What is Electrochemistry?
Electrochemistry is the part of chemistry which deals with the chemical reactions which result in the production of electrical energy. In this part, how chemical energy is converted into electrical energy and vice-versa is studied. This energy is used in various kinds of cells, batteries and appliances. It mainly deals with such reactions that involve oxidation and reduction. These are fast, efficient, non-polluting and of great importance at the industrial level.
Types of Cells
The cell in which the chemical reaction occurs and chemical energy is used to convert into electrical energy is known as an electrochemical cell. This reaction can be spontaneous or non-spontaneous. In non-spontaneous cells, the energy is supplied from the outside to initiate the chemical reaction. Depending upon this, there are types of cells: Galvanic cells and electrolytic cells.
It is the cell in which chemical energy is converted into electrical energy. The chemical reaction is spontaneous and operates in two half cells. The half-cell in which oxidation occurs is known as an anode. The other half cell in which the reduction occurs is known as the cathode. These two half cells contain metal electrodes and they are dipped into their respective solutions. These two half cells are connected via a salt bridge. When the reaction proceeds, electrons flow from the anode to the cathode and electricity flows in the cells.
The common example of a galvanic cell is the Daniel cell in which Zn and Cu electrodes work as anode and cathode respectively. At anode, oxidation occurs and Zn acts as the reducing agent and Cu acts as the oxidizing agent. It can be detected from the oxidation state of each ion.
Oxidation-reduction process is the main key to the electrochemistry of these cells. These reactions occur at the metal electrodes and energy is produced due to the redox reaction.
As the name suggests, electrolysis occurs in such kinds of cells. In such cells, electrical energy is required to support the non-spontaneous reaction. The electrode at which reduction happens is known as cathode. The electrode at which oxidation happens is known as the anode. The chemicals used in such cells are known as electrolytes.
An example of an electrolytic cell is the electrolysis of water to gaseous hydrogen and oxygen.
Salt bridge is inserted in between the anodic and cathodic parts. It is used to connect the two different half cells. It maintains the charge flow in between these two half-cell electrolytes. The salt bridge has an inert electrolyte inserted in it. It can be sodium sulphate or potassium nitrate. The chemicals inserted in the salt bridge are inert in nature. Inert electrolytes are used so that they won't interfere with the reactions of anode and cathode. The negative ions or anions of the electrolytes move to the anodic part and positive ions or cations move to the cathodic part. This is how the charge is balanced in the electrochemical cells using a salt bridge.
In the electrochemical or electrolytic cells, which element will act as cathode or anode is decided by the standard electrode potential values. Standard electrode potential values are measured using hydrogen as the reference electrode. The table in which elements are arranged in the order of increasing electrode potential values comprises the electrochemical series.
Electrode potential is the property or the tendency of an element to gain or lose the electrons when placed in the solution of its own ions. When an electron is removed from an atom, it becomes a cation and when an electron is added to the atom, it becomes the anion. Depending upon the oxidation or reduction process an atom is undergoing, the electrode potential is often termed as oxidation potential or reduction potential.
Both the reduction and oxidation potential have the same values, they only have the opposite sign. The unit of electrode potential is Volt (V).
EMF of the Cell
EMF or electromotive force is the difference between the electrode potentials of the half cells in an electrochemical cell. It is calculated when there is no current flowing in the circuit of the electrochemical cell.
Ecell= Ecathode + Eanode
For cathode, the reduction potential value is taken and for anode, oxidation potential value is used.
IUPAC provided the notations and conventions for the electrochemical cells. The cell is represented as follows:
For a galvanic cell:
Zn (s)I Zn2+ (aq)(M) II Cu2+ (aq)(M)I Cu(s)
- | represents the cell boundary. The left side of these lines indicates the oxidation half-cell and the right side of the cell indicates the reduction half-cell.
- (s) and (aq) represents the state of the element, i.e., whether it is in a solid, aqueous or gaseous state.
- || represents the salt bridge.
- (M) represents the molar concentration of the individual ions in the solution.
The Nernst equation relates the electrode potential of the cell with the concentration. When current flows through the cell, EMF values change. This changed EMF value is calculated using Nernst equation.
For a galvanic cell, the Nernst equation can be written as:
- R = Gas constant
- F= Faraday’s constant
- n= number of electrons exchanged
- M1= concentration of species which is oxidized.
- M2= concentration of species which is reduced.
Applications of Nernst equation
- EMF of the cell can be calculated using the Nernst equation.
- Concentration of the ions in the solutions can be determined using the equation.
- Number of electrons exchanged in the solution can be calculated using the equation.
- Equilibrium constant for the reaction can be calculated using the equation.
- Once equilibrium constant is calculated, Gibbs free energy and other thermodynamic variables can be calculated.
It is a type of electrolytic cell. In this cell, two electrodes of the same element are dipped into the same solutions but at different concentrations. These two half cells are only different in concentrations, so they are called concentration cells. There will be a concentration difference between the two half cells and it will lead to the voltage generation so that the equilibrium is attained between the cells.
It is of two types:
- Electrode concentration cell
- Electrolyte concentration cell
When electrochemical cells are connected in the series to generate energy, they are known as batteries. They are highly important for commercial use. Batteries are one of the most important inventions of chemistry.
- There are two types of batteries- Primary and secondary.
- Examples of the batteries are dry cell, mercury cells, and lead storage batteries. The chemical used in batteries decides whether the battery is primary or secondary.
- Primary batteries are those which cannot be used after the materials have been consumed. They cannot be recharged while secondary batteries can be recharged and can be used for a long time.
Fuels cells are also a part of electrochemistry. They differ from the batteries in the aspect that the reactants are supplied from the outside whereas in batteries, the reactant is present inside the cell. The fuels are used in space vehicles.
The chemical energy is converted into electrical energy in the fuel cell. Example of the fuel cell is a hydrogen cell. Electrolyte present in the fuel cell is commonly KOH. Hydrogen is used as the common reactant or fuel.
Corrosion is the formation of a layer of oxides over the metal surface. This involves a redox reaction and it mostly occurs on the surface of the iron metal. In the presence of moisture and air, at the uneven part of the metal surface, the reaction gets initiated. Reduction of the oxygen forms water at the cathode. Oxidation occurs at the anode; Fe gets oxidized and reacts with water to form the rust.
Context and Applications
This topic is significant in the professional exams for both graduate and postgraduate courses, especially for Bachelors and Masters in Chemistry.
1) Is mercuric cell rechargeable?
Answer: The mercuric cell belongs to the category of primary batteries. This is used as long as the reactants are present inside the cell. As the reactants are consumed, the cell stops working. It cannot be recharged. It has been used in watches, calculators, and hearing aids. But they are now banned due to toxicity of the mercury.
2) What are fuel cells used for in electrochemistry?
Answer: They are mainly used in space shuttles and vehicles. They are the primary source of energy in space vehicles. Also, they are used in automobiles, cell vehicles, submarines, various industries and commercial buildings, and in remote areas as energy source.
3) Are galvanic cells spontaneous?
Answer: A spontaneous cell has negative Gibbs free energy. It does not require an external source of energy. The redox reaction releases the energy. This belongs to the category of electrochemical cells. So, a galvanic cell is spontaneous in nature. Electrochemistry deals with such kind of cells.
4) What is the use of a reference electrode?
Answer: In electrochemistry, a reference electrode is used to set an electrode potential value to the elements in the electrochemical cells. A hydrogen electrode is used for this purpose. In the electrochemical series, the electrode potential of every element is calculated with respect to the hydrogen electrode. It is also known as the standard hydrogen electrode (S.H.E.).
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