Monday, December 12, 2011

Galvanic Cell (Part 2)

1. Standard reduction/electrode potential (Eo)
    (a) Electrons in different types of metals have different potential energy. Electrode potential (E) is a measure of the half-cell to attract electrons. Its unit is volt (V).
    (b) Standard electrode potential (Eo) is a measure of the half-cell to attract electrons at standard conditions.
    Example:
    Cu2+ (aq) + 2e- → Cu (s)        Eo = +0.34 V
    Zn2+ (aq) + 2e- → Zn (s)         Eo = -0.76 V
    (c) Standard conditions refer to conditions where the temperature is 25oC, the pressure is 1 atm (for gases), and the concentration of ions in the electrolyte is 1.0 mol dm-3.

2. Standard cell potential (Eocell)
    (a) The potential difference between the two metals acts a driving force or ‘electrical pressure’ that pushes electrons through the wire. The potential difference is termed cell potential.
    (b) Standard cell potential (Eocell) is the difference between the standard reduction potential of the cathode and the standard reduction potential at the anode.
        Eocell = Eocathode - Eoanode
    @ Eocell = Eored + Eoox

    Example:
    Calculate the standard cell potential for the following reactions:
        Cu (s) + 2 Ag+ (aq) → Cu2+ (aq) + 2 Ag (s)

    Method 1
    Cu2+ (aq) + 2e- → Cu (s)        Eo = +0.34 V
    Ag+ (aq) + e- → Ag (s)           Eo = +0.80 V
   
    From the overall cell reaction equation, it shows that copper half-cell is the anode (since it undergoes oxidation: an increase in oxidation number) and silver half-cell is the cathode (since it undergoes reduction: a decrease in oxidation number).
    Eocell = Eocathode - Eoanode
            = +0.80 - (+0.34)
            = +0.46 V

    Method 2
    Cu (s) → Cu2+ (aq) + 2e-            Eo = -0.34 V
    2 Ag+ (aq) + 2e- → 2 Ag (s)        Eo = +0.80 V
    Cu (s) + 2 Ag+ (aq) → Cu2+ (aq) + 2 Ag (s)
   
    Eocell = Eored + Eoox
            = +0.80 + (-0.34)
            = +0.46 V

    (c) The electrode potential of zinc half-cell written as a reduction reaction is -0.76 V. When the zinc half-cell equation is written as an oxidation reaction, the sign of its Eo value is reversed. When a half-equation is written in reverse, the sign is reversed too, but the value remains the same.
        Zn2+ (aq) + 2e- → Zn (s)        Eo = -0.76 V
        Zn (s) → Zn2+ (aq) + 2e-        Eo = +0.76 V

    (d) The reduction half-cell reactions may be written with different coefficients for the species involved, but the values and signs of reduction potentials do not change.
        ½ Cl2 (g) + e- → Cl (aq)         Eo = +1.36 V
        Cl2 (g) + 2e- → 2 Cl- (aq)       Eo = +1.36 V

3. Relative strength of oxidising agents or reducing agents
    (a) Electrode potentials may be positive or negative. A half-cell with a positive electrode potential is able to attract electrons towards it. The more positive a half-cell’s electrode potential, the stronger is its attraction for electrons.
    (b) Species on the left-hand side of a reduction half-equation with a positive standard reduction potentials is an oxidising agent. The oxidising power is greater when the value of the reduction potential is more positive.
    (c) Species on the right-hand side of a reduction half-equation with a negative standard reduction potentials is a reducing agent. The reducing power is greater when the value of the reduction potential is more negative.
    Example:
    Ag+ (aq) + e- → Ag (s)           Eo = +0.80 V
    Cu2+ (aq) + 2e- → Cu (s)        Eo = +0.34 V
    Ni2+ (aq) + 2e- → Ni (s)          Eo = -0.25 V

    Increasing strength of oxidising agent: Ni2+ < Cu2+ < Ag+
    Increasing strength of reducing agent: Ag < Cu < Ni

4. Standard hydrogen electrode (SHE)
    Standard hydrogen electrode (SHE) is made up of a platinum electrode coated with finely divided platinum (to act as catalyst for the half-cell reaction), immersed in an aqueous solution of H+ ions (normally hydrochloric acid) of concentration 1.0 mol dm-3 and bubbled with hydrogen gas at 1 atm pressure. The temperature is kept constant at 25oC.
    

    (a) The standard reduction potential of SHE is assigned exactly 0 V.
    (b) The direction of the half-reaction of SHE depends on the other half-cell connected to it. SHE serves as a reference to determine the standard electrode (reduction) potential of other half-cell.

5. Determination of standard reduction electrode of zinc (Zn) half-cell
    

    (a) The voltmeter reading gives +0.76 V with a deflection of the needle indicating electrons are flowing from zinc electrode to SHE.
    (b) The half-cell equations and overall cell reaction equation
        Anode:     Zn (s) → Zn2+ (aq) + 2e-
        Cathode:  2 H+ (aq) + 2e→ H2 (g)
        Overall reaction:  Zn (s) + 2 H+ (aq) → Zn2+ (aq) + H2 (g)

    (c) Cell notation
        Zn (s) | Zn2+ (aq) || H+ (aq) | H2 (g) | Pt (s)

    (d) Standard reduction potential of zinc half-cell
        Eocell = Eocathode - Eoanode
        +0.76 = 0.00 - EoZn2+/Zn
        EoZn2+/Zn = -0.76 V

6. Determination of standard reduction electrode of copper(Cu) half-cell
   

    (a) The voltmeter reading gives +0.34 V with a deflection of the needle indicating electrons are flowing from SHE to copper electrode.
    (b) The half-cell equations and overall cell reaction equation
        Anode:     H2 (g) + 2e→ 2 H+ (aq)
        Cathode:  Cu (s) → Cu2+ (aq) + 2e-
        Overall reaction: H2 (g) + Cu (s) → 2 H+ (aq) + Cu2+ (aq)
   
    (c) Cell notation
        Pt (s) | H2 (g) | H+ (aq) || Cu (s) | Cu2+ (aq)

    (d) Standard reduction potential of copper half-cell
        Eocell = Eocathode - Eoanode
        +0.34 = EoCu2+/Cu - 0.00
        EoCu2+/Cu = +0.34 V

7. Spontaneity of a redox reaction
    A redox reaction is spontaneous when its cell potential (Ecell) is positive. Conversely, a redox reaction is non-spontaneous when its cell potential is negative.

    Example:
    Predict the spontaneity of the following reaction under standard conditions.
    Zn (s) + Sn4+ (aq) → Zn2+ (aq) + Sn2+ (aq)
    [EoZn2+/Zn = -0.76V; EoSn4+/Sn2+ = +0.13 V]

    Method 1
    From the overall cell reaction equation, it shows that zinc half-cell is the anode (since it undergoes oxidation: an increase in oxidation number) and tin half-cell is the cathode (since it undergoes reduction: a decrease in oxidation number).
    Eocell = Eocathode - Eoanode
            = +0.13 - (-0.76)
            = +0.89 V
    Thus, the redox reaction is spontaneous since the Eocell is positive.

    Method 2
    Zn (s) → Zn2+ (aq) + 2e-          Eo = +0.76 V
    Sn4+ (aq) + 2e- → Sn2+ (aq)    Eo = +0.13 V
    Zn (s) + Sn4+ (aq) → Zn2+ (aq) + Sn2+ (aq)

    Eocell = Eored + Eoox
            = 0.13 + 0.76
            = +0.89 V
    Thus, the redox reaction is spontaneous since the Eocell is positive.

1 comment:

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