Electrochemistry : 10.1 Galvanic Cell (continued)

 SPONTANEOUS REACTION

• occurs as the result of different ability of metal to give up their electron to flow through the circuit.
  

 
CELL POTENTIAL (E_Cell)

• different in electrical potential of electrodes
  
• also called voltage or electromotive force (e.m.f)
  

 
E_cell > 0

• sponteneous reaction
  
  • The more positive E_cell
    
  • The more work the can do
    
• The further the reaction proceed to right
  

 
E_cell < 0

• Non spontaneous cell reaction
  

 
E_Cell = 0

• The reaction has reach equilibirum
  
• The cell can do no more work.
  

 
SI UNIT CELL POTENTIAL

• unit = volt (V)
  
• 1V = 1J/C
  

C = coulumb (SI unit of electrical charge)


 
STANDARD CELL POTENTIAL (E⁰_cell)

• Different in electrical potential of electrodes measured at a specified temperature (usually 298k) with all components in their standard states.
  
• standard state 
• 1 atm for gaseous
• 1 M for solution
• Pure solid for electrodes

STANDARD ELECTRODE (HALF CELL) POTENTIAL (E⁰_half-cell)

• potential associate with a given half-reaction (electrode compartment) when all component are in their standard states.
  

E⁰_{half cell =} E⁰_anode or E⁰_cathode

• also call standard reduction potential.
  
• example : 

                         Zn²⁺(aq) + 2e^- ------ Zn(s) E⁰_zinc (E⁰_anode)

                        Cu²⁺(aq) + 2e^- ------ Cu(s) E⁰_copper(E⁰_cathode)

                    Zn(s) + Cu²⁺ (aq) ------ Zn²⁺ (aq) + Cu(s)


*changing the balancing coefficients of a half-reaction does not change E⁰ value because electrode potential are intensive properties –does not depend on amount



E⁰_cell AND E⁰_{half cell}

• Example : 
  

Half cell reation

                            Cu²⁺(aq) + 2e^- ------ Cu(s)
                         Zn(s) + Cu²⁺(aq) ------ Zn²⁺(aq) + Cu(s)

                         Zn(s) + Cu²⁺(aq) ------ Zn²⁺(aq) + 2e^-
 

• E⁰_cell = E⁰_cathode – E⁰_anode
  

 
STANDARD HYDROGEN ELECTRODE

• specially prepared platinum electrode immersed in a 1M aqueous solution of a strong acid,H⁺(aq) or H₃O⁺(aq), through which H₂ gas at 1 atm is bubled
  

DETERMINING E⁰_{half cell}

• Use standard hydrogen electrode (SHE)
• standard reference half-cell
  
  2H⁺ (aq, 1m) + 2e^- ------ H₂ (g,1atm) E⁰ ref= OV
  

         Zn²⁺ (aq, 1m) + 2e^- ------ Zn (s, 1atm) E zinc= ?

Zn(s) │ Zn²⁺ (1M) ││ H⁺ (1M) │ H₂ (1atm) │ Pt (s)

E⁰ cell  = E⁰ cathode – E⁰ anode

              = E⁰ ref – E⁰ zinc

0.76 V  = 0.00 V – E⁰ zinc

E⁰ zinc  = -0.76 V

Pt(s) │ H₂ (1atm) │ H⁺ (1M) │ Cu²⁺ (1M) │ Cu (s)

  

Anode : H₂ (1atm) 2H⁺ (1M) +2e^-

Cathode : 2e^- + Cu²⁺ (1M)     Cu (s)

H₂ (1atm) + Cu²⁺ (1M)     Cu (s) + 2H⁺ (1M)

 

RELATIVE STRENGH OF OXIDIZING AND REDUCTING AGENT

• Example:
  

Cu²⁺(aq) + 2e^- ------ Cu (s) E⁰ = 0.34 V

2H⁺ (aq) + 2e^- ------ H₂ (g) E⁰ = 0.00 V

Zn²⁺ (aq) + 2e^- ------ Zn (s) E⁰ = -0.76 V

• The more positive E⁰ value, the more tendency to be reduced
  
• Strength of oxidizing agent (reactant)
  
  Cu²⁺ > H⁺ > Zn²⁺
• Strength of reducing agent (product)
  
  Zn > H₂ > Cu
  
  

STANDARD REDUCTION POTENTIAL

• All value are relative to hydrogen electrode
  
• Strength of oxidizing agent – increase up
  
• Strength of reducing agent – increase down
  
• Half-cell component usually appear in the same order as in the half-reaction
  
                                               Cu²⁺ (aq) + 2e^- ------ Cu (s)                        (reducing)
  
                                                                Zn (s) ------ Zn²⁺ (aq) + 2e^-        (oxidizing)
  
                                           Cu²⁺ (aq) + Zn (s) ------ Zn²⁺ (aq) + Cu(s)    (overall)

Cell notation : the coefficient is not involve


 
WRITING SPONTANEOUS REDOX REACTION



Zn(s) + Cu²⁺ (aq) ------ Zn²⁺ (aq) + Cu(s)


 
Zn     - stronger reducing agent

Cu²⁺ - stronger oxidizing agent

Zn²⁺  - weaker (0.9)

Cu     - weaker (0.9) 

• stronger oxidizing agent : E⁰ larger (more positive)
  
• stronger reducing agent : E⁰ smaller (more negative)
  

Cu²⁺ (aq) + 2e^- Cu(s) E⁰: 0.34 V

Zn²⁺ (aq) + 2e^- Zn(s) E⁰: - 0.76 V

• How to determine anode (oxidation) and cathode (reduction) for spontaneous reaction, E⁰_cell > 0 ?
  
• Strong reducing agent = E⁰ larger (positive)
  

                                                 = E⁰ cathode (reduction)


Reduction: Cu²⁺ (aq) + 2e^- ------ Cu (s) E⁰ =0.34V

Oxidation :                  Zn (s) ------ Zn²⁺ (aq) + e^- E = - 0.76V

• Stronger reducing agent     = E⁰ smaller
  

                                                         = E⁰ anode (oxidation)



PREDICTING SPONTANEOUS REDOX REACTION USING DIAGONAL RULE

• Under standard – state condition, any species on the left of a given half-cell reaction will react spontaneously with a species that appear on the-right of any half-cell reaction located below it,
  
• diagonal rule !!!
  

Cu²⁺ (aq) + 2e^- ------ Cu (s)     E⁰ = 0.34V

 Zn²⁺ (aq) + 2e^- ------ Zn (s)      E⁰ = - 0.76V