3.1.11 3.1.11 Electrode potentials and electrochemical cells (A-level only)

Question 1

What are the components of an electrochemical cell?

[4]:

Answer 1

• 2 half cells joined by a wire
• Voltmeter
• Salt bridge

Question 2

How does a salt bridge work?

Answer 2

It has mobile ions which can conduct a charge (allows ions to flow through)

Question 3

What is a salt bridge usually made from?

Answer 3

Piece of filter paper soaked in a salt/potassium nitrate

Question 4

What is a property that the salt in the salt bridge is required to have?

Answer 4

Must be unreactive with the electrodes and electrode solutions

Question 5

Why do we not use a wire instead of a salt bridge?

Answer 5

because the metal wire would set up its own electrode system with the solutions

Question 6

Why is a high resistance voltmeter used?

[2]:

Answer 6

• 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)

Question 7

How is potential difference measured?

Answer 7

With a high resistance voltmeter

Question 8

E=

Answer 8

Potential difference

Question 9

The Oxidation reaction is the

Answer 9

most negative one

Question 10

OILRIG

Answer 10

Oxidation is loss of electrons and hydrogen, reduction is gain

Question 11

Why does voltage form in a cell? [3]:

Answer 11

• 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

Question 12

What will the most positive electrode always undergo?

Answer 12

Reduction

Question 13

What will the most negative electrode always undergo??

Answer 13

Oxidation

Question 14

What is used when a system does not include a metal that can act as an electrode?

Answer 14

A Platinum electrode

Question 15

Why is a platinum electrode used? [3]:

Answer 15

• Provides a conducting surface for electrons to transfer
• Unreactive
• conducts electricity

Question 16

What is an electrochemical cell =

Answer 16

2 half cells joined together

Question 17

Why do we flip the oxidation reaction?

Answer 17

Oxidation is loss so we flip to show the electrons being lost

Question 18

What is electrode potential measured in?

Answer 18

Volts

Question 19

What do you do to get the overall cell equation?

Answer 19

Reduction equation + FLIPPED oxidation equation

Question 20

What are the standard conditions for standard hydrogen electrode potential E° [3]:

Answer 20

• 298K
• 100kPa
• 1.00 mol dm⁻³

Question 21

What is the standard hydrogen electrode (SHE) used for?

Answer 21

used as a reference to measure standard electrode potentials

Question 22

What is the E° of the SHE

Answer 22

0.00V

Question 23

What is E° measured in?

Answer 23

Volts

Question 24

standard electrode potential equation:

Answer 24

E° = E° reduced - E° oxidised

Question 25

Cell notations [3]:

Answer 25

• double line (||) represents the salt bridge
• single lines (|) show a physical state change
• Most NEGATIVE electrode on the LEFT +ive on RIGHT

Question 26

Cell notation structure:

Answer 26

Reduced A | Oxidised A || Oxidised B | Reduced B

Question 27

How to predict feasibility using E° [4]:

Answer 27

• 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

Question 28

The E° of a cell has to be …. to be feasible

Answer 28

Positive

Question 29

Non- rechargeable batteries vs rechargeable batteries [2]:

Answer 29

• Non- rechargeable batteries are cheaper

- Rechargeable batteries are reversible so they last longer

Question 30

What is an example of a rechargeable battery?

Answer 30

A lithium ion battery

Question 31

How do rechargeable batteries work? [3]:

Answer 31

• 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

Question 32

What is an example of a fuel cell?

Answer 32

An alkaline hydrogen-oxygen fuel cell

Question 33

How do hydrogen fuel cells work when hydrogen is added? [3]:

[step 1]

Answer 33

1. Hydrogen enters the fuel cell and reacts with OH⁻ ions in solution
2. Electrons produced in reaction 1 travel through a platinum electrode
3. Flow of electrons is used to power sumn e.g. a car

Question 34

What is the reaction when hydrogen enters the fuel cell?

Answer 34

2H₂₍𝓰₎ + 4OH⁻ → 4H₂O₍ₗ₎ + 4e⁻

Question 35

How do hydrogen fuel cells work when Oxygen is added? [3]:

[Step 2]

Answer 35

1. Oxygen reenters the fuel cell where it reacts with water and the 4e⁻ produced in step 1
2. Electrons flow to negative electrode/ cathode which is made from platinum
3. Electrolyte is made from KOH solution which carries OH⁻ from cathode to anode

Question 36

What is the reaction when oxygen reenters the fuel cell?

Answer 36

O₂₍𝓰₎ + 4H₂O₍ₗ₎ + 4e⁻ → 4OH⁻

Question 37

Hydrogen oxygen fuel cells (step 3) [3]:

Answer 37

• 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

Question 38

Overall equation for hydrogen-oxygen fuel cells:

Answer 38

2H₂₍𝓰₎ + O₂₍𝓰₎ → 2H₂O₍𝓰₎

Question 39

Advantages of fuel cells [3]:

Answer 39

+ 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

Question 40

Disadvantages of fuel cells [3]:

Answer 40

• 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

Question 41

What electrode does oxidation occur at?

Answer 41

The anode

Question 42

What electrode does reduction occur at?

Answer 42

The cathode