Electrochemistry
Introduction to Electrochemistry
Electrochemistry is a branch of Physical Chemistry that studies the relationship between chemical reaction and production of electricity. In other words the Electrochemistry deals with the interaction between electrical energy and chemical change.
Batteries and fuel cells are worked on the theory of electrochemistry, using which chemical energy is changed into electrical energy.
Electricity produced by chemical changes in chemical reaction is environmental friendly also as it produces very less pollution or almost pollution free.
This subject has great importance; as in our body also transmission of sensory signals through cells to brain and from brain to cells and communication between cells are known to have electrochemical origin.
Electrochemical Cells
A device which can generate electrical energy from chemical reaction or facilitate chemical reaction through the introduction of electrical energy is known as Electrochemical Cell.
In an electrochemical cell redox reaction takes place and releases energy.
Redox Reaction
A chemical reaction in which oxidation and reduction both take place simultaneously is called Redox Reaction.
In a Redox Reaction one substance loses electrons while other gains electrons. Substance which loses electrons undergoes oxidation and which gains electrons undergoes reduction.
An electrochemical cell convert the chemical energy liberated during redox reaction is called a Galvanic or a Voltaic Cell.
And when energy is supplied from external source to an electrochemical cell, i.e. a Galvanic Cell, this electrical energy is used to carry non-spontaneous chemical reaction the electrical cell is called ELECTROLYTIC CELL.
Galvanic or Voltaic Cell
A Galvanic Cell is a Electrochemical cell that is used to converts the chemical energy obtained from a spontaneous redox reaction into electrical energy.
This Galvanic cell was named after Luigi Glavani an Italian physician, biologist and philosopher, or Alessandro Giuseppe Antonio Anastasio Volta, an Italian physicist and chemist.
A typical Galvanic Cell consists of two different metals, generally zinc and copper, connected by a salt bridge placed in two separate pot or individual half cells separated by a porous membrane. Zinc metal in the rod shape is placed in zinc sulphate solution and copper rod is placed in the copper sulphate solution. The cell converts the chemical energy liberated during the redox reaction to electrical energy and has an electrical potential equal to 1.1 V when concentration of Zn+2 and Cu+2 ions is unity (1 mol dm–3).
The Redox Reaction involves in a typical Galvanic or Voltaic Cell can be written as follows:
Zn(s) + Cu+2(aq) → Zn+2(aq) + Cu(s)
Redox Couples or Half Cells
The redox reaction in a Galvanic Cell takes place in two different portions.
The reduction half reaction takes place on the copper electrode while the oxidation half reaction takes place on the zinc electrode.
These two portions of the cell are called Half Cells or Redox Couples also.
Reduction half reaction
Cu2+ + 2e– → Cu(s)
Oxidation half reaction
Zn(s) → Zn2+ + 2e–
Combined form of above two half reactions
Zn(s) + Cu+2(aq) → Zn+2(aq) + Cu(s)
Anode
In an electrochemical cell the half cell in which oxidation takes place is called Anode. It has a negative potential with respect to the solution.
Cathode
In an electrochemical cell the half cell in which reduction takes place is called Cathode. It has a positive potential with respect to the solution.
Electrode Potential
An electrochemical cell releases energy because of potential difference developed during the redox reaction.
The potential difference develops between the electrode and the electrolyte is called Electrode Potential.
Standard Electrode Potential
The electrode potential is known as Standard Electrode Potential when the concentrations of all the species involved in a half cell become unity or is unity.
Standard Reduction Potentials are now called Standard Electrode Potentials according to IUPAC convention.
Cell Potential
The potential difference between two electrodes of a galvanic cell is called the Cell Potential. Cell potential is measured in volts.
Cell Electromotive Force (emf)
When no current is drawn through the cell the cell potential is called Cell Electromotive Force (emf).
Representation of cell
Anode is kept on the left and cathode is at right and a vertical line is put between metal and electrolyte solution while representing a Galvanic Cell conventionally. Two electrolyte connected with a salt bridge is represented by a double vertical line.
Example:
Cu(s) | Cu+2(aq) || Ag+ | Ag(s)
Emf of a cell is positive and is given by the potential of the half cell on the right hand side minus the potential of the half cell on the left side.
Ecell = Eright – Eleft
Example: Let consider a redox reaction of a Galvanic Cell
Cu(s) + 2Ag+(aq) → Cu2+(aq) + 2Ag(s)
Half cell reaction:
Cathode (reduction)
2Ag(aq)+ + 2e– → 2Ag(s)
Anode (oxidation)
Cu(s) → Cu2+(aq) + 2e–
Above reaction can be represented as
Cu(s) | Cu+2(aq) || Ag+ | Ag(s)
Thus, Electromotive force (emf)
Ecell = Eright – Eleft
= EAg+|Ag – ECu2+|Cu
Measurement of Electrode Potential
However, Electrode Potential is the potential difference between the electrode and electrolyte, but electrode potential of a half cell cannot be measured. Because to connect voltmeter between electrode and electrolyte is not possible, so electrode potential of a half cell can be measured using a standardized system called reference electrode.
The system to measure electrode potential is called The Standard Hydrogen Electrode
Structure of Standard Hydrogen Electrode
The standard hydrogen electrode consists of a platinum electrode coated with platinum black. The electrode is dipped in an acidic solution and pure hydrogen gas is bubbled through it. The concentration of both the reduced and oxidized forms of hydrogen is maintained at unity. This implies that the pressure of hydrogen gas is one bar and the concentration of hydrogen ion in the solution is one molar.
Measuring the Electrode Potential
The standard Hydrogen Electrode is attached to another half cell electrode potential of which cell is to be measured. By convention standard hydrogen electrode (reference cell) is taken as anode and other half cell is taken as cathode. A voltmeter is attached with the cell to measure the volt.
Thus electrode potential of cell, i.e. the cell potential (E0cell) will be equal to electrode potential of right cell (cathode cell) minus electrode potential of left cell (anode).
⇒ E0cell = E0R – E0L
Where, E0cell = electrode potential of cell, i.e. cell potential
E0R = Electrode potential of right cell (anode)
E0L = Electrode potential of left cell (cathode)
If the concentrations of the oxidized and the reduced forms of the species in the right hand half cell are unity, then the cell potential is equal to the standard electrode potential.
As E0L for standard hydrogen electrode is zero
∴ E0cell = E0R – 0
⇒ E0cell = E0R
Measuring of Electrode Potential of half cell formed by copper metal
Let the Electrode Potential formed by copper metal is to be measured.
The standard Hydrogen Electrode is attached to half cell of copper metal. By convention standard hydrogen electrode is taken as anode (reference cell) and half cell formed by copper metal is taken as other half cell and taken as cathode.
The cell is constructed as follows:
Pt(s) | H2(g) | H+(aq 1M) || Cu2+(aq 1M)|Cu
The two half cell at equilibrium
At anode
2H+ + 2e– ⇌ H2At cathode
Cu2+(aq 1M) + 2e– ⇌ Cu(s)
The electrode potential of cell (E0cell), i.e. cell potential comes to 0.34 V.
Here positive value of the standard electrode potential indicates that Cu2+ ions get reduced more easily than H+ ions.
Measuring of Electrode Potential of half cell formed by zinc metal
Let the Electrode Potential of half cell formed by zinc metal is to be measured.
The standard Hydrogen Electrode is attached to half cell formed by zinc metal. By convention standard hydrogen electrode is taken as anode (reference cell) and half cell formed by zinc metal is taken as other half cell and taken as cathode.
The cell constructed can be represented as follows:
Pt(s) | H2(g) | H+(aq 1M) || Zn2+(aq 1M)|Cu
The two half cell at equilibrium
At anode
2H+ + 2e– ⇌ H2
At cathode
Zn2+(aq 1M) + 2e– ⇌ Zn(s)
The electrode potential of cell (E0cell), i.e. cell potential comes to –0.76 V.
Here negative value of standard electrode potential indicates that hydrogen ion can oxidize zinc or zinc can reduce hydrogen ions.
Emf of a Daniel Cell
The half reaction for a Daniel cell can be written as
Left electrode
Zn(s) → Zn2+(aq) + 2e–
Right electrode
Cu2+(aq) + 2e– → Cu(s)
The overall reaction
Zn(s) + Cu2+ → Zn2+(aq) + Cu(s)
Thus, emf of the cell (E0cell)
= E0R – E0L
= 0.34V – (– 0.76) = 1.10 V
Importance of Standard Electrode Potential
A lot of information can be drawn out by knowing the value of standard electrode potential of half cell reduction reaction such as
(a) If the standard electrode potential of an electrode is greater than zero, then its reduced form is more stable compare to that of hydrogen gas.
(b) And if the standard electrode potential of an electrode is less than zero, the hydrogen gas is more stable than the redued form of the species.
Example:
(a) The standard electrode potential for fluorine is 2.87 V and is the highest. This shows that fluorine gas has the maximum tendency to get reduced to fluoride ions (F–) and therefore fluorine gas is the strongest oxidizing agent and fluoride ion is the weakest reducing agent.
(b) The standard electrode potential for lithium is lowest and is equal to –3.05. This indicates that the lithium ion is the weakest oxidizing agent while lithium metal is the most powerful reducing agent in an aqueous solution.
(c) If the standard electrode potential value for a specie is positive, then this will get reduced more easily than H+ ions, and hydrogen cannot oxidize the specie. And that specie does not dissolve in HCl. For example electrode of Cu has positive value of standard electrode potential and thus, Cu does not dissolved in HCl.
In nitric acid Copper is oxidized by nitrate ion and not by hydrogen ion.
(d) If value of standard electrode potential of a specie is negative, this shows that hydrogen ion (H+) can oxidize the specie or the specie can reduce hydrogen ions. Such specie will dissolve in HCl. For example Zinc has negative value of standard electrode potential and thus get dissolved in HCl.
Use of Electrochemical Cells
To determine the pH of solutions
To determine the solubility product
To determine the equilibrium constant
To determine many thermodynamics properties
Used in potentiometric titrations
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