Electrochemistry Flashcards
What is an electrode and how does it work?
4
- Metal dipped in solution of its metal ions
- equilibrium is set up
- metal has a tendency to form positive ions and go into solution
- metal ions have tendency to gain electrons and form metal
What does it mean if the electrode potential equilibrium is left?
Is the electrode potential positive or negative?
Redox?
4
- Metal has a negative charge
- build up of electrons on metal
- so negative electrode potential
- anode is oxidised
What does it mean if the electrode potential equilibrium is right?
Is the electrode potential positive or negative?
Redox?
4
- metal has a positive charge
- electrons used up to form metal
- so has positive electrode potential
- cathode is reduced
What are the standard conditions for half cells?
Conc
Temp
Pressure
1.0 moldm-3 of ions involved in the half equation
298K
100kPa (only affects half-cells with gases
What is an electode?
two words
Half cell
What is a metal electrode?
1
-Metal surrounded by a solution of its ions
What is a gas electrode?
2
- gas with a solution of its ions
- inert metal (which is the actual electrode) for flow of electrons
What is a redox electrode?
2
- two different ions of the same element
- two types present with inert metal electrode
What is the purpose of the salt bridge?
2
- allow ions in ionic solution to flow through
- maintains charge balance/completes circuit
How does salt bridge maintain equilibrium of positive ions?
1
there is a build up of positive charge at the anode due to the loss of electrons so negative salt bridge ions are attracted to it which maintains flow of ions
How does salt bridge maintain equilibrium of negative ions?
1
ions are being removed at the cathode as they are attracted to electrons so form solid metal, positive salt bridge ions are attracted so maintains flow of ions
Which electrode is traditionally more positive?
Left or right?
Right, cathode
How do you measure the electrode potential of a half cell?
3
- Connect with a half-cell of known potential
- measure potential difference using a high resistance voltmeter
- this is so potential can be measured under zero-current conditions as no current can be drawn from the cell
What is the SHE?
use?
What is looks like?
3
Standard hydrogen electrode
used as primary standard so all potentials are compared to it so it has 0 volts
Pt(s) | H2(g) | H+(aq)
Why are standard conditions needed?
1
Position of the redox equilibrium will change with conditions
(remember equilibria)
What is the set up of an electrochemical cell?
3/4
- Two half cells
- the two metals joined with a wire (e- flow)
- two solutions joined with salt bridge (flow of ions)
- voltmeter to measure potential difference (emf)
Which electrode is oxidised and which is reduced?
anode vs cathode
Anode = oxidised Cathode = reduced
What is emf?
2
including equation
Electromotive force
E* Cell = E* right - E* left
(E* = electrode potential under standard conditions)
What is the conventional representation of cells?
1
R | O || O | R
What are the | lines and why are they important?
3
- the are phase boundaries
- use them whenever there is a change in state
- if none then use comma
What does || represent?
Salt bridge
What does ROOR stand for?
2
Reduced, oxidised, oxidised, reduced
-species with highest oxidation state near salt bridge
Which electrode is the standard electrode?
right or left?
Left
Why are standards other than the SHE used sometimes?
What is a secondary standard?
3
- SHE is difficult to use as it involves and gas and is flammable so other standards are used
- they are calibrated against the SHE and are known as
- secondary standards
What happens at the anode?
Redox
3
- The metal loses electrons so is oxidised to its metal ions
- these electrons make the electrode negative
- these electrons travel through the wire to the other electrode, adding to ions to produce metal ions
What happens at the cathode?
redox
- The metal ions gain electrons and so are reduced to metal atoms
- as electrons are used up this makes the electrode positive
What is the electrochemical series?
2
List of half cell potentials in order of decreasing/increasing potential
compared against SHE
What does it mean if an electrode is negative in the electrochemical series?
It is worse than Hydrogen at gaining electrons / less reductive / more oxidative
What does it mean if an electrode is positive in the electrochemical series?
It is better than Hydrogen at gaining electrons / more reductive / less oxidative
What are the half equations in the electrochemical series written as? Why
2
Reduction half equations
measure of how easily it reduces
What are three types of commercial cells?
- Non-rechargeable
- rechargeable
- fuel cells
What happens in non-rechargeable cells?
2
- The chemicals are used up over time and the emf drops
- once one or more of the chemicals have been used up the cell is flat and emf is 0 volts
What happens in rechargeable cells?
2
- the reactions are reversible
- they are reversed by applying an external current and regenerating the chemicals
What happens in fuel cells?
2
- have a continuous supply of the chemicals so don’t run out of chemicals and don’t need recharging
- do need constant supply of chemicals
What is an example of a rechargeable cell? what is it used for?
Lithium ion
phones, tablets, cameras, laptops etc
What is the equation at the positive electrode of a lithium ion cell?
This is also what happens during discharge
Li+ + CoO2 + e– → Li+[CoO2]–
What is the equation at the negative electrode of a lithium ion cell?
This is also what happens during re-charge
Li → Li+ + e–
What is an example of a fuel cell?
Alkaline Hydrogen Fuel Cell
What happens at the anode and cathode of an alkaline hydrogen fuel cell?
Anode = H2 in cathode = O2 in and H2O out
How/where do the ions travel in an alkaline hydrogen fuel cell?
2
H+ ions move between electrodes through the electrolyte
electrons move between electrodes through a wire
What reaction happens at the anode of an alkaline hydrogen fuel cell? equation
H2 + 2OH- –> @H2O + 2e-
What reaction happens at the cathode of an alkaline hydrogen fuel cell? equation
O2 + 2H2O + 4e- —> 4OH-
What is the overall equation for an alkaline hydrogen fuel cell?
2H2 + O2 –> 2H2O
What the benefits of using non-rechargeable cells?
1
Cheap
What the risks of using non-rechargeable cells?
1
plus explanation
Waste issues
-they leak and corrode and chemicals sink into the soil contaminating ground water
What the benefits of using rechargeable cells?
3
less waste
cheaper in the long run
lower environmental impact
What the risks of using rechargeable cells?
1
plus explanation
some waste issues at end of useful life
-they leak and corrode and chemicals sink into the soil contaminating ground water
What the benefits of using hydrogen fuel cells?
3
only waste product is water
do not need recharging
very efficient- less energy wasted and it converts more of the available energy from hydrogen into kinetic energy than other cells
What the risks of using hydrogen fuel cells?
4
need constant supply of fuels
hydrogen is flammable and explosive
hydrogen usually made using fossil fuels
high cost of fuel cells
