Electrode Potential

Question 1

What happens when a rod of a metal is dipped into a solution of its own ions ?

Answer 1

An equilibrium is set up between the solid metal and the aqueous metal ions

Question 2

Write a half-equation for zinc (s) to zinc (2+)

Answer 2

Zn (s) = Zn2+(aq) + 2e-

Question 3

Write a half-equation for copper (II) to copper (lI).

Answer 3

Cu2+(aq) = Cu3+(aq) + e-

Question 4

What is the simplest salt bridge made out of ?

Answer 4

Filter paper soaked in a saturated solution of KNO3 (potassium nitrate)

Question 5

What is the purpose of a salt bridge ?

Answer 5

• complete the circuit
• allows the ions to move to balance the charge
• do not react with electrodes

Question 6

What is electrode potential?

Answer 6

Measure of how easily a metal is oxidised

Question 7

What electrode potential does a metal that is easily oxidised have ?

Answer 7

Negative

Question 8

What electrode potential does a metal that is hard to oxidise have ?

Answer 8

Positive

Question 9

Why is there an equilibrium within a cell ?

Answer 9

There is a tendency for metal ions to form + ions and go into solution, but there is also the tendency for for metal ions to gain electrons in solution .

Question 10

What happens if the equilibrium shifts to the left ?

Answer 10

The metals acquires a negative charge due to the build up of electrons on the metal

Question 11

What happens when the equilibrium shifts to the right ?

Answer 11

Positive charge will build up on the metal as electrons have been used up to form metal from the metal ions

Question 12

What happens at the left hand electrode ?

Answer 12

• oxidation
• is the half cell with the most negative Eº value

Question 13

What happens at the right hand electrode ?

Answer 13

• reduction
• electrode with the most positive Eº value

Question 14

What conditions is the standard hydrogen electrode used in ?

Answer 14

• 298K
• 100kPa
• [H+] 1.00 mol dm-3

Question 15

What is the standard hydrogen electrode used for ?

Answer 15

• Comparing other cells against it because the Eº of SHE is 0

Question 16

Why might you use other standard electrodes ?

Answer 16

They are cheaper/easier/quicker to use and can provide just as good a reference.
Platinum is expensive

Question 17

If an E° value is more negative, what does it mean in terms of oxidising/reducing power?

Answer 17

Better reducing agent (easier to oxidise)

Question 18

If an E° value is more positive, what does it mean in terms of oxidising/reducing power?

Answer 18

Better oxidising agent (easier to reduce)

Question 19

What factors will change E° values?

Answer 19

Concentration of ions
Temperature

Question 20

What happens if you reduce the concentration of the ions in the left hand half cell?

Answer 20

Equilibrium moves to the left to oppose the change of removing ions; this releases more electrons, the E° of the left hand cell becomes more negative, so the e.m.f. Of the cell increases.

Question 21

How do you calculate the emf of a cell from E° values?

Answer 21

E°cell = E° right - Eº left

Question 22

When would you use a Platinum electrode?

Answer 22

When both the oxidised and reduced forms of the metal are in aqueous solution

Question 23

Why is Platinum chosen?

Answer 23

Inert so does not take part in the electrochemistry
Good conductor to complete circuit

Question 24

How would you predict if a reaction would occur?

Answer 24

• Take the 2 half equations.
• Find the species that is being reduced (this is effectively the right hand electrode)
• Calculate its E° value minus the E° value of the species that is being oxidised (effectively the left hand cell).
• If E° overall > 0, reaction will occur.

Question 25

How does Concentration and Pressure Effect the EMF?

Answer 25

• Increasing the concentration of the solutions used in the electrochemical cell makes the cell
  EMF more positive as fewer electrons are produced in the reaction.
• Increasing the pressure of the cell will make the cell EMF more negative as more electrons
  are produced.

Question 26

What are the two reactions that take place in zinc/carbon cells (disposable batteries)?

Answer 26

Zn oxidised to Zn2+
NH4+ reduced to NH3 at carbon electrode

Question 27

What are the reactions that occur in a lead/acid
battery (car batteries)?

Answer 27

Pb + SO42- → PbSO4 (S) + 2e-
PbO2 + 4H+ + SO42- + 2e- → PbSO4 + 2H20

Question 28

How are cells recharged (if they are rechargeable)?

Answer 28

Reactions are reversible and are reversed by running a higher voltage through the cell than the cell’s E°

Question 29

What is a fuel cell ?

Answer 29

A cell that is used to generate an electrical current and does not require charging as it has a constant supply of the chemicals it needs

Question 30

What are the reactions that take place at the two electrons in an alkaline hydrogen fuel cell?

Answer 30

2H2 + 40H-→ 4H20 + 4e-
O2 + 2H20 + 4e- → 40H-

Question 31

Why is it better to use a fuel cell than to burn H, in air, even though the same overall reaction occurs?

Answer 31

In combustion, sulfur containing compounds (SO2, SO3) and nitrogen containing compounds (NO2, NOx) are produced due to the high temperatures and the S and N in air. These are bad for the environment.
This does not occur in a fuel cell; the only product is water.
More efficient

Question 32

Disadvantages of fuel cells?

Answer 32

Hydrogen is a flammable gas with a low b.p. → hard and dangerous to store and transport → expensive to buy
Fuel cells have a limited lifetime and use toxic chemicals in their manufacture

Question 33

Why might the e.m.f. Of a cell change after a period of time?

Answer 33

Concentrations of the ions change - the reagents
are used up

Question 34

How can the e.m.f. Of a cell be kept constant?

Answer 34

Reagents are supplied constantly, so the concentrations of the ions are constant; E° remains constant