Electrochemistry

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 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 E⁰value because electrode potential are intensive properties –does not depend on amount

E⁰_cellAND 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 H₂gas 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

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Electrochemistry