Electromotive Force (EMF) of Electrochemistry Cells

Electromotive force (EMF) of an electrochemistry cell is the quantitative property which can be used to measure the performance of a cell for a real cell. EMF value must be a positive value. Cells with negative EMF values are not practically possible. EMF value is measured in volts (V).


Electromotive force is given by the following Expression.

Ecell (EMF) = ERHS - ELHS

E cell (EMF) = Ecathode - EAnode



What is ERHS and ELHS?

  • ERHS: Reduction potential of the electrode at the right hand side of the IUPAC convention
  • ELHS: Reduction potential of the electrode at the left hand side of the IUPAC convention

In a real cell EMF must always be positive (+). Cell with a negative (-) EMF is practically impossible.

Zn(s)|Zn2+(aq, 1 moldm-3,298K)||Cu2+(aq, 1 moldm-3, 298K) |Cu(s)


In the above cell,

  • Ecell (EMF) = 0.34V - (-0.76)
  • Ecell (EMF) = 1.10 V



Factors affecting the EMF of a cell

EMF of a cell depends on the following factors.

  • Concentration of the electrolyte
  • Nature of the electrolyte
  • Temperature
  • Partial pressure of a gas (For a gaseous electrode)


Important facts about electromotive force of a cell

  • EMF of a cell does not depend upon the size of the electrode. However it depends upon the type of the electrode.
  • Since the enthalpy of the cell reaction is always exothermic, the EMF increases with the decreasing temperature of the cell according to the Le-chatelier's principle. (Greater the rate of forward reaction, higher the EMF of the cell)
  • Since Zn and Cu are solid metals, their concentrations do not involve in the equilibrium. Therefore the EMF is independent from the surface area of the electrode.
  • Increased concentration of Cu2+ ion can favor the forward reaction. Therefore the EMF of the cell could be increased.
  • Similarly decreased Zn2+ concentration can maximize the EMF of the cell by favoring the forward reaction.



Example: Consider the following electrodes.

Al3+(aq) + 3e ⇌ Al(s), E = -1.66V

Ag+(aq) + e ⇌ Ag(s), E = 0.80V

  1. Write the anode reaction
  2. Write the cathode reaction
  3. Write the cell reaction
  4. Write the cell reaction during the discharge of the cell
  5. Draw the cell diagram (IUPAC convention)
  6. Calculate the EMF of the cell
  7. Explain the effect of the temperature on EMF of the cell


Answers


1. Anode reaction

Al(s) ⇌ Al3+(aq) + 3e


2. Cathode reaction

Ag+(aq) + e ⇌ Ag(s)


3. Cell reaction

Al(s) + 3Ag+(aq) ⇌ 3Ag(s) + Al3+(aq)


4. Cell reaction during the discharge of the cell

Al(s) + 3Ag+(aq) → 3Ag(s) + Al3+(aq)


5. Cell diagram (IUPAC convention)

Al(s)|Al3+(aq, 1 moldm-3,298K)||Ag+(aq, 1 moldm-3, 298K) |Ag(s)


6. Electromotive force (emf) of cell

  • Ecell (EMF) = 0.80V - (-1.66V)
  • Ecell (EMF) = 2.46V

7. Effect to EMF value by increasing and decreasing temperature of cell

If we increase the temperature of the system, the backward reaction will be more favored. Therefore EMF decreases upon increasing temperature. Therefore decreased temperature can increase the EMF of the cell by favoring the forward reaction.




Example: Consider the following electrodes.

Mg2+(aq) + 2e ⇌ Mg(s), E = -2.37V

Cl2(g) + 2e ⇌ Cl-(aq), E = 1.36V

  1. Write the anode reaction
  2. Write the cathode reaction
  3. Write the cell reaction
  4. Write the cell reaction during the discharge of the cell
  5. Draw the cell diagram (IUPAC convention)
  6. Calculate the EMF of the cell
  7. Explain the effect of the temperature on EMF of the cell


Answers


1. Anode reaction

Mg(s) ⇌ Mg2+(aq) + 2e


2. Cathode reaction

Cl2(g) + 2e ⇌ 2Cl-(aq)


3. Cell reaction

Mg(s) + Cl2(g) ⇌ Mg2+(aq) + 2Cl-(aq)


4. Cell reaction during the discharge of the cell

Mg(s) + Cl2(g) → Mg2+(aq) + 2Cl-(aq)


5. Cell diagram (IUPAC convention)

Mg(s)|Mg2+(aq, 1 moldm-3,298K)||Cl-(aq, 1 moldm-3, 298K) |Cl2(g)|Pt(s)


6. Electromotive force (emf) of cell

  • Ecell (EMF) = 1.36V - (-2.37V)
  • Ecell (EMF) = 3.73V




Questions






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