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How can we find emf of the half cell reaction??? And another question is how can we find cell potential in the case in which two two chemical equation shows reduction ??

How can we find emf of the half cell reaction??? And another question is how can we find cell potential in the case in which two two chemical equation shows reduction ??

Grade:9

1 Answers

jyoti bhatia
202 Points
5 years ago

The cell electromotive force, or cell EMF, is the net voltage between the oxidation and reduction half-reactions taking place between two redox half-reactions. Cell EMF is used to determine whether or not the cell is galvanic. The electromotive force (EMF) is the maximum potential difference between two electrodes of a galvanic or voltaic cell. This quantity is related to the tendency for an element, a compound or an ion to acquire (i.e. gain) or release (loss) electrons. For example, the maximum potential between Zn and Cu of a well known cell

Zn (s) | Zn2+ (1 M) || Cu2+ (1 M) | Cu (s)
has been measured to be 1.100 V. A concentration of 1 M in an ideal solution is defined as the standard condition, and 1.100 V is thus thestandard electromotive force, DEo, or standard cell potential for the Zn-Cu galvanic cell.

The standard cell potential, DEo, of the a galvanic cell can be evaluated from the standard reduction potentials of the two half cells Eo. The reduction potentials are measured against the standard hydrogen electrode (SHE):

Its reduction potential or oxidation potential is defined to be exactly zero. The reduction potentials of all other half-cells measured in volts against the SHE are the difference in electrical potential energy per coulomb of charge. Note that the unit for energy J = Coulomb volt, and the Gibbs free energy G is the product of charge q and potential difference E: G in J = q E in C V

Pt (s) | H2 (g, 1.0 atm) | H+ (1.0 M).
When a metal electrode is dipped in a solution containing its metal ions, a potential difference is developed at the metal solution interface. This potential difference is called the electrode potential.

For example, when a zinc rod is dipped in a solution containing Zn2+ ions, it oxidizes, and the Zn2+ ions pass from the zinc rod to the solution leaving excess of electrons at the zinc rod. Thus the zinc rod becomes negatively charged with respect to the solution and a potential difference is set up between the zinc rod and the solution. This potential difference is called the electrode potential of zinc. Similarly, when a copper rod is dipped in a solution containing Cu2+ions, the Cu2+ ions gain electrons from the copper rod leaving positive charge on the copper rod. As a result a potential difference is set up between the copper rod and the solution and is called the electrode potential of copper.

In an electrochemical cell, the anode has a negative potential and cathode has a positive potential.The potential of each individual half cell cannot be measured. We can measure only the difference between the potential of the two half cells.

The potential of a half cell can be measured by connecting it with Standard Hydrogen Electrode (SHE). The standard electrode potential of a SHE is assumed to be zero.

The electrode potential at standard conditions such as 25°C temperature, 1 atm pressure, 1 M concentration of electrolyte, is called the standard electrode potential. It is denoted by the symbol E0. The electrodes are arranged in the increasing order of their standard reduction potential and are called electrochemical series.Ions of opposite charge tend to associate into loosely-bound ion pairs in more concentrated solutions, thus reducing the number of ions that are free to donate or accept electrons at an electrode. For this reason, the Nernst equation cannot accurately predict half-cell potentials for solutions in which the total ionic concentration exceeds about 10–3 M.

 

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