electrons
Equations for hydrogen-oxygen fuel cell
2H₂(g) + 4 OH- (aq) —> 4H₂O (l) + 4 e-
O₂ (g) + 2H₂O (l) + 4e- —> 4 OH- (aq)
Combined equation:
2H₂ (g) + O₂ (g) —> 2H₂O (g)
Advantages of fuel cells
-more efficient than an internal combustion engine in machines
-more energy is converted into kinetic energy
-less is wasted as thermal energy
-don’t need to be recharged
-no CO2 is directly emitted
-water is the only byproduct
Disadvantages of fuel cells
-H2 is highly flammable
-expensive to store and transported
-energy is required to make H2 and O2
-fossil fuels are used to pass water through an electrolysis process
-fossil fuels contribute to CO2 emissions
Why are certain substances not used as a salt bridge?
The ions would react with one of the ions in solution
Alters cell e.m.f.
What side is SHE always on?
Left hand side
Suggest why the current in the external circuit of this cell may fall to zero after the cell has operated for some time (1 mark)
Concentrations become equal
Why does a fuel cell not need to be recharged?
Hydrogen is continuously supplied
Explain why rechargeable cells are connected to solar cells
Suggest why waste disposal centres contain a separate section for cells and batteries
Prevents pollution to the environment due to toxic components
