Electrochemical Cell

Question 1

What to do when balancing half-equations?

Answer 1

try use data booklet for relevant half-eqn

Question 2

Describe what happens when strip of Mg metal is placed in Mg2+ solution

Answer 2

(half-cell config)
- Mg atom on strip surface may lose e- bcome ion OR Mg2+ ion may gain 2 e-, deposit as Mg atom on strip
- When rate Mg2+ ion re-join surface = rate leaving surface, dynamic eqm established
- at eqm, const -ve charge on Mg metal, const no. Mg cations in sol n ard metal
=> separat n of charge across metal-sol n interface, so pd btw Mg metal & Mg2+ sol n. Pd aka electrode potential (E) of metal

Question 3

What is needed for measuring electrode potential of a half-cell?

Answer 3

• standard condit n (25 deg C/ 298K, Pa = 1 bar, 1 mol dm-3 conc)
• standard electrode as reference electrode

Question 4

Describe standard hydrogen electrode (SHE)

Answer 4

• reference hydrogen half-cell assigned as zero
• used to find out Eθ value
  SHE consists of
• platinised platinum electrode (inert)
• immersed in sol n where [H+] = 1mol dm-3
• H2 gas at 1 bar bubled over Pt electrode
• temp kept at 298K

*platinised mean coated w platinum black increase surface area so eqm btw H2 & H+ established rapidly vs polished surface

Question 5

Define standard electrode potential of a half-cell, Eθ

Answer 5

Eθ is electromotive force, measured at 298K, btw half-cell & standard hydrogen electrode, in which conc of any reacting species in sol n is 1 mol dm-3, any gaseous species at Pa 1 bar

Question 6

What does Eθ tell?

Answer 6

*can be refer as reduct n potential
- measure tendency reduct n rxn occur relative to SHE
- more +ve, greater tendency reduct n occur
- less +ve, greater tendency oxidat n occur

Question 7

How to write Eθ fully?

Answer 7

+/- Eθ Mn+/M

where
+/- indicate polarity of electrode wrt SHE,
Mn+/M refer to oxidised & reduced species respectively (subscript)

Question 8

What are 3 common types of half-cells?

Answer 8

1. metal-ion half cell
2. gas-ion half cell
3. ion-ion half cell

• must draw all types + label + standard condit n present

Question 9

What must be the conditions to measure standard electrode potential?

Answer 9

• 298K, 1 bar
• all reacting ions hv conc 1 mol dm-3
• any gas must hv Pa 1 bar
• Pt (bcos inert, for gas-ion, ion-ion) as electrode when half cell no include metal
• salt bridge completes circuit (inverted u-shaped tube w high conc non reacting ion in gel) (prevent mixing of 2 sol n but allow ion pass thru, maitain electrical neutrality)

Question 10

Describe experimental procedure to determine Eθ M n+/M

Answer 10

• use standard condit n 298K, 1 bar, conc of reacting ion (H+, M n+) 1 mol dm-3 each
• SHE used as refer electrode, 2 half cell connected by salt bridge
• pd set up when 2 half cell connected can b measured by high resis voltmeter
• value shown will be standard electrode potential Eθ M n+/M

Question 11

How does salt bridge maintain electrical neutrality in each half-cell?

Answer 11

• w/o salt bridge, Cl2/Cl- half cell wld slowly bcome more -ve charged as more Cl- produced, while H2/H+ half cell more +ve charged as more H+ produced. Charge imbalance stop cell operat n
• thus, w salt bridge, ions present in salt bridge (eg Na2+, SO4 2-) diffuse out, ensure e- neutrality
  (Na+ ion move fr salt bridge to sol n Cl2/Cl- half cell, SO4 2- ion move fr salt bridge to sol n H2/H+ half cell)

Question 12

What factors affect standard electrode potential? Elaborate concept

Answer 12

By LCP, affected by
- conc ions
- temp
- Pa of gas species (size/surface area of electrode no affect Eθ cell, since M(s) no shift eqm pos n)

Concept:
M n+(aq) + ne- ⇌ M (s)
- value of electrode potential Eθ M n+/M can be affected by above factors, shift eqm pos n
- if shift right, reduct n favoured, so Eθ more +ve
- if shift left, oxidat n favoured, so Eθ less +ve

Question 13

By convention, electrode potentials of half cells are written as …

Answer 13

reduction processes

Question 14

What happens to change in Eθ value?

Answer 14

1. more +ve Eθ value,
   - eqm pos n shift right
   - forward rxn favoured
   - reduct n more likely occur
   - higher tendency gain e-
   - stronger OA
   eg Cu2+ higher tendency reduced as Cu vs Zn
2. less +ve/more -ve Eθ value,
   - eqm pos n shift left
   - backward rxn favoured
   - oxidat n more likely occur
   - higher tendency lose e-
   - stronger RA
   eg Zn higher tendency oxidised to Zn2+ vs Cu

Question 15

How to prevent rusting?

Answer 15

• can b prevented/slowed down by galvanising (eg iron galvanised w zinc)
• iron connected to metal eg Zn w more -ve Eθ (Zn more easily oxidised)
• since Eθ Zn2+/Zn more -ve than Eθ Fe2+/Fe, zinc undergo oxidat n, termed as sacrificial corros n instead of Fe

Question 16

Describe relative stabilities of metal ions in different mediums, using Fe2+(aq) as example

Answer 16

Fe2+(aq) usually stabilised against oxidat n by add n small amt acid
bcos,

in acidic medium
Fe3+ + e- ⇌ Fe2+ Eθ = +0.77V
- Eθ value more +ve, so Fe2+ less likely oxidised to Fe 3+
- Fe2+ more stable than Fe 3+ in acidic medium

in basic medium
Fe(OH)3 + e- ⇌ Fe(OH)2 + OH-, E= -0.56V
- Eθ value more -ve thus Fe(OH)2 more likely oxidised to Fe(OH)3
- Fe2+ less stable than Fe3+ in basic medium

Question 17

Define galvanic/voltaic cell

Answer 17

electrochemical cell in which spontaneous rxn occur to convert chem energy to electrical energy

Question 18

What occurs at anode and cathode of galvanic cell?

Answer 18

anode: oxidation (An Ox)

cathode: reduction (Red Cat)

Question 19

Describe what happens in a galvanic cell

Answer 19

• no direct transfer e- fr eg Zn metal to Cu2+ ion as separated fr each other (oni travel thru wire)
• e- flow fr anode to cathode thru external ciruit always
• convert chem to electrical energy
• An Ox
• Red Cat
• polarity of anode is -ve, of cathode is +ve (reverse of electrolytic cell)

Question 20

Define standard cell potential Eθcell/cell e.m.f.
Give formula

Answer 20

p.d. btw cathode & anode of cell

Eθcell = Eθred - Eθoxid

where
Eθred & Eθoxid is standard electrode potential of half-cell undergoing reduct n & oxidat n respectively
* No need multiply by stoichiometric coeff in rxn

Question 21

Describe steps to apply Eθcell formula

Answer 21

1. check Eθ of each half-eqn in data book
   - more +ve value will be Eθred
   - less +ve value will b Eθoxid
2. calculate w Eθcell = Eθred - Eθoxid
3. write overall eqn (if qn asks)

• if there is specie w multiple OS, calculate (no need show working unless needed) until get Eθcell value < 0 (non-spontaneous rxn)

Question 22

Describe steps to predict feasibility of redox reaction in cell

Answer 22

1. determine if starting rxt oxidised or reduced in eqn based on change in ON (not Eθ value)
2. choose relevant half eqn fr data book
3. calculate Eθcell = Eθred - Eθoxid
4. predict feasibility:
   - Eθcell > 0, spontaneous rxn
   - Eθcell < 0, not spontaneous rxn
   - Eθcell = 0, rxn reached eqm
   => write concluding statement (eg thus Mg2+ (aq) cnt oxidise Zn(s) to Zn2+(aq) )

Question 23

Explain limitations of standard electrode potentials, Eθ

Answer 23

1. energetics vs kinetic feasibility
   - Eθ value predict whether rxn energetically feasible but NOT rate rxn
   - even if rxn hv Eθ > 0, rxn unlikely occur as slow rate (too kinetically slow) rxn due to vv high Ea
2. non-standard conditions
   - Eθ value relate oni to standard condit n (otherwise, invalid value)
   - factors affecting eqm pos n (ie conc, temp, Pa) affect Eθ value
   => if Eθ value change, no more standard condit n

Question 24

Give Gibbs free energy and electrode potential relationship formulae. What can be deduced?

Answer 24

ΔGθ = -nFEθcell

where
n is no of e- transferred per mole rxn,
F is Faraday const (9.65E4 C mol-1)
*NOTE: units of ΔGθ is J mol-1, NOT kJ mol-1

if combine ΔGθ = -RTlnK w above eqn, we get,

Eθcell = RT(lnK)/(nF)
- since Eθcell depend on temp & conc, non-spontaneous rxn under standard condit n can b made feasible under non-standard condit n (eg increase temp, conc)

Question 25

Describe signs of ΔGθ and Eθcell for spontaneous and non spontaneous reactions

Answer 25

• spontaneous: ΔGθ < 0, Eθcell > 0
• non-spontaneous: ΔGθ > 0, Eθcell < 0

Question 26

Describe 2 main battery types

Answer 26

1. primary cell
   - electrochem cell in which chem rxn generate electrical energy
   - redox rxn spontaneous, irreversible (NOT rechargeable)
   eg daniel cell, dry cell, alkaline battery
2. secondary cell
   - redox rxn spontaneous, reversible (rechargeable)
   eg lead-acid storage cell, nickel-cadmium cell

Question 27

Describe fuel cell

Answer 27

• consist of fuel eg H oxidised directly by oxidant (usually O). Energy produced is converted to electrical energy
• rxt continuously replaced as they r consumed & pdt continuously removed

Question 28

What can be deduced from having fuel cell in acidic and alkaline medium?

Answer 28

• Eθcell is const for fuel cell in both acidic, alkaline medium (ie rxn equally spontaneous for both medium)

Question 29

Name main advantages, disadvantages and extent of application of fuel cells

Answer 29

advantage:
- pollu n free (produce water)
- high pwr to mass ratio (small size, low mass but generate high voltage)
- highly eff (> 70% chem energy converted to electrical energy vs ~ 30% for traditional pwr stat n

disadvantages:
- expensive due to large amt Ni & Pt needed as catalyst for electrode rxn
- catalyst easily poisoned by impurities in fuel & oxygen
- most fuel cell require high temp to react in fuel cell

applicat n:
- limited due to high cost, temp involved (eg store H2 gas difficult)
- now used to supply electrical pwr in space shuttle. Final pdt of water removed, consumed by astronauts

Question 30

Name main advantages and disadvantages of zinc-carbon dry cell

Answer 30

advantage
- cheap, portable source energy
- supplies reliable voltage

disadvantage
- not rechargeable (gas pdt eg H2 cnt b consumed rapidly enough when current drawn rapidly)
- poor shelf-life when cell is used, zinc casing disintegrate (due to oxidat n of zinc when react w acidic electrolyte NH4Cl give Zn2+ & hydrogen). Paste will leak through outer casing

Question 31

Name advantage and disadvantage lead-acid storage battery

Answer 31

advantage
- provide initial high voltage needed to start car engine

disadvantage
- large, heavy weight due to lead (provides low pwr compared to mass it hv)