3.1.11 3.1.11 Electrode potentials and electrochemical cells (A-level only) Flashcards
What are the components of an electrochemical cell?
[4]:
- 2 half cells joined by a wire
- Voltmeter
- Salt bridge
How does a salt bridge work?
It has mobile ions which can conduct a charge (allows ions to flow through)
What is a salt bridge usually made from?
Piece of filter paper soaked in a salt/potassium nitrate
What is a property that the salt in the salt bridge is required to have?
Must be unreactive with the electrodes and electrode solutions
Why do we not use a wire instead of a salt bridge?
because the metal wire would set up its own electrode system with the solutions
Why is a high resistance voltmeter used?
[2]:
- The voltmeter needs to be of very high resistance to stop the current from flowing in the circuit.
- In this state, it is possible to measure the maximum possible potential difference (E)
How is potential difference measured?
With a high resistance voltmeter
E=
Potential difference
The Oxidation reaction is the
most negative one
OILRIG
Oxidation is loss of electrons and hydrogen, reduction is gain
Why does voltage form in a cell? [3]:
- The half cell that is oxidised releases electrons
- More electrons will build up on the oxidised electrode than the reduced electrode
- A potential difference is created between the two electrodes
What will the most positive electrode always undergo?
Reduction
What will the most negative electrode always undergo??
Oxidation
What is used when a system does not include a metal that can act as an electrode?
A Platinum electrode
Why is a platinum electrode used? [3]:
- Provides a conducting surface for electrons to transfer
- Unreactive
- conducts electricity
What is an electrochemical cell =
2 half cells joined together
Why do we flip the oxidation reaction?
Oxidation is loss so we flip to show the electrons being lost
What is electrode potential measured in?
Volts
What do you do to get the overall cell equation?
Reduction equation + FLIPPED oxidation equation
What are the standard conditions for standard hydrogen electrode potential E° [3]:
- 298K
- 100kPa
- 1.00 mol dm⁻³
What is the standard hydrogen electrode (SHE) used for?
used as a reference to measure standard electrode potentials
What is the E° of the SHE
0.00V
What is E° measured in?
Volts
standard electrode potential equation:
E° = E° reduced - E° oxidised
Cell notations [3]:
- double line (||) represents the salt bridge
- single lines (|) show a physical state change
- Most NEGATIVE electrode on the LEFT +ive on RIGHT
Cell notation structure:
Reduced A | Oxidised A || Oxidised B | Reduced B
How to predict feasibility using E° [4]:
- Find which one is oxidised and flip equation
- Combine oxidised and reduced equation to get the feasible reaction
- Calculate E° of the cell
- E° HAS TO BE +IVE TO BE FEASIBLE
The E° of a cell has to be …. to be feasible
Positive
Non- rechargeable batteries vs rechargeable batteries [2]:
- Non- rechargeable batteries are cheaper
- Rechargeable batteries are reversible so they last longer
What is an example of a rechargeable battery?
A lithium ion battery
How do rechargeable batteries work? [3]:
- Plugging them in supplies a current
- Current forces electrons to flow in opposite direction
- All we do is reverse the overall decharge equation to show that it is charging
What is an example of a fuel cell?
An alkaline hydrogen-oxygen fuel cell
How do hydrogen fuel cells work when hydrogen is added? [3]:
[step 1]
- Hydrogen enters the fuel cell and reacts with OH⁻ ions in solution
- Electrons produced in reaction 1 travel through a platinum electrode
- Flow of electrons is used to power sumn e.g. a car
What is the reaction when hydrogen enters the fuel cell?
2H₂₍𝓰₎ + 4OH⁻ → 4H₂O₍ₗ₎ + 4e⁻
How do hydrogen fuel cells work when Oxygen is added? [3]:
[Step 2]
- Oxygen reenters the fuel cell where it reacts with water and the 4e⁻ produced in step 1
- Electrons flow to negative electrode/ cathode which is made from platinum
- Electrolyte is made from KOH solution which carries OH⁻ from cathode to anode
What is the reaction when oxygen reenters the fuel cell?
O₂₍𝓰₎ + 4H₂O₍ₗ₎ + 4e⁻ → 4OH⁻
Hydrogen oxygen fuel cells (step 3) [3]:
- Electrons flow from anode which is made of platinum
- Product from step 1 is released into surroundings
- OH⁻ produced in step 2 are carried back to anode via electrolyte KOH
Overall equation for hydrogen-oxygen fuel cells:
2H₂₍𝓰₎ + O₂₍𝓰₎ → 2H₂O₍𝓰₎
Advantages of fuel cells [3]:
+ More efficient than an internal combustion engine cus more energy converted into kinetic
+ Water is a less harmful product than carbon dioxide
+ Does not need to be recharged just need hydrogen and oxygen
Disadvantages of fuel cells [3]:
- Hydrogen is highly flammable so must be stored and transported correctly
- Expensive to store hydrogen correctly
- energy Is required to make the oxygen and hydrogen
What electrode does oxidation occur at?
The anode
What electrode does reduction occur at?
The cathode
