Electrode potentials

Question 1

What is the golden rule?

Answer 1

The more positive the electrode potential value, the easier it is for the species to gain electrons (to be reduced).

Question 2

Definition of standard electrode potential

Answer 2

The voltage measured under standard conditions (298K, 101kPa, 1moldm^-3) when the half-cell is connected to a standard hydrogen half-cell/electrode.

Question 3

Why is platinum used as the electrode for the standard hydrogen electrode?

Answer 3

It is inert (unreactive), so is not involved in the reaction.

Question 4

Write the equilibrium half reaction equation for a hydrogen half-cell.

Answer 4

2H+ + 2e- …../….. H2

Question 5

Draw the diagram representing the standard hydrogen half-cell.

Answer 5

*Refer to electrode potentials notes.

Question 6

What are the two possible reactions that may occur in the standard hydrogen half-cell?

Answer 6

1. H+(aq) + e- ….. 1/2H2(g)
2. 1/2H2(g) ….. H+(aq) + e-
   The one that occurs depends on the species in the other half-cell.

Question 7

Describe how to measure a standard electrode potential

Answer 7

*The half-cell to be measured is connected to a standard hydrogen electrode.
1. The electrodes are connected by a wire (via a high resistance voltmeter) to allow the electrons to flow (and for the voltage to be measured).
2. The two solutions are connected with a salt bridge to complete the circuit and to allow the ions to flow into the half-cells to maintain the neutrality of the half-cells.

Question 8

Draw a diagram to represent the apparatus used for measuring the standard electrode potential for a Cu^2+(aq)/Cu(s) half-cell.

Answer 8

*Refer to electrode potentials notes.

Question 9

What chemical is typically used for a salt bridge?

Answer 9

Made of filter paper soaked in KNO3(aq).

Question 10

Draw a diagram to represent the apparatus for measuring the standard electrode potential value for a Cr^2+(aq)/Cr(s) half-cell.

Answer 10

*Refer to electrode potantials notes.

Question 11

What does the examiner look for when marking half-cell diagrams?

Answer 11

1. Half-cell on the left is fully labelled.
2. Half-cell on the right is fully labelled.
3. Complete circuit- no gaps!
4. Standard conditions to be stated.

Question 12

Draw a diagram to represent the apparatus for measuring the electrode potential for a half cell with a Zn^2+(aq)/Zn electrode system and a Cu^2+(aq)/Cu(s) electrode system. Write the half equs for these electrodes.

Answer 12

*Refer to electrode potentials notes.

Question 13

Draw a diagram to represent the apparatus for measuring the electrode potential for a half-cell with a Zn^2+(aq)/Zn(s) electrode system and a Cl2(g)/Cl-(aq) electrode system.

Answer 13

*Refer to electrode potentials notes.

Question 14

What chemical is the electrode if the non-metal is a gas?

Answer 14

Platinum.

Question 15

Draw a diagram to represent the apparatus for measuring the electrode potential for a half-cell with a Cu^2+(aq)/Cu(s) electrode system and a S(s)/S^2_(aq) electrode system.

Answer 15

*Refer to electrode potentials notes.

Question 16

What chemical is the electrode if the non-metal is solid?

Answer 16

Platinum as the non-metal will not conduct, so the electrode is in contact with the solid non-metal and the aqueous layer.

Question 17

Draw a diagram to represent the apparatus for measuring the electrode potential for a half-cell with a Fe^2+(aq)/Fe(s) electrode system and a Fe^3+(aq)/Fe^2+(aq) electrode system.

Answer 17

*Refer to electrode potentials notes.

Question 18

Describe how to measure the standard cell potentials.

Answer 18

1. Prepare two standard half-cells.
2. Connect the two electrodes of the half-cells to a voltmeter using wires.
3. Prepare a salt bridge by soaking a strip of filter paper in a saturated solution of KNO3(aq).
4. Connect the two solutions of the half-cells with a salt bridge.
5. Immediately record the standard cell potential from the voltmeter.

Question 19

What is the equ to calculate a standard cell potential?

Answer 19

Standard cell potential = (standard electrode potential of reduction) / (standard electrode potential of oxidation).

Question 20

What 3 things can a standard electrode potential value be used for?

Answer 20

1. Predicting the feasibility pf a redox reaction.
2. Identifying the strongest oxidising agent.
3. Identifying the strongest reducing agent.

Question 21

Define Le Chatelier’s principle.

Answer 21

When any of the conditions affecting the position of the dynamic equilibrium are changed, then the position of the equilibrium will shift to minimise that change.

Question 22

What are the 3 limitations to the predictions used in calculating standard electrode potentials for half-cells?

Answer 22

1. Non-standard conditions- if any of the conditions are altered, the lectrode potential values are also altered, so will affect the reaction.
2. Reaction rate- It may be that the reaction does not happen as the activation energy is too high causing the rate of reaction to be slow.
3. Concentration.

Question 23

Describe primary/non-rechargeable cells.

Answer 23

They provide electrical energy until the chemicals have reacted and the voltage falls, so is discarded.

Question 24

Describe secondary/rechargeable cells.

Answer 24

Chemicals in the cell react to provide electrical energy, the cell reaction can then be revered during recharging (the chemicals are regenerated).

Question 25

Describe fuel cells.

Answer 25

Uses the energy from the reaction of fuel with oxygen to create a voltage. Both fuel and oxidant are used up during the reaction and needs to be continuously provided if the cell is to continue to provide energy.