Do you know what are electrochemical cells? Why are they so important? Electrochemistry is a branch of chemistry which deals with the study of the production of electricity from energy released during spontaneous chemical reactions and the use of electrical energy to bring about non-spontaneous chemical transformations. Let’s find out more.
Importance of Electrochemistry
- Production of metals like Na, Mg, Ca and Al
- Purification of metals
- Batteries and cells used in various instruments
Browse more Topics under Electrochemistry
- Conductance of Electrolytic Solutions
- Electrolytic Cells and Electrolysis
- Fuel Cells
- Galvanic Cells
- Nernst Equation
- Variation of Conductivity and Molar Conductivity with Concentration
- Metallic Conductors or Electronic Conductors: Those substances that allow the electric current to pass through by way of the movement of electrons are known as the metallic conductors, e.g.. metals.
- Electrolytic Conductors or Electrolytes: Those substances which allow the passage of electricity by way of the aqueous solution or the fused state and undergo chemical decomposition are called electrolytic conductors. Some examples are the aqueous solution of acid, bases and salts.
Types of Electrolytes
- Strong electrolytes: Those electrolytes which completely ionise or dissociate into ions are known as strong electrolytes. Some of the examples of strong electrolytes are HCl, NaOH, K2SO4.
- Weak electrolytes: Those electrolytes that dissociate partially (ex < 1) are known as weak electrolytes. Some of the examples of weak electrolytes are CH3COOH, H2CO3, NH4OH, H2S, etc.
General Representation of Electrochemical Cells
|Sign||Positive due to consumption of electrons||Negative due to release of electrons|
|Movement of electrons||Into the cell||Out of Cell|
Features of Electrochemical cells
Some of the features of electrochemical cells are:
- There is no evolution of heat.
- The solution remains neutral on both sides.
- The reaction and now of electrons stops after sometime.
An electrochemical cell of zinc and copper metals is known as Daniel cell. It is represented as below:
By convention, the Cathode is represented on the RHS and Anode on the LHS.
Functions of Salt Bridge
- Salt bridge allows the flow of current by completing the circuit.
- It assures electrical neutrality on both sides.
- The salt bridge usually comprises of the solution of strong electrolyte known as KNO3, KCL etc. In most cases, KCI is preferred because the transport numbers of K+and Cl– are almost same.
Transport Number or Transference Number
The current that passes through an electrolytic solution is carried via the ions. Now, the degree or the fraction of the current that is carried by the ion is known as the transport number or transference number. So,
Transport number of the cation nc = (current carried by cation/total current)
Transport number of the anion na = (current carried by anion/total current)
Evidently nc + na = 1
When an electrode is in direct contact with the solution of its ions in the half-cell, it has the tendency to either lose or gain electrons. This property of the electrode is known as the electrode potential. It is an intensive property i.e. independent of the number of species in the reaction. And is expressed in volts.
Now, when the electrode has the tendency to lose electrons, it is known as the oxidation potential. The oxidation potential of half-cell is inversely proportional to the concentration of ions in the solution.
In case the electrode has the tendency to gain electrons, it is known as the reduction potential. According to IUPAC convention, the reduction potential alone is called as the electrode potential unless it is specifically mentioned.
E°red = – E°oxidalion
It is actually not possible for you to determine the absolute value of the electrode potential. For this, a reference electrode [NHE or SHE] is required. In real terms, the electrode potential is mainly the difference between the potentials of two electrodes. This can be measured by combining them to give a complete cell.
Standard Electrode Potential
Standard electrode potential is the potential difference developed between a metal electrode and solution of ions of unit molarity (1M) at 1 atm pressure and 25°C (298 K). It is denoted by E°.
If the electrode of the potential is known, then it is called as the reference electrode. It can be a primary reference electrode such as hydrogen electrode or a secondary reference electrode such as calomel electrode.
Standard Hydrogen Electrode (SHE)
Standard Hydrogen Electrode (SHE) i.e. also known as the hydrogen electrode (NHE), comprises of platinum wire, carrying platinum foil coated with finely divided platinum black. The wire is sealed in a glass tube and is placed in a beaker that has 1 M HCl. The hydrogen gas at 1 atm pressure is bubbled through the solution at 298K. Half-cell is pt H2 (1 atm) H+ (1 M)
Solved Examples for You
Q: What is the electrode potential of Standard hydrogen electrode (SHE)?
Ans. The electrode potential of SHE has been fixed as zero at all temperatures.