Redox And Standard Electrode Potentials

Question 1

Cu2+(aq) | Cu(s)

Answer 1

Blue –> Red-brown

Question 2

Zn2+(aq) | Zn(s)

Answer 2

Colourless –> Silver

Question 3

H+(aq) | H2(g) | Pt(s)

Answer 3

Colourless

Question 4

Fe3+(aq), Fe2+ (aq) | Pt (s)

Answer 4

Yellow –> Pale green

Question 5

MnO4-(aq), Mn2+(aq) | Pt(s)

Answer 5

Purple –> Colourless / Pale pink

Question 6

Cl2 (g) | Cl- (aq) | Pt(s)

Answer 6

Green/ Yellow –> Colourless

Question 7

Br2(aq), Br-(aq) | Pt(s)

Answer 7

Orange/Brown –> Colourless

Question 8

I2(g) | I-(aq) | Pt(s)

Answer 8

Purple –> Colourless

Question 9

Steps for working out half equations

Answer 9

1. Balance formulae of species before and after the change
2. Calculate oxidations states before and after
3. Add electrons to one side to balance oxidation states
4. Add H+ to balance charges
5. add H2O tto balance equation

Question 10

What is the purpose of electrochemical cells?

Answer 10

To produce electricity through chemical reactions

Question 11

In what direction do electrons flow?

Answer 11

Towards the strongest oxidising agent (more positive E°)

Question 12

EMF =

Answer 12

E° for reduction - E° for oxidation

Question 13

What is the purpose of the salt bridge?

Answer 13

This completes the circuit by allowing ions to move, without the two solutions mixing

Question 14

Define standard electrode potential

Answer 14

The potential difference when any half-cell is connected to the standard hydrogen electrode under standard conditions

Question 15

What are the standard conditions?

Answer 15

Concentration ~ 1mol dm-3
Temperature ~ 298K
Pressure ~ 1atn

Question 16

What is the purpose of an inert platinum electrode in an electrochemical cell?

Answer 16

To allow for the transfer of electrons

Question 17

Cr2O7^2-(aq) in acidified solution

colour change, product, redox?

Answer 17

= Cr3+
Orange –> green
Reduction

Question 18

Cr2O7^2- in alkaline solution

colour change, product, redox?

Answer 18

= 2CrO4^2-
Orange –> Yellow
Neither, the oxidation state of chromium does not change

Question 19

Observations for the reaction between copper (II) ions and iodide ions

Answer 19

~ White solid of CuI

~ Brown solution of iodine

Question 20

What indicator is used in the iodine clock reaction? What is the colour change?

Answer 20

Starch, goes blue-black in the presence of iodine, which then goes colourless when all iodine has been reduced to iodide ions

Question 21

Equations at the anode, the cathode, and overall for hydrogen fuel cells

Answer 21

Anode : H2 –> 2H+ + 2e-
Cathode : O2 + 4H+ + 4e- –> 2H2O
Overall : 2H2 + O2 –> 2H2O

Question 22

3 advantages of hydrogen fuel cells

Answer 22

1. Water is the only product, so no CO2
2. Highly efficient, as less energy is wasted as heat
3. Hydrogen gas can be produced using renewable resources by the electrolysis of water
4. Hydrogen can be produced from water, which is a sustainable resource

Question 23

3 disadvantages of hydrogen fuel cells

Answer 23

1. Hydrogen gas is highly flammable
2. Hydrogen gas is difficult to store due to low density and therefore large volume
3. Hydrogen gas is usually produced from fossil fuels; net energy loss