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NCERT Class 12 Chemistry Textbook Chapter 3 With Answer PDF Free Download
Chapter 3: Electrochemistry
Electrochemistry is the study of the production of electricity from the energy released during spontaneous chemical reactions and the use of electrical energy to bring about non-spontaneous chemical transformations.
The subject is of importance both for theoretical and practical considerations.
A large number of metals, sodium hydroxide, chlorine, fluorine, and many other chemicals are produced by electrochemical methods.
Batteries and fuel cells convert chemical energy into electrical energy and are used on a large scale in various instruments and devices.
The reactions carried out electrochemically can be energy efficient and less polluting. Therefore,
the study of electrochemistry is important for creating new technologies that are eco-friendly.
The transmission of sensory signals through cells to the brain and vice versa and communication between the cells are known to have an electrochemical origin.
Electrochemistry, is, therefore, a very vast and interdisciplinary subject. In this Unit, we will cover only some of its important elementary aspects.
These reactions occur in two different portions of the Daniell cell. The reduction half-reaction occurs on the copper electrode while the oxidation half-reaction occurs on the zinc electrode.
These two portions of the cell are also called half-cells or redox couples. The copper electrode may be called the reduction half cell and the zinc electrode, the oxidation half-cell.
We can construct an innumerable number of galvanic cells on the pattern of the Daniell cell by taking combinations of different half-cells.
Each half-cell consists of a metallic electrode dipped into an electrolyte. The two half-cells are connected by a metallic wire through a voltmeter and a switch externally.
The electrolytes of the two half-cells are connected internally through a salt bridge as shown in Fig. 3.1. Sometimes, both the electrodes dip in the same electrolyte solution, and in such cases, we do not require a salt bridge.
At each electrode-electrolyte interface, there is a tendency for metal ions from the solution to deposit on the metal electrode trying to make it positively charged.
At the same time, the metal atoms of the electrode have a tendency to go into the solution as ions and leave behind the electrons at the electrode trying to make it negatively charged.
At equilibrium, there is a separation of charges and depending on the tendencies of the two opposing reactions, the electrode may be positively or negatively charged with respect to the solution.
A potential difference develops between the electrode and the electrolyte which is called electrode potential.
When the concentrations of all the species involved in a half-cell are unity then the electrode potential is known as the standard electrode potential According to the IUPAC convention, standard
reduction potentials are now called standard electrode potentials.
In a galvanic cell, the half-cell in which oxidation takes place is called the anode and it has a negative potential with respect to the solution.
The other half-cell in which reduction takes place is called the cathode and it has a positive potential with respect to the solution.
The standard electrode potentials are very important and we can extract a lot of useful information from them.
The values of standard electrode potentials for some selected half-cell reduction reactions are given in Table 3.1. If the standard electrode potential of an electrode is greater than zero then its reduced form is more stable compared to hydrogen gas.
Similarly, if the standard electrode potential is negative then hydrogen gas is more stable than the reduced form of the species.
It can be seen that the standard electrode potential for fluorine is the highest in the Table indicating that fluorine gas (F2 ) has the maximum tendency to get reduced to fluoride ions (F–) and therefore fluorine gas is the strongest oxidising agent and fluoride ion is the weakest reducing agent.
Lithium has the lowest electrode potential indicating that lithium-ion is the weakest oxidizing agent while lithium metal is the most powerful reducing agent in an aqueous solution.
It may be seen that as we go from top to bottom in Table 3.1 the standard electrode potential decreases and with this, decreases the oxidizing power of the species on the left and increases the reducing power of the species on the right-hand side of the reaction.
Electrochemical cells are extensively used for determining the pH of solutions, solubility product, equilibrium constant, and other thermodynamic properties and for potentiometric titrations.
| Author | NCERT |
| Language | English |
| No. of Pages | 30 |
| PDF Size | 1296 KB |
| Category | Chemistry |
| Source/Credits | ncert.nic.in |
NCERT Solutions Class 12 Chemistry Chapter 3 Electrochemistry
Q 3.1:
Arrange the following metals in the order in which they displace each other from the solution of their salts. Al, Cu, Fe, Mg, and Zn
Answer:
According to their reactivity, the given metals replace the others from their salt solutions in the said order: Mg, Al, Zn, Fe, Cu.
Mg: Al: Zn: Fe: Cu
Q 3.2:
Given the standard electrode potentials,
K+/K = –2.93V,
Ag+/Ag = 0.80V,
Hg2+/Hg = 0.79V
Mg2+/Mg = –2.37 V,
Cr3+/Cr = – 0.74V
Arrange these metals in their increasing order of reducing power.
Ans:
The reducing power increases with the lowering of the reduction potential. In order of given standard electrode potential (increasing order) : K+/K < Mg2+/Mg < Cr3+/Cr < Hg2+/Hg < Ag+/Ag
Thus, in the order of reducing power, we can arrange the given metals as Ag< Hg < Cr < Mg < K
Q 3.3 :
Depict the galvanic cell in which the reaction
Zn(s) + 2Ag+(aq) →Zn2+(aq) + 2Ag(s) takes place. Further show:
(i) Which of the electrode is negatively charged?
(ii) The carriers of the current in the cell.
(iii) Individual reaction at each electrode.
Ans :
The galvanic cell in which the given reaction takes place is depicted as:[latex]Zn_{ ( s ) } | Zn^{ 2+ }_{( aq )}||Ag^{ + }_{( aq )}|Ag_{( s )}[/latex]
(i) The negatively charged electrode is the Zn electrode (anode)
(ii) The current carriers in the cell are ions. Current flows to zinc from silver in the external circuit.
(iii) Reaction at the anode is given by :[latex]Zn_{ ( s ) }\rightarrow Zn^{ 2+ }_{( aq )} + 2 e^-[/latex]
Reaction at the anode is given by :[latex]Ag^{+}_{ ( aq ) } + e^- \rightarrow Ag_{( s )}[/latex]
Electrochemistry NCERT With Solution PDF Free Download