Electrochemistry : 10.1 Galvanic Cell

Electrochemistry

• Study of relationship between chemical change & electric work
  
  
  
  
  
  Oxidation
  
• Loss of electron by species accompanied byn an increase in oxidation number
  
  Ex:
  
  
  
  
  
  
  
  
  
  Reduction
  
• Gain electron by a species accompanied by a decrease number of oxidation
  
  Ex:
  
  
  
  
        Redox reaction
  
• Process which there are net movement of electrons from one reactant to another
  
• Also called oxidation – reduction process.
  
• Oxidation and reduction occur at the same time.
  
  Ex:
  
  
  
  
  
  
  
  Oxidizing agent
  • Substance that accepts electron in redox reaction and undergoes decrease number of oxidation.
    
    
    O.A is Fe₂O₃
    
    
    
    
    Reducing agent
    
  • Substance that donate electron in redox reaction and undergoes an increase oxidation number.
    
    Ex:
    
    
    
    CO is an reducing agent
    
    
    
    Three key point
    
  • Oxidation always accompany by reduction
    
  • Oxidizing agent reduced reducing agent oxidised

ELECTROCHEMICAL CELL

There are two type :

• Voltaic cell
  
• Electrolytic cell
  

VOLTAIC CELL

• Use spontaneous reaction to generate electric energy
  
• System does work on surrounding
  
• All batteries contains voltaic cell
  

ELECTROLYTIC CELL

• Use electrical energy to drive non-spontaneous reaction
  
• Surrounding do work on system
  
• Ex: electroplating & recovering metal from ores
  

ELECTRODES

• Object that conduct electricity between cell and surrounding
  
• 2 electrode (anode & cathode) are dipped into electrolyte
  
• ACTIVE ELECTRODES
  
• Involve in half reaction
  
• Ex: zinc (Zn), copper (Cu), Iron (Fe)

INACTIVE ELECTRODE

• If no reactant or product can be uses as electrode
  
• Ex: graphite (C), Platinum (Pt)
  
• The electrolyte solution contain all species involved in the half – reaction 

ANODE & CATHODE

• Oxidation half reaction takes place at cathode
  
• Electron given up by substance being oxidised (reducing agent) and leave the cell at anode
  
• Reducing half reaction takes place at cathode
  
• Electron are taken up by substance being reduced (oxidizing agent) & enters the cell at cathode
  

ELECTROLYTE

• Mixture of ions (usually in aqueos solution) that are involved in reaction or that carry charge.
  

ESSENTIAL IDEA OF VOLTAIC CELL

• Component of each half reaction (half – cell) are placed in a seperate container
  
• The two half cell are joined by circuit which consist a wire and a salt bridge
  

OXIDATION HALF CELL

• Metal bar (anode) is immersd in Zn²⁺ electrolyte
  
  Ex: ZnSO₄
  
• Zn is reactant in oxidation half reaction
  
• Zn conduct electricity and release electron out of its half cell
  
   
• Mass of Zn electrode decrease
  

REDUCTION HALF CELL

• A Cu metal bar (cathode) is immersed in a Zn²⁺ electrolyte
  
• Ex: CuSO₄ solution
  
• Cu is a product in reduction half reaction
  
• Cu conducts electricity and released electron into itself.
  
  
• Mass of copper electrode increase
  

RELATIVE CHARGE ON ELECTRODES

• Anode:
  

  
• Electron flow from anode to cathode through wire
  
• Cathode
  
  
• Electron are continously generated at anode * consume at cathode
  

SALT BRIDGE

• Contains a solution of non-reacting ion such as KCl,KNO_3, Na₂SO₄ acts as liquid wire (allowing ions to flow & complete the circuit.
  
  
  
  
  
  
  HOW DOES THE CELL MAINTAIN ITS ELECTRICAL NEUTRALITY 

Anode Half Cell	Cathode Half Cell
Zn 2+ ions enters the solution causing the excess of positive charge.	Cu2+ ions leave the solution causing the excess of negative charge
Cl- ions from the salt bridge move into anode (Zn) half cell	K+ ions from salt bridge move into cathode Cu half cell

  

CELL NOTATION

• Ex : Zn(s) l Zn²⁺ (aq) l l Cu²⁺ (aq) l Cu(s)
  
• Anode left, cathode right
  
• 'l' represent phase boundary
  
• Use ',' for component that are in the same phase
  
  Graphite l I^- (aq) l I(s) l l H⁺ (aq), MnO₄^- (aq), Mn²⁺ (aq) l graphite
  
• Sometimes we specify the concentration of dissolved component
  
  Zn(s) l Zn²⁺ (1 M) l Cu²⁺ (1 M) l Cu(s)
  
• Electrode appear ait far right and left notation.
  
• Half-cell component usually appear in the same order as in the half-reaction
  
  
  
      Cu ²⁺ (aq) + 2^e- ------ Cu (s)        (reducing)
                      Zn(s) ------ Zn²⁺ (aq) + 2e^- (oxidizing)
  
  Cu ²⁺ (aq) + Zn(s) ------ Cu (s) + Zn²⁺ (aq) (overall)

Cell notation: the coefficient is not involved.