Nernst Equation-EMF of a Cell, Equilibrium Constant, Relation between Gibbs Free Energy and EMF

The electrical potential disparity across the cell membrane of all living cells is called the membrane potential, the inner part of the cell being negative compared to the outside. The magnitude of the membrane potential varies from cell to cell and in an exceptional cell following its functional state. For example, a nerve cell has a membrane potential of -70mv at rest, but the membrane potential drops to about +30mv when excited. The membrane potential at rest is called the resting potential. RMP is basically due to-

  • Uneven distribution of ions across the cell membrane due to its selective permeability.
  • Due to the combined effect of forces acting onions. This origin of RMP is dependent

Selective Permeability of Cell Membrane

The cell membrane is selectively permeable, i.e. it is freely permeable to K+ and Cl, from the medium to Na+, and is impermeable to proteins and organic phosphates which are negatively charged ions. Usually, the intracellular cation is K+ and major intracellular anions are proteins and organic phosphate. General extracellular cation is Na+ and anion is Cl

The presence of gated protein channels in the cell membrane is responsible for the variable permeability of ions. The forces executive on the ions across the cell membrane Production of variations in the membrane potential. The magnitude of forces acting thenceforward the cell membrane on each ion can be analyzed by the Nernst equation.

Concentration gradient: The Donnan effect results in an uneven distribution of diffuse ions across the cell membrane, in the form of additional diffuse cations, resulting in a concentration gradient.

Electrical gradient: As a consequence of concentration gradient cation K+, will try to disseminate back into ECF from ICF. But it is counteracted by an electrical gradient which will be created due to the impendence of nondiffusible anions within the cell. The membrane potential at which the electric force is equal in magnitude but opposite in the direction of the concentration force is called the equilibrium potential for that ion. The magnitude of the equilibrium potential is determined by the Nernst equation.

E = -RT/ZF In Cin/Cout

37 °C at normal body temperature, substituting for the constants (R, T, and F) and converting to the normal logarithm,

Em= -61.5 log Cin/Cout

Nernst equation

The Standard electrode potentials are measured at their standard states when the concentration of the electrolyte solution is fixed as 1M and the temperature is 298 K. Despite this, in actual practice, electrochemical cells do not always have a fixed concentration on the electrolyte solution. The electrode potential depends on the concentration of the electrolyte solution. The Nernst equation gave a relationship between the electrode potential and the concentration of electrolyte solutions, known as the Nernst equation. For a general electrode:

Mn+ + ne–  → M

Ecell = E°cell–(RT/nF)lnQ

Ecell = E°cell–(RT/nF)ln([C]c[D]d / [A]a[B]b)

where,

  • R  = The gas constant  (8.314JK-1mol-1)
  • F = Faraday constant (96,500Cmol-1)
  • T = Temperature in kelvin 
  • Q = Reaction quotient
  • n = Total number of moles of electrons translocate

It should be remembered that when writing the Nernst equation for the overall cell reaction, the log term is the same as the expression for the equilibrium constant for the reaction. The relation of both is similar.

Ecell = E°cell – (2.303 RT/nf) ln([C]c[D]d/ [A]a[B]b)

Ecell = E°cell  + 2.303 RT [A] [B]ln([A]a[B]b/ [C]c[D]d

Similarly, for the electrode reaction:

Mn+ + ne–  → M

The Nernst equation is- 

Ecell = E°cell – (2.303 RT/nf)log[1/Mn+]

or Ecell = E°cell  + (2.303RT / nF)log[1/Mn+]

when T = 273K, F=96500 Cmol-1, R=8.314JK-1mol-1 and concentration of solid M is taken as unit

Ecell = E°cell – (0.059/n)log[1/Mn+]

Relationship between equilibrium constant and standard potential of a cell

Ecell = Ecell° – (2.303 RT/nF)log[Kc], [Kc = equilibrium constant]

At equilibrium, Ecell =0

E°cell = (2.303 RT/nF)log[Kc

Kc = antilog[nEcell°/0.0591]

Limitations of Nernst Equation

  • The Nernst equation applies exclusively because no current flows through the electrodes. When current flows, the movement of ions at the electrode surface changes, and the conditions of excess potential and resistance loss contribute to the measured potential.
  • At very low concentrations of commutation potential-determining ions, the potential approach ± is found using the Nernst equation. This is corporeally useless, because, underneath such a situation, the commutation current density is reduced and tends to control the electrochemical behaviour of the system more than other effects.
  • Since the active coefficients are close to unity in dilute solutions, the Nernst equation can be expressed in the directly implicit form of the concentration. But in the case of higher concentrations, the actual activities of the ions must be used. This creates complexity for the use of the Nernst equation because estimating the non-ideal activities of ions usually requires experimental measurements.

Sample Questions

Question 1: Will the Eº value change when the coefficients in the chemical equation change?

Answer:

The Eº value does not depend on the coefficient in the chemical equation i.e. when we double or triple the coefficient, the E° value does not change.

For example:

  • Zn²+ 2e → Zn; E° =-0.76 V
  • 2Zn²+ 4e → 2Zn; E° =-0.76 V
  • 3Zn²+ 6e → 3Zn; E° =-0.76 V

In the half-reaction, if the coefficients change, the number of electrons will change to cancel out the effect of the change in n coefficients.

Question 2: Which reference electrode is used to measure the electrode potential of other electrodes?

Answer:

The standard hydrogen electrode is used as a reference electrode whose electrode potential is assumed to be zero. The electrode potential of the other electrode is measured concerning it.

Question 3: Zinc rod is dipped in 0.1M solution of ZnSO4. The salt is 95% dissociated at this dilution at 298 K. Calculate the electrode potential given that E (Zn²+ | Zn) = -0.76 V.

Answer:

The electrode reaction is :

Zn2+ + 2e⇆ Zn(s)

According to Nernst equation, at 298 K

E(Zn2+ |Zn)=E° (Zn2+ |Zn)- (0.059 /n) log [ angle n] [Zn]/[Zn2+ (aq)]

E° (Zn2+ |Zn)=-0.76 V, [Zn] = 1,

[Zn2+(aq)]=0.1*95/100=0.095 M

E(Zn2+ |Zn)=-0.76- (0.009/n )log1/-0.095

=-0.76-0.03=-0.79V

Question 4: What advantage do the fuel cells have over primary and secondary batteries?

Answer:

Primary batteries restrain delimited congeries of reactants and are destroyed when the reactants have been consumed. Secondary batteries can be recharged but retract a longer to recharge. The fuel cell is conducted consecutive as long as reactants are recoupment to it and products are continuously removed.

Question 5: How will the pH of the brine (aq. NaCl solution) be affected when it is electrolyzed?

Answer:

Since NaOH is formed during electrolysis, the pH of the brine solution will increase

Er. Neeraj K.Anand is a freelance mentor and writer who specializes in Engineering & Science subjects. Neeraj Anand received a B.Tech degree in Electronics and Communication Engineering from N.I.T Warangal & M.Tech Post Graduation from IETE, New Delhi. He has over 30 years of teaching experience and serves as the Head of Department of ANAND CLASSES. He concentrated all his energy and experiences in academics and subsequently grew up as one of the best mentors in the country for students aspiring for success in competitive examinations. In parallel, he started a Technical Publication "ANAND TECHNICAL PUBLISHERS" in 2002 and Educational Newspaper "NATIONAL EDUCATION NEWS" in 2014 at Jalandhar. Now he is a Director of leading publication "ANAND TECHNICAL PUBLISHERS", "ANAND CLASSES" and "NATIONAL EDUCATION NEWS". He has published more than hundred books in the field of Physics, Mathematics, Computers and Information Technology. Besides this he has written many books to help students prepare for IIT-JEE and AIPMT entrance exams. He is an executive member of the IEEE (Institute of Electrical & Electronics Engineers. USA) and honorary member of many Indian scientific societies such as Institution of Electronics & Telecommunication Engineers, Aeronautical Society of India, Bioinformatics Institute of India, Institution of Engineers. He has got award from American Biographical Institute Board of International Research in the year 2005.

CBSE Class 12 Chemistry Syllabus Download PDF

Below is the CBSE Class 12 Syllabus along with the marking scheme and time duration of the Chemistry exam.

S.NoTitleNo. of PeriodsMarks
1Solutions107
2Electrochemistry129
3Chemical Kinetics107
4d -and f -Block Elements127
5Coordination Compounds127
6Haloalkanes and Haloarenes106
7Alcohols, Phenols and Ethers106
8Aldehydes, Ketones and Carboxylic Acids108
9Amines106
10Biomolecules127
Total70

CBSE Class 12 Chemistry Practical Syllabus along with Marking Scheme

The following is a breakdown of the marks for practical, project work, class records, and viva. The total number of marks for all parts is 15. The marks for both terms are provided in the table below.

Evaluation Scheme for ExaminationMarks
Volumetric Analysis08
Salt Analysis08
Content-Based Experiment06
Project Work and Viva04
Class record and Viva04
Total30

CBSE Class 12 Chemistry Syllabus (Chapter-wise)

Unit -1: Solutions

  • Raoult's law.
  • Colligative properties - relative lowering of vapour pressure, elevation of boiling point, depression of freezing point, osmotic pressure, determination of molecular masses using colligative properties, abnormal molecular mass.
  • Solutions, Types of solutions, expression of concentration of solutions of solids in liquids, solubility of gases in liquids, solid solutions.
  • Van't Hoff factor.

Unit -2: Electrochemistry

  • Redox reactions, EMF of a cell, standard electrode potential
  • Nernst equation and its application to chemical cells
  • Relation between Gibbs energy change and EMF of a cell
  • Kohlrausch's Law
  • Electrolysis and law of electrolysis (elementary idea)
  • Dry cell-electrolytic cells and Galvanic cells
  • Conductance in electrolytic solutions, specific and molar conductivity, variations of conductivity with concentration.
  • Lead accumulator
  • Fuel cells

Unit -3: Chemical Kinetics

  • Rate of a reaction (Average and instantaneous)
  • Rate law and specific rate constant
  • Integrated rate equations and half-life (only for zerfirst-order order reactions)
  • Concept of collision theory (elementary idea, no mathematical treatment)
  • Factors affecting rate of reaction: concentration, temperature, catalyst;
  • Order and molecularity of a reaction
  • Activation energy
  • Arrhenius equation

Unit -4: d and f Block Elements  

  • Lanthanoids- Electronic configuration, oxidation states, chemical reactivity and lanthanoid contraction and its consequences.
  • Actinoids- Electronic configuration, oxidation states and comparison with lanthanoids.
  • General introduction, electronic configuration, occurrence and characteristics of transition metals, general trends in properties of the first-row transition metals – metallic character, ionization enthalpy, oxidation states, ionic radii, color, catalytic property, magnetic properties, interstitial compounds, alloy formation, preparation and properties of K2Cr2O7 and KMnO4.

Unit -5: Coordination Compounds  

  • Coordination compounds - Introduction, ligands, coordination number, color, magnetic properties and shapes
  • The importance of coordination compounds (in qualitative analysis, extraction of metals and biological system).
  • IUPAC nomenclature of mononuclear coordination compounds.
  • Bonding
  • Werner's theory, VBT, and CFT; structure and stereoisomerism

Unit -6: Haloalkanes and Haloarenes  

  • Haloarenes: Nature of C–X bond, substitution reactions (Directive influence of halogen in monosubstituted compounds only). Uses and environmental effects of - dichloromethane, trichloro methane, tetrachloromethane, iodoform, freons, DDT.
  • Haloalkanes: Nomenclature, nature of C–X bond, physical and chemical properties, optical rotation mechanism of substitution reactions.

Unit -7: Alcohols, Phenols and Ethers   

  • Phenols: Nomenclature, methods of preparation, physical and chemical properties, acidic nature of phenol, electrophilic substitution reactions, uses of phenols.
  • Ethers: Nomenclature, methods of preparation, physical and chemical properties, uses.
  • Alcohols: Nomenclature, methods of preparation, physical and chemical properties (of primary alcohols only), identification of primary, secondary and tertiary alcohols, mechanism of dehydration, and uses with special reference to methanol and ethanol.

Unit -8: Aldehydes, Ketones and Carboxylic Acids   

  • Carboxylic Acids: Nomenclature, acidic nature, methods of preparation, physical and chemical properties; uses.
  • Aldehydes and Ketones: Nomenclature, nature of carbonyl group, methods of preparation, physical and chemical properties, mechanism of nucleophilic addition, the reactivity of alpha hydrogen in aldehydes, uses.

Unit -9: Amines    

  • Diazonium salts: Preparation, chemical reactions and importance in synthetic organic chemistry.
  • Amines: Nomenclature, classification, structure, methods of preparation, physical and chemical properties, uses, and identification of primary, secondary and tertiary amines.

Unit -10: Biomolecules     

  • Proteins -Elementary idea of - amino acids, peptide bond, polypeptides, proteins, structure of proteins - primary, secondary, tertiary structure and quaternary structures (qualitative idea only), denaturation of proteins; enzymes. Hormones - Elementary idea excluding structure.
  • Vitamins - Classification and functions.
  • Carbohydrates - Classification (aldoses and ketoses), monosaccharides (glucose and fructose), D-L configuration oligosaccharides (sucrose, lactose, maltose), polysaccharides (starch, cellulose, glycogen); Importance of carbohydrates.
  • Nucleic Acids: DNA and RNA.

The syllabus is divided into three parts: Part A, Part B, and Part C. Part A consist of Basic Concepts of Chemistry, which covers topics such as atomic structure, chemical bonding, states of matter, and thermochemistry. Part B consists of Topics in Physical Chemistry, which includes topics such as chemical kinetics, equilibrium, and electrochemistry. Part C consists of Topics in Organic Chemistry, which covers topics such as alkanes, alkenes, alkynes, and aromatic compounds.

Basic Concepts of Chemistry:

  • Atomic structure: This section covers the fundamental concepts of atomic structure, including the electronic configuration of atoms, the Bohr model of the atom, and the wave nature of matter.
  • Chemical bonding: This section covers the different types of chemical bonds, including ionic, covalent, and metallic bonds, as well as the concept of hybridization.
  • States of the matter: This section covers the three states of matter - solid, liquid, and gas - and the factors that influence their properties.
  • Thermochemistry: This section covers the principles of thermochemistry, including the laws of thermodynamics and the concept of enthalpy.

Chapters in Physical Chemistry:

  • Chemical kinetics: This section covers the study of the rate of chemical reactions and the factors that influence it, including the concentration of reactants, temperature, and the presence of catalysts.
  • Equilibrium: This section covers the principles of chemical equilibrium, including the concept of Le Chatelier's principle and the equilibrium constant.
  • Electrochemistry: This section covers the principles of electrochemistry, including the concept of half-cell reactions, galvanic cells, and electrolysis.

Chapters in Organic Chemistry:

  • Alkanes: This section covers the properties and reactions of alkanes, including their structure, isomerism, and combustion.
  • Alkenes: This section covers the properties and reactions of alkenes, including their structure, isomerism, and addition reactions.
  • Alkynes: This section covers the properties and reactions of alkynes, including their structure, isomerism, and addition reactions.
  • Aromatic compounds: This section covers the properties and reactions of aromatic compounds, including their structure, isomerism, and electrophilic substitution reactions.

In addition to the topics covered in the syllabus, the CBSE Class 12 Chemistry exam also tests students on their analytical and problem-solving skills, as well as their ability to apply the concepts learned in the classroom to real-world situations.

Students can also check out the Tips for the Class 12 Chemistry Exam. They can easily access the Class 12 study material in one place by visiting the CBSE Class 12 page at ANAND CLASSES (A School Of Competitions). Moreover, to get interactive lessons and study videos, download the ANAND CLASSES (A School Of Competitions) App.

Frequently Asked Questions on CBSE Class 12 Chemistry Syllabus

Q1

How many chapters are there in the CBSE Class 12 Chemistry as per the syllabus?

There are 10 chapters in the CBSE Class 12 Chemistry as per Syllabus. Students can learn all these chapters efficiently using the study materials provided at ANAND CLASSES (A School Of Competitions).

Q2

What is the marking scheme for CBSE Class 12 Chemistry practical exam according to the syllabus?

The marking scheme for CBSE Class 12 Chemistry practical exam, according to the syllabus, is 8 marks for volumetric analysis, 8 marks for salt analysis, 6 marks for the content-based experiment, 4 marks for the project and viva and 4 marks for class record and viva.

Q3

Which is the scoring chapter in Chemistry as per CBSE Class 12 syllabus?

The chapter Electrochemistry in Chemistry is the scoring chapter as per CBSE Class 12 syllabus.