electrochemical cells pmt

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 (II).

Answer 2

Zn (s) ⇌ Zn2+(aq) + 2e-

Question 3

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

Answer 3

Cu2+(aq) ⇌ Cu3+(aq) + e-

Question 4

What is the simplest salt bridge made of?

Answer 4

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

Question 5

Why are salt bridges necessary?

Answer 5

Complete the circuit, but avoid further metal/ion potentials as does not perform electrochemistry. Allows ion movement to balance the charge. Do not react with electrodes

Question 6

What symbol is used to represent a salt bridge in standard notation?

Answer 6

||

Question 7

What type of species goes on the outside (furthest from the salt bridge) in standard cell notation?

Answer 7

The most reduced species

Question 8

What does | indicate?

Answer 8

Phase boundary (solid/liquid/gas)

Question 9

How would an Aluminium/Copper cell be represented?

Answer 9

Al(s) | Al3+(aq) || Cu2+(aq) | Cu(s)

Question 10

What happens at the left-hand electrode?

Answer 10

Left hand electrode is where oxidation occurs. Left hand electrode is the half cell with the most negative Eo value

Question 11

What happens at the right-hand electrode?

Answer 11

right hand electrode is where reduction occurs. Right hand electrode is the half cell with the most positive Eo value

Question 12

Which side of the cell has the most negative E° value? what happens to the metal with the most negative Eo
value?

Answer 12

Oxidation - left hand electrode

Question 13

What conditions is the standard hydrogen electrode used in?

Answer 13

Temperature = 298 K Pressure = 100 kPa [H+] = 1.00 mol dm-3

Question 14

What is the standard hydrogen electrode used for?

Answer 14

Comparing other cells against. EO of SHE is defined as 0, so all other Eo values are compared against it.

Question 15

Why might you use other standard electrodes occasionally?

Answer 15

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

Question 16

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

Answer 16

Better reducing agent (easier to oxidise)

Question 17

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

Answer 17

Better oxidising agent (easier to reduce)

Question 18

What factors will change Eo values?

Answer 18

Concentration of ions
Temperature

Question 19

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

Answer 19

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

Question 20

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

Answer 20

Eo cell = Eo right - Eo left

Question 21

When would you use a Platinum electrode?

Answer 21

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

Question 22

Why is Platinum chosen?

Answer 22

Inert so does not take part in the electrochemistry

Good conductor to complete circuit

Question 23

How would you predict if a reaction would occur?

Answer 23

Take the 2 half equations.

Find the species that is being reduced (this is effectively the right hand electrode)

Calculate its Eo value minus the Eo value of the species that is being oxidised (effectively the left hand cell).

If Eo overall > 0, reaction will occur.

Question 24

What was the first commercial cell made from

Answer 24

Zinc/copper (II)

Question 25

What are zinc/carbon cells more commonly known as?

Answer 25

Disposable batteries

Question 26

What are the two reactions that take place in zinc/carbon cells?

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 + SO4 2- → PbSO4 (s) + 2e- PbO2 + 4H+ + SO4 2- + 2e- → PbSO4+ 2H2O

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 Eo

Question 29

Where are lithium-ion cells used?

Answer 29

Mobile phones Laptops

Question 30

What reactions occur on discharge in lithium-ion cells?

Answer 30

Li+ + CoO2 + e- → Li+[CoO2]- Li → Li+ + e-

Question 31

What is a fuel cell?

Answer 31

A cell that is used to generate electric current; does not require electrical recharging

Question 32

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

Answer 32

2H2 + 4OH- → 4H2O + 4e- O2 + 2H2O + 4e- → 4OH-

Question 33

Why is it better to use a fuel cell than to burn H2 in air, even though the same overall reaction occurs?

Answer 33

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 34

Disadvantages of fuel cells?

Answer 34

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 35

How do you find the weakest reducing agent from a table of electrode potential data?

Answer 35

Most positive Eθ value. Then it is the PRODUCT of the reduction equation i.e. imagine equation going from right to left

Question 36

What is the reason that some cells cannot be recharged?

Answer 36

Reaction of the cell is not reversible - a product is produced that either dissipates or cannot be converted back into the reactants

Question 37

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

Answer 37

Concentrations of the ions change - the reagents are used up

Question 38

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

Answer 38

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