3.1.11: Electrode potentials

Question 1

What are electrode potentials?

Answer 1

• made from 2 different metals dipped in salt solutions of their own ions connected by a wire (external circuit)
• redox process (2 reactions occur)
• electrons flow through wire from more reactive to less reactive metal
• half cell/ electrode- metal dipping into solution of its ions

Question 2

What is a metal electrode?

Answer 2

a metal surrounded by a solution of its ions

Question 3

What is a gas electrode?

Answer 3

• for gas and a solution of its ions
• inert metal (e.g. Pt) used as actual electrode to measure flow of electrodes

Question 4

What is a redox electrode?

Answer 4

• 2 different ions of the same element
• where the 2 types of ions present in solution 2/ an inert metal electrode to allow flow of electrodes

Question 5

How do you measure potential of an electrode?

SHE

Answer 5

• comparison to SHE (standard hydrogen electrode)
• connect half-cell to another half-cell of known potential and measure p.d. beteen 2 half cells
• 2 half cells produces electrochemical cell
• SHE- potential of 0V (primary standard which all other potentials compared too)

Question 6

How do you set up an e-cell?

Answer 6

• two half cells joined, giving a complete circuit
• two metals joined by wire- electrons flow through
• two solutions joined by salt bridge- where ions flow through
• voltmeter inclused to measure EMF
• salt bridge- paper soacked in uncreactive ions (e.g. KNO3) or tube w/ions in agar gel

Question 7

What are standard conditions of an electrode?

Answer 7

• concentration- 1.0 moldm-3
• temperature- 298K
• pressure- 100 kPa

Question 8

What is the SHE?

Answer 8

• the standard in which all electrodes are measured by
• 2H+(aq) + 2e- ⇌ H2 (g)
• E = +0.00V
• Pt (s) | H2 (g) |H+ (aq) ||

Question 9

How do you write a cell notation?

Answer 9

• left side- oxidised, (-) EMF
• right side- reduced, (+) EMF
• R O || O R
• highest oxidation state in centre
• solid electrodes at ends, if no solid, Pt used
• ||- salt bridge, |- phase boundary

Question 10

How do you calculate cell EMF?

Answer 10

• E= E right- E left
• E= more positive- more negative

Question 11

Why is there sometimes a second standard?

Answer 11

• used as standard electrode
• calibrated against primary standard
• as SHE uses flammable gas

Question 12

How does REDOX reactions occur in an electrode?

Answer 12

• on left side, metal atoms lose e-
• travel through wire to other electrode on right side
• adding to ions producing metal atoms (reduction)

OILRIG

Question 13

What is the electrochemical series?

Answer 13

a list of electrode potentials in order of decreasing (or increasing) potential

Question 14

What does a very (+) potential mean?

Answer 14

• best oxidising agent
• worst reducing agent

Question 15

What does a very (-) potential mean?

Answer 15

• worst ocidising reagent
• best reducing agent

Question 16

What are uses of the electrochemical series?

Answer 16

• predict direction of redox reactions
• more postive gains electrons (remains forward reaction)
• more negative goes in backwards reaction

Question 17

Why are electrochemical cells used as commercial cells?

Answer 17

• can be used as commercial source of electrical energy (portable)
• e.g. non-rechargeable cells, rechargeable cells, fuel cells

Question 18

What is a battery?

Answer 18

more than one cell joined together

Question 19

How do non-rechargeable cells work?

Answer 19

• chemicals used over time, EMF drops, when one or more chemicals used, cell goes flat (EMF = 0V)
• can’t be recharged, have to be disposed of after single use
• standard, chap, non-rechargeable cell, but has fairly short life

Question 20

What is the structure of an alkaline non-rechargeable cell?

Answer 20

• KOH
• Zn
• MnO2
• graphite

Question 21

How do rechargeable cells work?

Answer 21

• reversible reaction used to recharge
• done by applying external current, regenerating chemicals
• e.g. lithium ion, lead acid, nickel cadmium

Question 22

What is the use of a lithium ion rechargeable cell?

Answer 22

• used in phones, tablets, cameras, laptops etc.
• (+) electrode- Li+ + CoO2 +e- ⇌ LiCoO2 | E= +0.60V
• (-) electrode- Li+ + e- ⇌ Li | E= -3.00V

Question 23

What is the use of a lead acid rechargeable cell?

Answer 23

made of 6 cells, used in cars

Question 24

How do fuel cells work?

Answer 24

• used to generate electric current, doesn’t need recharge
• have continuous supply of chemicals into cell
• efficient as only gives water as waste

Question 25

How do hydrogen- oxygen fuel cells work?

Answer 25

- H2 and O2 in on opposite sides - OH- ions move through/ between electrodes through **electrolyte** - H2O out - e- moves between electrodes through wire

Question 26

What is the difference between acidic and alkaline H fuel cells?

Answer 26

**alkaline:** - (-) electrode- 2H2 + 4OH- -> 4H2O + 4e- | **E= -0.83 V** - (+) electrode- O2 + 2H2O + 4e- -> 4OH- | **E= +0.40V** - overall equation- 2H2 + O2 -> 2H2O - **EMF- +1.23V** **acidic:** - (-) electrode- 2H2 -> 4H+ + 4e- | **E= 0.00V** - (+) electrode- O2 + 4H+ + 4e- -> 2H2O | **E= 1.23V** - overall equation- 2H2 + O2 -> 2H2O - EMF- +1.23V

Question 27

What are benefits and risks of cells?

Answer 27

**benefits:** - portable chemical energy source **risks:** - waste issues

Question 28

What are benefits and risks of non-rechargeable cells?

Answer 28

**benefits:** - standard - cheap **risks:** - waste issues

Question 29

What are benefits and risks of rechargeable cells?

Answer 29

**benefits:** - less waste - cheaper overall - lower environmental impact **risks:** - some waste issues (at end of useful life)

Question 30

What are benefits and risks of hydrogen fuel cells?

Answer 30

**benefits:** - only waste product is water - don't need recharging - very efficient **risks:** - need constant fuel supply - H flammable - H expensive - H usually made of fossil fuels - high cost of fuel cells