Electrochemical Cells

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

What is the simplest salt bridge made of

Answer 3

Filter paper soaked in saturated solution of KNO3

Question 4

Why are salt bridges necessary

Answer 4

They 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 5

What symbol is used to represent a salt bridge In standard notation

Answer 5

II

Question 6

What type of species goes on the outside in standard cell notation

Answer 6

The most reduced species

Question 7

What does I indicate

Answer 7

Phase boundary

Question 8

What happens at the left side electrode

Answer 8

Is where oxidation occurs

Half cell with mist negative E nought value

Question 9

What happens at the right hand electrode

Answer 9

Reduction occurs

Half cell with most positive E nought value

Question 10

What conditions is the SHE used in

Answer 10

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

Question 11

What is SHE used for

Answer 11

Comparing other cells against

E nought value is 0

Question 12

Why might you use other standard electrodes occasionally

Answer 12

Cheaper
Easier
Quicker
Platinum is expensive

Question 13

What factors will change E nought values

Answer 13

Concentration of ions

Temperature

Question 14

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

Answer 14

Equilibrium moves to the left to oppose the change
This release more electrons
So E nought value becomes more negative
So EMF increases

Question 15

How do you calculate the EMF of a cell from E nought values

Answer 15

E nought of the right - E nought of the left

Question 16

When would you use a platinum electrode

Answer 16

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

Question 17

Why is platinum chosen

Answer 17

Inert is does not take part in the electrochemistry

Good conductor to complete the circuit

Question 18

How would you predict if a reaction would occur

Answer 18

Find the species being reduced, calculate its E nought value
Calculate the E nought value of the species being oxidised
Reduction - oxidation
If E nought value > 0 then the reaction will occur

Question 19

What are zinc/carbon cells more commonly known as

Answer 19

Disposable batteries

Question 20

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

Answer 20

Zn oxidised to Zn2+

NH4+ reduces to NH3 at carbon electrode

Question 21

What are the reactions that occur in a lead/acid battery

Answer 21

Pb + SO42- -> PbSO4 + 2e-

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

Question 22

How are cells recharged

Answer 22

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

Question 23

Where are lithium ion cells used

Answer 23

Mobile phones

Laptops

Question 24

What is a fuel cell

Answer 24

A cell that is used to generate electric current

Does not require electrical recharging

Question 25

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

Answer 25

In combustion sulfur contains compounds and nitrogen containing compounds are produced due to the high temperatures
These are bad for the environment
In a fuel cell the only product is water

Question 26

Disadvantages of fuel cells

Answer 26

Hydrogen is a flammable gas with a low bp
Hard and dangerous to transport expensive to buy
Fuel cells have a limited life time
Use toxic chemicals in their manufacture

Question 27

Why might the EMF of a cell change after a period of time

Answer 27

Concentration of ions change

Question 28

How can the EMF of a cell be kept constant

Answer 28

Reagents are supplied constantly so the concentrations of the ions are constant