3.1.11 Electrode potentials and electrochemical cells

Question 1

Conventional representation of:
Zn –> Zn2+ + 2e-
Cu2+ + 2e- –> Cu

Answer 1

Zn l Zn2+ ll Cu2+ l Cu

Question 2

How do cells measure electrode potential

Answer 2

By comparing them to a reference, normally the Standard Hydrogen Electrode (SHE)

Question 3

Standard electrode potential conditions

Answer 3

100kPa
298K
1 moldm-3

Question 4

Parts of the SHE electrode

Answer 4

High resistance voltmeter
Salt bridge (KNO3)
1 moldm-3 ionic solutions
Solid electrode on non hydrogen side
Platinum wire, with hydrogen in at 1 atm

Question 5

Why is a salt bridge used

Answer 5

The ions in the ionic substance in the salt bridge move through the salt bridge to complete the circuit and maintain charge balance

Question 6

Conventional representation

Answer 6

l represents phase change
ll represents salt bridge
, represents no phase change
Pt electrode goes on outside
Most oxidised species is closest to salt bridge
RHS is reduction, LHS is oxidation (unless SHE)

Question 7

Why is a high resistance voltmeter used

Answer 7

To stop any current flowing, whilst maintaining maximum potential difference. In this state electrode potentials can be measured.

Question 8

Electrochemical series definition

Answer 8

List of electrode potentials in numerical order

Question 9

Suggest why two complex ions containing the same metal have different electrode potentials

Answer 9

Different ligands

Question 10

Electrode reactions in a lithium cell

Answer 10

Li+ + CoO2 + e- –> Li+[CoO2]- (+ve electrode)
Li –> Li+ + e- (-ve electrode)

Question 11

3 types of cell

Answer 11

Non-rechargeable
Rechargeable
Fuel cell

Question 12

EMF equation

Answer 12

EMF = E right – E left

Question 13

What electrode potentials are good oxidising agents

Answer 13

Positive electrode potentials as these have been reduced

Question 14

What electrode potentials are good reducing agents

Answer 14

Negative electrode potentials as these have been oxidised

Question 15

Increasing concentration in an electrochemical cell effect

Answer 15

If side with electrons concentration is increased, cell voltage becomes more positive
If side without electrons concentration is increased, cell voltage becomes more negative

Question 16

Increasing pressure in an electrochemical cell effect

Answer 16

If side with electrons pressure is increased, cell voltage becomes more positive
If side without electrons pressure is increased, cell voltage becomes more negative

Question 17

Rechargeable cell

Answer 17

Reaction is a reversible reaction
Lithium ion batteries
In order ot be recharge current must be applied, forcing the elctrons in the opposite direction

Question 18

Increasing surface area in an electrochemical cell effect

Answer 18

Increases rate at which equilibrium is established but not position of equilibrium

Question 19

Reasons for different values of the same electrode potential

Answer 19

Impurities in chemicals
Resistance not high enough
Different conditions
Solutions may not be exactly one mol

Question 20

Anode definition

Answer 20

Electrode where oxidation occurs

Question 21

Cathode definition

Answer 21

Electrode where reduction occurs

Question 22

Dry cells

Answer 22

Have electrolyte paste
More portable
Allows current to flow

Question 23

Non-rechargeable cells

Answer 23

Concentration decreases over time, so voltage does too

Question 24

Alkaline hydrogen fuel cell electrode reactions

Answer 24

2 H2 + 4 OH- –> 4 H2O + 4e- ANODE
O2 + 2 H2O + 4e- –> 4OH- CATHODE

Question 25

How do fuel cells keep producing a voltage

Answer 25

They have to be supplied by an external source of fuel e.g. hydrogen and an oxidant e.g. air/oxygen

Question 26

Fuel cells pros and cons

Answer 26

Pros Non-polluting Stable voltag Cons Producing hydrogen requires a lot of energy No infrastructure for charging Difficult to store hydrogen Too large Hydrogen is very flammable