Redox and Standard Electrode Potential

Question 1

What is a salt bridge?

Answer 1

A tube of unreactive ions that can move move between the solutions to carry the flow of charge but not interfere with the reaction.

Question 2

State the rules for when formulating a standard cell
(4 marks)

Answer 2

• The half cell with the move negative potential goes on the left (oxidised)
• The most oxidised species from each half-cell goes next to the salt bridge
• Salt bridge is always a double line
• Always include state symbols

Question 3

What is standard electrode potential and why is it used?

Answer 3

• The difference between the Standard hydrogen electrode and any other half cell under standard conditions of 298K and 1atm and concentrations of 1.00 mol dm⁻³
• Electrode potential cannot be directly measured, so a reference cell (SHE) is used having a potential of 0V

Question 4

Draw a diagram of a standard hydrogen electrode

Answer 4

Should include:
- H₂ (g) at 1 atm
- Pt (s) electrode which is inert
- 1 mol dm⁻³ of HCl (aq) / H⁺ (aq)

Question 5

Where must the standard hydrogen electrode always be placed when determining the standard electrode potential of any cell?

Answer 5

On the left

Question 6

How is cell EMF calculated?

Answer 6

E right - E left
or the most positive potential (reduced species) - the most negative potential (oxidised species)

Question 7

Show the overall cell reaction in a hydrogen fuel cell (include state symbols)

Answer 7

2H₂ (g) + O₂ (g) <-> 2H₂O (l)
or 1/2O₂ + 2H⁺ + 2e⁻ <-> H₂O (l)

Question 8

State the components of the hydrogen fuel cell
- What is fed into which side at which cathode
- Metal catalyst used

Answer 8

• Hydrogen is fed into the anode
• Oxygen is fed into the cathode
• Platinum catalyst

Question 9

What are the advantages and disadvantages of the hydrogen fuel cell
(3 ad, 3 dis)

Answer 9

AD
- Clean technology as water is the only product
- High efficiency as chemical energy is converted directly into electrical energy
- Simple construction
DIS
- Storage of hydrogen problematic as its very flammable and explosive
- Expensive to store/transport
- Energy is required to produce a supply of hydrogen and oxygen (electrolysis)

Question 10

What is the significance of using a hydrogen fuel cell

Answer 10

Generate an electrical current without needing to be recharged/reaction occurs spontaneously. Continuous current

Question 11

Show the ion-electron half equation for
acidified manganate (VII) ions (show state symbols) and state the colour change

Answer 11

MnO₄⁻ (aq) + 8H⁺ (aq) + 5e⁻ <-> Mn²⁺ (aq) + 4H₂O (l)
Purple to colourless

Question 12

Show the ion-electron half equation for
acidified dichromate ions (show state symbols) and state the colour change

Answer 12

Cr₂O₇²⁻ + 14H⁺ (aq) + 6e⁻ <-> 2Cr³⁺ (aq) + 7H₂O (l)
Orange to green

Question 13

Show the ion-electron half equation for
iodine to iodide ions (show state symbols) and state the colour change

Answer 13

I₂ (aq) + 2e⁻ <-> 2I⁻ (aq)
Brown to colourless

Question 14

Show the ion-electron half equation for
thiosulfate to tetrathionate (show state symbols)

Answer 14

2S₂O₃²⁻ (aq) <-> S₄O₆²⁻ (aq) + 2e⁻

Question 15

Describe and explain the reaction (titration experiment) of aqueous thiosulfate ions and aqueous iodine.
- Explain and describe (2 marks)
- State observations (2 marks)
- Give indicator used and why (1 mark)
- Give any relevant equations (1 mark)

Answer 15

• S₂O₃²⁻ (aq) is oxidised by I₂ (aq).
• S₂O₃²⁻ in placed in burette and ran into a flask containing iodine until the colour turns pale-yellow/straw due to iodine being reduced to iodide ions (colourless)
• At this point starch is added as an indicator turning the mixture dark blue
• The end point is when the blue colour decolourised after all the iodine ions have been used up
• I₂ (aq) + 2S₂O₃²⁻ (aq) -> S₄O₆²⁻ (aq) + 2I⁻ (aq)