Chapter 20 (Electrode Potential and Electrochemical Cells)

Question 1

What is oxidation?

Answer 1

Loss of electrons/increase in oxidation number

Question 2

What is reduction?

Answer 2

Gain of electrons/decrease in oxidation number

Question 3

What does an oxidising agent do?

Answer 3

Accepts electrons

Question 4

What does a reducing agent do?

Answer 4

Donates electrons

Question 5

What are the steps for writing a redox reaction?

Answer 5

1) Write more negative reaction as oxidation
2) Write more positive reaction as a reduction
3) Balance the number of electrons being transferred between the two reactions
4) Combine the two half equations

Question 6

How is an electrode potential between a metal and solution formed?

Answer 6

When a metal in placed in a solution of its ions, some of the metal atoms form ions and go into the solution, resulting in a slight build up of electrons on the metal. This build up of charge produces an electrode chemical potential between the metal and its solution.

Question 7

What is a half reaction?

Answer 7

The equilibrium between the metal and its ions.

Question 8

How is an electrochemical cell formed?

Answer 8

Two half reactions are connected using a salt bridge.

Question 9

What factor increases a potential difference?

Answer 9

A greater tendency of a metal to produce ions

Question 10

What are the standard conditions for electrode potentials?

Answer 10

Concentration = 1 mol dm-3 solution of ions
Temperature = 298 K
Pressure(if gas involved) = 100kpa
Zero current

Question 11

Why is a high resistance voltage meter used?

Answer 11

The high resistance stops the current from flowing in the circuit which stops reactions from occurring. This allows it to measure the maximum possible potential difference.

Question 12

Where must the S.H.E be when measuring standard electrode potential?

Answer 12

On the left`

Question 13

How is standard electrode potential measured?

Answer 13

The difference between the standard electrode potential between a given electrode under standard conditions and the standard hydrogen electrode.

Question 14

What does a vertical bar mean for cell notation?

Answer 14

Phase boundary

Question 15

What does a double vertical bar mean for cell notation?

Answer 15

Salt bridge

Question 16

What is used to calculate standard cell potentials?

Answer 16

Standard electrode potentials of half cells

Question 17

What type of cell is a lithium ion cell?

Answer 17

Rechargeable cell

Question 18

How do lithium ion cells work?

Answer 18

Lithium ions move between the graphite electrode (-) and the CoO2 electrode (+) depending on whether it charging or recharging.

During discharge - > +
During recharge + > -

Question 19

Why are fuel cells reliable?

Answer 19

There are no moving parts

Question 20

Why can fuel cells operate virtually continuously?

Answer 20

The reactants and products (O2 and H2) are pumped in and out but the electrolyte remains in the cell.

Question 21

How is E° cell calculated?

Answer 21

E° cell = E° forward/right - E° backwards/left

Question 22

What conducts charge in electrochemical cells?

Answer 22

Free moving ions

Question 23

How are salt bridges made?

Answer 23

A piece of filter paper soaked in salt solution , usually potassium nitrate.

The salt should be unreactive with the electrodes and electrode solutions.

Question 24

Why is a salt bridge used instead of a wire?

Answer 24

A wire would set up its own electrode solution.

Question 25

What is used when a system does not include a solid metal and why?

Answer 25

A platinum electrode is used because it provides a conducting surface for electron transfer, is unreactive and can conduct electricity.

Question 26

What is the effect of changing concentration on Eº?

Answer 26

Increasing concentration of one of the reactants increases Eº and decreasing concentration decreases Eº

Question 27

What is the effect of increasing temperature on Eº?

Answer 27

Most cells are exothermic so increasing the temperature would decrease Eº because equilibrium reaction would shift backward.

Question 28

Why does a fuel cell maintain a constant voltage?

Answer 28

Continuous supply of O2 and H2 so constant concentration of reactants.

Question 29

What are the half and overall equations for a hydrogen cell in alkaline conditions and the Eº?

Answer 29

4e- + 4H2O > 2H2 + 4 OH- Eº = -0.83 v 4e- + 2H2O + O2 > 4OH- Eº = + 0.4 v Overall: 2H2 + O2 > 2H2O Eº = + 1.23 v

Question 30

What is the reaction at the positive electrode of a lithium-ion cell and Eº?

Answer 30

Li+ + CoO2 + e- > Li+[CoO2]- Eº = + 0.6 v

Question 31

What is the reaction at the negative electrode of a lithium-ion cell and Eº?

Answer 31

Li > Li+ + e- Eº = -3.0 v

Question 32

What is the overall discharge equation for lithium-ion cells and Eº?

Answer 32

Li + CoO2 > LiCoO2 Eº = + 3.6 v

Question 33

What are the advantages of fuel cells over conventional petrol vehicles?

Answer 33

* Less pollution and less CO2 emitted as fuel cells only emit water * Greater efficiency (more thermal energy transferred to kinetic energy)

Question 34

What are the limitations of using fuel cells over petrol vehicles?

Answer 34

* Expensive * Storing and transporting hydrogen is difficult as has to be a pressurised liquid and is flammable * Limited lifetime (regular replacement and disposal) and high production costs * Use of toxic chemicals in production

Question 35

Write the conventional representation for an alkaline hydrogen-oxygen fuel cell

Answer 35

Pt|H2(g)|OH–(aq),H2O(l)||O2(g)|H2O(l),OH–(aq)|Pt

Question 36

Why is the EMF of a hydrogen-oxygen fuel cell the same in both acid and alkaline conditions?

Answer 36

Same overall reaction

Question 37

Why might a porous platinum electrode be used rather than a platinum rod

Answer 37

Increase surface area so faster reaction

Question 38

Suggest why the recharging of a lithium cell may lead to release of carbon dioxide into the atmosphere.

Answer 38

Electricity for recharging the cell may come from power stations burning (fossil) fuel.

Question 39

Describe a standard hydrogen electrode (S.H.E)

Answer 39

Hydrogen gas with a solution on 1 mol dm-3 of HCl/H+ at 298K and 100kPa with a platinum electrode,