Electrode Potential

Electrochemistry is the branch of chemistry which deals with the production of electricity when a chemical reaction takes place. The whole process is carried in a container or vessel called as a cell. When we construct a cell using a pattern of Daniel cell. We combine two different half cell. Each half cell consists of metallic electrode dipped in the electrolyte. If an electrode potential established. This will result in the formation of a battery. These batteries provide electrical current to start automobiles to power a host of products such as pocket calculator digital watches radio and tape recorder etc.

Must Read: What are Electrolytes and Types.

What is Electrode Potential

  • Oxidation – Loss of electrons.
  • Reduction – gaining of electrons.

Electrode potential develops only when there is a potential difference between the electrode and the electrolyte. If we place a  copper metal rod in the CuSO4 solution. The copper atoms have a behavior (tendency) to lose electrons to form Cu2+ ions which go into the solution as a result of oxidation.

Cu(s) → Cu2+ + 2e

Soon, excess electrons will accumulate on the electrode and it will acquire a negative charge. The electrons present at the electrode will also have a tendency to attract the positive ions (Cu2+ ions) present in the solution. This means that the electrode will not keep on losing electrons and an equilibrium will be ultimately established.Electrode Potential

When such an equilibrium is reached, the separation of positive and negative charges will occur leads to a potential difference between the metal electrode and its ions present in the solution called electrode potential.

Standard Reduction Potential

It should be remembered that:

  • An Electrode can undergo oxidation.
  • An electrode can also undergo reduction by gaining electrons.

An electrode can undergo oxidation by losing electrons, the positive ions present in the solution can also take up electrons from the electrode and the resulting lack of electrons on electrode than the initials. Thus, imparting a positive charge on it.  As a result, the electrode will now attract electrons from the solution and its electron attracting or accepting tendency is called reduction potential. Thus, there are two types of electrode potentials i.e., oxidation potential and reduction potential.

Oxidation potential. It is the tendency of the electrode to lose electrons and as a result, it gets oxidized.

Reduction potential. It is the tendency of the electrode to accept electrons and as a result, it gets reduced.

Standard Electrode Potential Definition

The potential difference established between the metal electrode and the electrolyte of its ions of unit molarity (1M) at 298 K.

In general, an electrode is said to be under standard conditions if:

  • Concentration of electrolyte solution is 1 molar (1M)
  • The pressure of gas/gases involved is one atmosphere
  • Temperature is constant (generally 298 K).

How to Calculate Standard Cell Potential

It should be noted that we cannot measure the cell potential directly due to the following reasons:

  •  A half cell whether oxidation or reduction half cell cannot work of its owns and can work only when connected to the other half cell.
  • The electron releasing or accepting tendency of an electrode only relative tendency and not absolute tendency.

Thus, we cannot determine the absolute standard electrode potential of an electrode. To solve the problem, a reference electrode is needed and must be assigned to it. The commonly used reference electrode is standard hydrogen electrode (SHE) also called normal hydrogen electrode (NHE) and its standard electrode potential (oxidation as well as reduction) is taken as zero.Electrode Potential

Determination of Eo value of zinc

An electrochemical cell is set up in which the zinc electrode placed in 1M ZnSO4 solution constitutes one-half cell while a standard hydrogen electrode as the other half cell. In this case, the reading by the voltmeter is 0.76 V and direction of flow of electrons is from zinc to hydrogen electron (opposite to the flow of current as indicated by the voltmeter). Thus, the zinc will act as anode and standard hydrogen electrode acts as a cathode.

Electron away = Anode.

Electrode Potential

  • EoCell = Eocathode – Eoanode
  • 0.76 = 0 + Eoanode
  • Eoanode= -0.76V
  • Thus, the standard reduction potential of zinc (Zn2+/Zn) is -0-76 V.
Cell Notation
Zn (s) | Zn2+ (aq) || 2H+ (aq) | H2  1 bar

In case, the deflection is towards the hydrogen electrode, this means that the flow of electrons is from hydrogen electrode towards the metal electrode. In such a case, hydrogen electrode will act as anode and the metal electrode as the cathode.

Standard Electrode Potential Table

The values represent the standard reduction potentials of the electrode. Their standard oxidation potential will have the same value but with an opposite sign. These values are the latest convention adopted by the IUPAC, both the standard reduction and oxidation potentials are also called as standard electrode potential. Electrode Potential

Standard Hydrogen Electrode

In a standard hydrogen electrode, a platinum wire coated with finely divided platinum called platinum black is sealed in a glass tube. It carries a platinum foil at one end. The wire is dipped in an aqueous solution containing 1 molar H+ ion concentration (normally 1 M HCl is used). Pure hydrogen gas under a pressure of 1 bar is constantly bubbled through the solution: The temperature of the solution is maintained at 298 K. A standard hydrogen electrode can act both as cathode and anode. When it is to act as an anode oxidation is to take place, and act as cathode reduction takes place.

The standard hydrogen electrode is also known as a reversible electrode.

Refer to the following video for standard hydrogen electrode

Normal Hydrogen Electrode

The earlier developments in the field of electrochemistry.There is a use the normal hydrogen electrode(NHE) having the zero electrode potential. The normal hydrogen electrode (NHE) is constructed in such a way having platinum electrode into a solution of 1 N strong acid. With the passage of time, it has been changed.

In short summary,

  • Normal hydrogen electrode – The Electrode potential of a platinum in terms of 1 N acid solution.
  • Standard hydrogen electrode- The Electrode potential of a platinum in terms of 1 M acid solution.

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