Electrochemistry

Question 1

What are the two types of half cells?

Answer 1

• Metal/metal ion half cell: metal rod dipped into an aqueous solution of its ions. Metal rod allows for e- mvmt.
• Ion/ion half cell: aqueous ions of the same element in 2 different oxidation states. Inert platinum electrode used.

Question 2

What do electrode/reduction potentials show?

Answer 2

• How readily an element will gain electrons.
• More positive = greater tendancy to gain electrons.

Question 3

What is the definition of a standard electrode potential?

Answer 3

• The EMF of a half-cell measured against the standard hydrogen electrode.

Question 4

What are the standard conditions for standard electrode potentials?

Answer 4

• 298 K
• 100 kPa
• 1.00 mol dm-3

Question 5

How do you measure the standard electrode potential of a half-cell? (metal/metal ion)

Answer 5

• On one side have SHE.
• On other side have zinc rod dipping into a 1.00 mol dm -3 of Zn2+.
• Wire connecting 2 electrodes. Voltmeter in middle.
• Salt bridge: strip of filter paper soaked in potassium nitrate. Allows a flow of ions to balance the charge difference between the two beakers.

Question 6

How do you measure the standard electrode potential of a half-cell? (ion/ion)

Answer 6

• SHE on one side.
• Other side: Solution of Fe2+ and Fe3+ ions with a platinum electrode.

Question 7

How do you measure the standard electrode potential of a half-cell? (gas)

Answer 7

• SHE
• Cl2 gas (100kPa) coming into 1.00 mol dm-3 solution of Cl- ions.
• Pt electrodes

Question 8

What happens when two half-cells are connected?

Answer 8

• The half equation with a more positive electrode potential will work in the forwards direction (reduction).
  e.g: Fe3+ + 3e- <–> Fe +0.77 V
  2e- + Zn2+ <–> Zn -0.76 V

Therefore, Fe and Zn2+ ions are produced. Overall equation:
2 Fe3+ + 3 Zn –> 2Fe + 3Zn2+

• Cell potential = most pos - least pos.

Question 9

How are the cathode and anode decided?

Answer 9

• Most positive half-cell is the positive electrode. Cathode. Reduction takes place.
• Least positive half-cell is the negative electrode. Anode. Oxidation takes place.

Question 10

How can you make predictions?

Answer 10

• Look at which direction the arrows need to go in for the reaction to occur.
• Compare reduction potentials to find actual directions.
• See if they are the same.