Tuesday, December 4, 2012

Galvanic Cell

1. Galvanic cell
    (a) Electrochemistry is the study of the relationship between electricity and chemical reactions.
    (b) Chemical reactions involved in electrochemistry are reduction and oxidation reactions (jointly termed ‘redox’).
    (c) Reduction
  • a species accept one or more electrons
  • oxidation number of the species decreases
  • takes place at cathode
    (d) Oxidation
  • a species loses one or more electrons
  • oxidation number of the species increases
  • takes place at anode
    Rules to remember: RED CAT (REDuction at CAThode)

    (e) Redox reaction in electrochemistry refers to reduction and oxidation reaction, in which electrons are transferred from a reducing agent (anode) to an oxidising agent (cathode).
    (f) There are two types of cells in electrochemistry, namely Voltaic or Galvanic cell and electrolytic cell.

2. Operation of a galvanic cell
    (a) Galvanic cell is an electrochemical cell that converts chemical energy from a spontaneous redox reaction into electricity.
    (b) A galvanic cell is built from one half-cell make up a meal (M) dipped into an aqueous solution of its ions (Mx+) in a container, and another half-cell made up of a metal (N) dipped into an aqueous solution of its ions (Ny+) in another container.
    (c) The two metals are connected by a wire, while the containers are connected by a salt bridge.
    (d) A voltmeter is normally used to detect the amount of electric current generated.
    As electric current is generated in the cell, the zinc electrode slowly dissolves. More copper is deposited on the copper electrode and the blue colour of CuSO4 solution fades.
    (e) At Zn electrode
  • Zinc metal loses electrons and oxidised to Zn2+ ions
  • Zinc metal is the anode
  • Anode is the negative electrode since it is a source of electorns
    (f) At Cu electrode
  • Copper (II) ions accept electrons and reduced to copper
  • Copper metal is the cathode
  • Cathode is the positive electrode because it attracts electrons
        Direction of the flow of electrons: anode → cathode
    (g) Half-cell equations and overall cell reaction equation
        Anode:    Zn (s) → Zn2+ (aq) + 2e-
        Cathode: Cu2+ (aq) + 2e- → Cu (s)
        Overall reaction: Zn (s) + Cu2+ (aq) → Zn2+ (aq) + Cu (s)

    (h) Cell notation
        Zn (s) | Zn2+ (aq) || Cu2+ (aq) | Cu (s)

        Rules to remember: ABC rule (Anode-Bridge-Cathode)

3. Function of the salt bridge
  • To complete the circuit by allowing the ions move from one half-cell to the others
  • To maintain the electrical neutrality   
    Without a salt bridge: The reaction will stop quickly and no voltage is produced. Thus, the reading of the voltmeter or galvanometer will drop to zero quickly.

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