Electrode Potentials

Question 1

Equilibrium Half Equation

Answer 1

Question 2

Half-Cell

Answer 2

Metal dipping into a solution of its ions

Question 3

Cell

Answer 3

Two half-cells combined

Oxidation must occur in one and Reduction in the other

Question 4

What must occur in any Electrochemical Reaction

Answer 4

Reduction and Oxidation

Question 6

What arrows are used for equations of reactions in half cells

Answer 6

Normal Arrows

Question 7

Standard Electrode Potential

Answer 7

the potential difference measured when a half-cell is connected to the standard hydrogen electrode under standard conditions

Question 8

Units of Electrode Potentials

Answer 8

V (volts)
must have - or +

Question 9

What does negative electrode potential indicate

Answer 9

Reduction reaction not feasible unless a more feasible oxidation takes place

Question 10

Electromotive Force (emf)

Answer 10

the potential difference measured when two half-cells are connected

Question 11

Calculate emf

Answer 11

Oxidation and reduction added together

Question 12

Positive vs Negative emf

Answer 12

+ = feasible
- = not feasible

Question 13

Features of Standard Hydrogen Electrode

Answer 13

•platinum electrode
•1.00moldm-3 H ions in solution
•H2 gas at 100kPa pressure
•temp 298K

Standard EP = 0.00V

Question 14

Combining of Half Cells

Answer 14

Using salt bridge and an external circuit containing a high resistance voltmeter - produce a cell

Salt bridge usually filter paper soaked in saturated potassium nitrate solution - dips into both half-cell solutions

Question 15

What does salt bridge allow

Answer 15

Electrical connection between the half-cells without allowing them to mix

Question 16

Why may Potassium Chloride not be used as salt bridge

Answer 16

Cl- ions can form complexes with transition metal ions

Question 17

Left Hand Half-Cell by Convention

Answer 17

Oxidation occurs
Negative electrode of cell

Question 18

Right Hand Half-Cell by Convection

Answer 18

Reduction Occurs
Positive electrode of the cell

Question 19

Diagram Combining 2 Half Cells

Answer 19

Question 20

How to work out which cell is LH or RH

Answer 20

Most negative is oxidation and should be on left - negative electrode

To calculate emf reverse sign of left hand cell

Question 21

Non-Standard Conditions in a Cell

Answer 21

2 cells the same but different conc of ions - solution show a reduction where the ion in solution is at greater conc

ammeter used to allow current to flow

Mass of LH increase - RH decrease

Question 23

Conventional Cell Representation

Answer 23

oxidation LH || reduction RH

|| = salt bridge

Incl state symbols

Pt incl if used as electrode - gas electrodes or two ions in solution

Question 24

Equation emf conventional cell

Answer 24

E°rhs - E°lhs

Question 25

What are | and when are they used

Answer 25

Phase boundaries Used when two species in half cell are in different states If same state a comma is used

Question 26

Displacement Reactions

Answer 26

E° value greater for one than the other - reduction more favourable for greater value so is displaced If positive emf (more negative value swap signs) means reaction feasible - displacement occurs

Question 27

Reactivity of Metals

Answer 27

E° for reverse oxidation reaction gives measure of reactivity - most reactive metal will form ions most easily Higher value = more reactive

Question 28

Reactivity of Mon-Metals

Answer 28

Recuced- most feasible with greater E° Explains reactivity decreases down group 7

Question 29

Reducing and Oxidising agents

Answer 29

Most negative = best reducing agent - easily oxidised Most positive = best oxidising agent - easily reduced

Question 30

E° Vanadium Chemistry

Answer 30

E° decreases down changes in OS 5-4 +1.00 4-3 +0.32 3-2 -0.26 Work out which is feasible only to the os wanted - not any further

Question 31

3 Types of Commercial Cells

Answer 31

**Primary** - non rechargeable (single use) **Secondary** - rechargeable (multiple use) **Fuel Cells** - continuously prod an electrical current as long as theyre supplied with fuel

Question 32

Zinc Carbon Cell

Answer 32

Primary - electrode: Zn -> Zn2+ +2e- + electrode: MnO2 + H2O + e- -> MnO(OH) + OH- Cell: Zn + 2MnO2 + 2H2O -> Zn2+ + 2MnO(OH) + 2OH- Carbon rod carries e- back to MnO2 paste Non-reversible - once discharged cell cannot be used again

Question 33

Nickel-Cadmium Cell

Answer 33

Secondary Cell - electrode: Cd + 2OH- -> Cd(OH)2 + 2 e- + elextrode: NiO(OH) + H2O + e- -> Ni(OH)2 + OH- Cell: Cd + 2NiO(OH) + 2H2O -> Cd(OH)2 + Ni(OH)2 ^equation reversed when cell recharged

Question 34

Lead-Acid Cell

Answer 34

Secondary Cell - used in cars - electrode: Pb + HSO4- -> PbSO4 + H+ + 2e- + electrode: PbO2 +3H+ + HSO4- + 2e- -> PbSO4 + 2H2O Cell: PbO2 + 2H2SO4 + Pb -> 2PbSO4 + 2H2O Lead(II) sulfate insoluble so if not recharged for long time - builds up and cannot be recharged

Question 35

Lithium Ion Cell

Answer 35

Secondary Cell - portable devices - electrode: Li -> Li+ + e- + electrode: Li+ + CoO2 + e- -> Li+[CoO2]- Cell: Li + CoO2 -> Li+[CoO2]- Graphite powder used as support medium - water react w/ Li Cell Rep: Li|Li+||Li+,CoO2|LiCoO2|Pt.

Question 36

Hydrogen Fuel Cell

Answer 36

- electrode: H2+ 2OH- ->2H2O + 2e- + electrode: O2 + 2H2O + 4e- -> 4OH- Cell: 2H2 + O2 -> 2H2O - can be doubled to balance

Question 37

Conv Cell Rep For Alkaline H Fuel Cell

Answer 37

Pt|H2|OH-,H2O||O2|H2O,OH-|Pt Emf always +1.23V

Question 38

Environmental issues of Cells

Answer 38

Primary: source of waste, some parts may be recycled Secondary: finite lifespan Fuel Cells: continuously prod current aslong as fuel supplied Cells contain metals that are dwindling resource

Question 40

Benefits H Fuel Cells

Answer 40

Use renewable source energy Silent when operating Do not produce any polluting substances However do require H gas which is flammable and difficult yo maintain safely

Question 41

Possible Alternative to H Fuel Cell

Answer 41

Ethanol Fuel Cells Ethanol prod from fermentation - considered carbon neutral fuel CO2 released offsets by CO2 taken in by photosynthesis