3.1.11 Electrode potentials and electrochemical cells Flashcards
What happens what a rod of a metal is dipped into a solution of its own ions?
and what is the equation
An equilibirum is set up between the solid aqueous metal ions
euation of equilibrium set up: M(s) ⇌ Mz+(aq) + ze-
z is charge so if 2+ obvs 2e-
Where are all the components of M(s) ⇌ Mz+(aq) + ze- found?
The +ve metal ions are in solution, the solid metal is a rod and the electrons are delocalised on the metal
M(s) ⇌ Mz+(aq) + ze- found?
what happens when equilbirum lies to the left
negatively charged due to electrons building up on it as metal cations are released into the solution (the solution becoming positively charged because of the
excess of cations. )
M(s) ⇌ Mz+(aq) + ze- found?
What happens when equilibrium lies to the right?
metal ions in the solution could take up delocalized electrons in the strip of metal and be discharged as metal atoms (increasing the mass of the foil). In this case, the metal strip would become positively charged.
What is a metal dipped into a solution of its ions called?
a half cell or an electrode
What are the three types of electrode
1) Metal electrode (metal surrounded by solution of its ions)
2) Gas electrodes ( gas and solution of its ions, usually platinium- inert- is the actual electrode to allow for flow of e-)
3) Redox electrodes (this is for two different ions of the same element eg Fe2+ and Fe3+, so both ions are present in solution and an inert electrode is used usually Pt to allow for flow of e-)
Can the actual potential of a half cell be measured directly
the potential indicates how it will react
no
How is the actual potential of a half cell measured?
it has to be connceted to another half cell of known potential and the potential difference between the two half cells is measured
what is an electrochemical cell
two combined half cells
what is the SHE
standard hydrogen electrode
Before the potential of any half-cells could be measured, a potential had to be assigned to one particular half-cell (then the potential of all the other electrodes could be measured against it).
* The electrode chosen was the standard hydrogen electrode (SHE) and this electrode is assigned the potential of 0 volts.
* The SHE is known as the primary standard as it is the potential to which all others are compared.
setting up an electrochemical cell
How are 2 half cells joined together to give a complete circuit
- the two metals are joined with a wire (electrons flow through the wire)
- the two solutions are joined with a salt bridge (ions flow through the salt bridge)
- a voltmeter is often included in the circuit to allow the potential difference (emf) to be measured
What can a salt bridge be made of?
- a piece of filter paper soaked with a solution of unreactive ions or
- a tube containing unreactive ions in an agar gel
Compounds such as KNO3 are often used in salt bridge as K+ and NO3
ions are quite unreactive.
what are the standard conditions?
cell concentration 1.0 mol dm-3 of the ions involved in the half-equation
cell temperature 298 K
cell pressure 100 kPa (only affects half-cells with gases)
The potential should also be measured under zero-current conditions [to measure the full potential difference (emf), no
current must be drawn from the cell - this is achieved by using a high resistance voltmeter.
Why are standard conditions required?
Standard conditions are required because the position of the redox equilibrium will change with conditions. For example,
in the equilibrium:
Mn+(aq) + n e– ⇌ M(s)
a decrease in the concentration of Mn+ would move the equilibrium to the left, so making the potential more negative as more electrons are released.
describe how shorthand for electrochemical cells works.
R O || O R
left hand half cell right hand half cell
Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)
phase boundary phase boundary
salt bridge
on which side of the electochemical cell are elctrons gained and which side looses them
and which is anode and which cathode
left looses = oxidises (anode)
right gains = reduced (cathode)
which side is the more positive half cell on
right
what are the three types of cells that can be used as a commercial source of energy
what is so good about them
- non rechargable
- rechargeable
- fuel cells
protable source of electricity
what is a battery
more than one cell joined together
how do non rechargeable cells work
what makes a cell non rechargeable
chemicals are used up over time and the emf drops. one one or more of the chemicals have been used y
up the cell is flate ad the emf is 0V
if the reaction that occurs is non reversilbe
what are the pros and cons of non rechargeable cells
pros: cheap
cons: waste issues/single use
what are rechargeable cells
the reactions are reversible by applying an external current (regenerates chemicals)
what is the rechargeable cell used in phones cameras laptops
lithium ion
what are the equations for a lithium ion cell
At the positive electrode:
Li+ + CoO2 + e– → Li+[CoO2]–
At the negative electrode:
Li → Li+ + e–
So, the overall reaction is:
Li + CoO2 → LiCoO2
How does the lithium ion cell get recharged
an electric current is passed through it in the opposite direction to that which flows when the cell is generating electricity. The original substances reform. This recharging is an example of electrolysis and is the reverse of discharge.
LiCoO2 → Li + CoO2
pros and cons of rechargeable cells
pros:
- less waste
- cheaper in long run
- less environmental impact
cons:
- some waste issues (at end of useful life)
What is a fuel cell?
Fuel cellshave a continuous supply of the chemicals into the cell and so neitjer run out of the chemicals nor need re charging (but do need constant supply of required chemicals)
what is the most common fuel cell
hydrogen oxygen fuel cell
What are the equations at each electrode, the oevrall equation and cell emf for hydrogen-oxygen fuel cells in alkaline and acidic conditions
pros of using hyrogen fuel cell
- only waste product is water
- do not need re-charging
- very efficient
cons of using hydrogen fuel cells
- need constant supply of fuel
- hydrogen is flammable and explosive
- hydrogen usually made using fossil fuels
- high cost of fuel cells
Explain how the salt bridge provides an electrical connection between the two solutions?
it allows ions to flow (by completing the circuit)
write the onventional representation for the SHE (half cell)
and what side is SHE always on
Pt (s) I H2 (g) I H+ (aq)
left
what is the function of an electrode
allows for e- to flow/be transferred
what effect on emf does increasing surface area of each platinum electrode have?
none - no change
Give one reason, other than cost, why the platinum electrodes are made by coating a
porous ceramic material with platinum rather than by using platinum rods.
Increases the surface area of the electrode
porous = more holes
