20.3 Electrochemical cells

Question 1

Give two examples of
non-rechargeable cells

Answer 1

. Zinc/copper cells
. Zinc/carbon cells

Question 2

What is the zinc/copper cell

Why is it not practical for portable devices

Answer 2

. used in 1800s to write telegraphs and morse code

. It is not practical for portable devices because of the liquids it contains

It works on the general principal of electrons being transferred from the more reactive metal to a less reactive one
Voltage is worked out from the difference between the electrode potentials in the electrochemical series

Question 3

What are zinc carbon cells
What material is its positive electrode made of

What are its uses

Answer 3

The positive electrode is carbon, which acts like an inert platinum electrode in the hydrogen electrode.
Because it doesn’t react.

These cells are ordinary disposable batteries

Question 4

What is an electrolyte

In zinc carbon cells, what consistency is the electrolyte and what is it

Answer 4

An electrolyte is the substance that dissociates in water to produce +ve and -ve ions.

It is a paste
Ammonium chloride (NH4Cl)

Question 5

Describe the structure of the zinc carbon cell

Answer 5

.Central carbon rod
. A zinc canister is filled with NH4Cl paste and water, and this is the electrolyte
. The central carbon rod is surrounded by a mixture of MnO2 and carbon

Question 6

What are the half equations for the zinc carbon electrodes

Why

Answer 6

At the zinc:
Zn(s) —> Zn2+ + 2e-
Because the

At the

Question 7

How do you remember which half cell is being oxidised or reduced from the Eº given

Answer 7

Mnemonic No Problem

So negative oxidised
Positive reduced

So the more negative half cell is the one oxidised

Question 8

How do you calculate the emf between two half cells with their Eº given

Answer 8

Think of Red – ox
So the reduction electrode subtract the oxidation electrode

Question 9

How does increasing concentration of Ag+ ions affect the overall emf value of the reaction

In reaction
Sn2+ + 2e- —> Sn E= -0.14V
Ag+ + 2e- —> 2Ag E= +0.8V

Answer 9

normal emf would be reduction - oxidation, and we can see that the Ag+ is more positive so is reduction.
So normal emf is 0.94V

Increasing conc of Ag+ ions will shift equilibrium to the right, so less electrons will be formed, so E value of Ag+ is more positive.
This means the overall emf of the cell will increase because there is a more positive number subtracting the same less positive number, so emf is bigger

So reaction is more feasible

Question 10

In reaction
Sn2+ + 2e- —> Sn E= -0.14V
Ag+ + 2e- —> 2Ag E= +0.8V

How would the emf value change if we increase concentration of Sn2+ ions

Answer 10

In the Sn2+ equation, if we increase concentration of it, the equilibrium will shift right so less electrons will be made for it, so this makes the E of the half cell more positive.
(when calculating this we don’t need to write out the equation in its oxidation form)
So normal emf of this cell is 0.94V, and now the new emf will be reduction - oxidation so there is a higher value for oxidation one (Sn2+) so 0.8V - 0.1V (example of higher value of E) will give an overall more negative emf

So reaction is less feasible

Question 11

How does emf affect feasibility of a reaction

Answer 11

If emf has a higher value, the reaction is more feasible

If emf is lower, reaction is less feasible

Question 12

What is a rechargable battery

Answer 12

You can reverse the cell reactions by applying an external voltage greater than the voltage of the cell to drive electrons in the opposite direction

Question 13

What are lead acid batteries

What are the cells made of

Answer 13

Re-chargable batteries used to operate starter motor of cars
Consist of six 2v cells in series to give 12v

Each cell consists of two plates dipped in sulfuric acid
The positive plate is lead coated in lead oxide (PbO2), and the negative plate is just made of lead

The battery is driving electrons from the lead plate to the lead oxide coated one

Question 14

Give the equations for the reactions occurring at the lead plate

What about at the lead dioxide plate

So overall reaction

Answer 14

Pb(s) + SO42- —> PbSO4 + 2e-

PbO2 + 4H+ + SO42- + 2e- –> PbSO4 + 2H2O

PbO2 + 4H+ +2SO42- +Pb –> 2PbSO4 + 2H2O

Question 15

What is the alkaline hydrogen oxygen fuel cell

Answer 15

The cell has two electrodes made of porous platinum based material

They are separated by a semi permeable membrane
The electrolyte is sodium hydroxide solution

Question 16

What is the reaction when hydrogen enters the NEGATIVE ELECTRODE

what about where oxygen enters at the POSITIVE ELECTRODE

So what is the overall reaction

Answer 16

2H2 + 4OH- —> 4H2O + 4e-
This releases electrons which flow through the circuit to the other electrode where oxygen enters..

O2 + 2H2O + 4e- –> 4OH-
This accepts electrons from the other electrode and

O2 + 2H2 —> 2H2O

It is the same reaction as burning hydrogen and oxygen, but it takes place at lower temperatures so no nitrogen oxides are produced which would usually form

Question 17

What is fuel from the hydrogen oxygen fuel cell used for

Answer 17

It is used to generate electricity on spacecraft because the only product is pure water which can be drunk
Produces no CO2 also

Question 19

Problems with the hydrogen oxygen fuel cell

Answer 19

Consider the source of the hydrogen!
Eg made from crude oil which is non renewable
Electrolysis of water can also be used to make hydrogen, but the electricity for this would be from burning fossil fuels which emit CO2

Hydrogen is very flammable so an infrastructure will need to be made to store it safely and transport it
Also because it is a gas it takes up lots of space