Year 13- Electrode potentials

Question 1

Why is a high resistance voltmeter used?

Answer 1

to stop the current from flowing in the circuit

Question 2

What is a salt bridge usually made from?

Answer 2

a piece of filter paper soaked in potassium nitrate- it unreactive with electrodes and electrode solutions

Question 3

Why can’t potassium chloride be used a s a salt bridge in a copper system?

Answer 3

chloride ions would form complexes with copper ions

Question 4

Why isn’t a metal wire used as a salt bridge?

Answer 4

it would create its own electrode systems with the solutions

Question 5

What type of reaction does the most positive electrode undergo?

Answer 5

reduction

Cu2+ + 2e- –> Cu (+ve as electrons are used up)

Question 6

What type of reaction does the most negative electrode undergo?

Answer 6

oxidation

Zn –> Zn2+ + 2e- (-ve as electrons are given off)

Question 7

In a cell diagram, where would you put the most positive half cell?

Answer 7

on the right if possible

Question 8

In a cell diagram, which form is put next to the double line?

Answer 8

the most oxidised form

Question 9

Why is a platinum electrode used in systems with no metals?

Answer 9

it provides a conducting surface for electron transfer, is unreactive and can conduct electricity

Question 10

How are the electrode potentials of all electrodes measured?

Answer 10

by comparing them to the standard hydrogen electrode which has a potential of 0V

Question 11

Cell diagram of hydrogen electrode

Answer 11

Pt l H2 (g) l H+ (aq)

Question 12

Why is platinum black used for SHE?

Answer 12

it is porous and absorbs hydrogen gas

Question 13

Why are standard conditions important?

Answer 13

the position of the redox equilibrium changes with conditions e.g. an increase in conc. can move it to the right or left

Question 14

Standard conditions

Answer 14

• all ion solutions at 1 mol sm-3

* 298K, 100kPa

Question 15

In half equations on a data book, what is found on the left side of the equation?

Answer 15

the most oxidised form

Question 16

Equation for the EMF of a cell

Answer 16

RHS-LHS

Question 17

When combining 2 half equations, what happens to the most negative half cell?

Answer 17

it will always oxidise (go backwards)

Question 18

When combining 2 half equations, what happens to the most positive half cell?

Answer 18

it will always reduces (go forward)

Question 19

Equation for Ecell of spontaneous changes

Answer 19

Ered - Eox (always will have a positive Ecell)

Question 20

As we go down the Ecell series, it gets more positive. What does this tell us?

Answer 20

it increases the tendency for the species on the left to reduce and act as oxidising agents

Question 21

As we go up the Ecell series, it gets more negative. What does this tell us?

Answer 21

it increases the tendency for the species on the right to oxidise and act as reducing agents

Question 22

Where is the most powerful reducing agent found on the series?

Answer 22

at the most negative end on the right

Question 23

Where is the most powerful oxidising agent found on the series?

Answer 23

at the most positive end on the left

Question 24

Effect of conc on Ecell

Answer 24

• increasing the conc of reactant will increase Ecell

- decreasing them will decrease it

Question 25

What are the forms of electrochemical cells as a commercial source for electrical energy?

Answer 25

as non-rechargeable (irreversible) , rechargeable and fuel cells

Question 26

What does a fuel cell do?

Answer 26

it uses energy from the reaction of a fuel with oxygen to create a voltage

Question 27

Positive electrode in lithium cell

Answer 27

Li+ + CoO2 + e- –> Li+
[CoO2
]-
E=+0.6V

Question 28

Negative electrode in lithium cell

Answer 28

Li+ + e- –> Li E=-3.0V

Question 29

Hydrogen fuel cell (potassium hydroxide electrolyte)- alkaline conditions equations

Answer 29

4e- + 4H2O –> 2H2 +4OH- E=-0.83V
4e- + 2H2O +O2 –> 4OH- E=+0.4V
Overall reaction 2H2 + O2 –> 2H2O E=1.23V

Question 30

How does a fuel cell maintain a constant voltage?

Answer 30

continuously fed with O2 and H2 so maintaining a constant conc of reactants

Question 31

Advantages of fuel cells over conventional petrol or diesel vehicles

Answer 31

• less pollution/CO2

- greater efficiency

Question 32

Limitations of hydrogen fuel cells

Answer 32

• expensive
• storing and transport H2 is dangerous
• limited lifespan
• high production costs that uses toxic chemicals

Question 33

Advantage of ethanol fuel cell over hydrogen

Answer 33

• made from renewable source, carbon neutral
• raw materials to produce it via fermentation are abundant
• less explosive, easier to store