Enthalpy is the measurement of heat or energy in the thermodynamic system. It is the most fundamental concept in the branch of thermodynamics. It is denoted by the symbol H.
In other words, we can say, Enthalpy is the total heat of the system.
Table of Contents
Let’s know more about Enthalpy in detail below.
Enthalpy Definition
Enthalpy is total energy of heat in the system which is equivalent to the sum of total internal energy and resulting energy due to its pressure and volume. The system has internal energy because of the molecule in motion and the state of molecules as well. The molecules in motion generate kinetic energy and due to vibrations and electric energy of atoms, the system can have energy in the potential form as well.
Other than this, internal energy also includes energy stored in the form of chemical bonds as we know the breaking of bonds releases energy in an exothermic reaction.
Enthalpy Symbol
In thermodynamics, Enthalpy is denoted by H. The enthalpy change (ΔH) plays a crucial role in quantifying the heat exchange between a system and its surroundings.
Enthalpy Units
Enthalpy is typically measured in units of energy per mole, such as joules per mole (J/mol) in the International System of Units (SI) or calories per mole (cal/mol) in the calorie-based system.
Enthalpy Formula
Enthalpy can be represented as:
H = U + PV
where,
Enthalpy Change
Enthalpy is a state function (those functions which are only dependent on the initial and final state of the process, not the path taken by the process) as its constituents U, P, and V are state functions. As enthalpy is a state function, change in enthalpy (ΔH) will depend on the initial and the final states of the system.
Thus, change in enthalpy is represented by ΔH and is given by the following formula:
ΔH = H2 – H1
Where,
H1 is the Initial State Enthalpy of System
H2 is the Final State Enthalpy of System
As we know, the formula for Enthalpy is H = U + PV, and then
H1 = U1 + P1V1
H2 = U2 + P2V2
Using, the values of H1 and H2, value of ΔH will be,
ΔH = (U2 + P2V2) – (U1+ P1V1)
⇒ ΔH = U2 + P2V2 – U1 -P1V1
⇒ ΔH = (U2 – U1) + (P2V2 -P1V1)
ΔH = ΔU + Δ(PV)
where,
ΔU is the Change in Internal Energy
Δ(PV) is the Change in Product of Pressure and Volume
Now, at a constant pressure P1 = P2 = P (Isobaric Process)
ΔH = ΔU + PΔV
Consider pressure inside and outside are the same for this isobaric process (i.e. Pex = P) then the formula for the isobaric process will become,
Qp = ΔU +PΔV
Thus from the above two equations, we get,
ΔH = Qp
Thus from this derived formula, we understand that the increase in enthalpy of a system is equal to the heat absorbed by it at a constant pressure.
Enthalpy of Fusion
Enthalpy of Fusion is the amount of heat energy required to convert a unit mass of a solid at its melting point into a liquid without an increase in temperature. It changes with the increase in temperature and other parameters.
Enthalpy of Vaporization
Enthalpy of Vaporization is the amount of heat energy required to convert a unit mass of a liquid at its boiling point into a vapor state without an increase in temperature. Its symbol is ∆H vap. Enthalpy of Vaporization changes with increases with temperature and other parameter.
Enthalpy of Freezing Water
Enthalpy of freezing water is the heat change required to change liquid ice to water and the its is equal to -6.00 kJ/mole.
Ionization Enthalpy
Ionization Enthalpy of an element is defined as the amount of energy required to remove an electron from the isolated gaseous in its gaseous state. Ionization energy depends on the force of attraction of electrons and the nucleus.
Ionization Enthalpy decreases from top to down in a Group.
Ionization Enthalpy increases from left to right in a Group.
Activation Enthalpy
Activation Enthalpy of the reaction is defined as the energy required to proceed a reaction. It is the minimum amount of energy that is necessary for the reactants in a chemical reaction to proceed and form the product.
Relationship between ΔH and ΔU
As we already established that ΔH and ΔU are related by the equation ΔH = ΔU + PΔV, at constant pressure. For reactions between solids and liquids, ΔV is very small because as pressure varies, solids or liquids won`t get affected significantly. So, for these reactions remove PΔV from the equation and write ΔH = ΔU
However, for the reactions involving gases, which are easily affected by the change in pressure, ΔV should strictly be considered.
ΔH = ΔU + PΔV
⇒ ΔH = ΔU + P(V2 – V1)
⇒ ΔH = ΔU + PV2 – PV1
where,
V1 is the Volume of Gas Reactants in Initial State
V2 is the Volume of Gas Products in Final State
Here we consider the reactants and the product to be ideal, so we can use the ideal gas equation (PV = nRT). Let’s consider there are n1 moles of gaseous reactants that produce n2 moles of gaseous products. The ideal gas equation becomes
PV1 = n1RT and PV2 = n2RT
⇒ ΔH = ΔU + n2RT – n1RT
⇒ ΔH = ΔU + RT (n2 – n1)
⇒ ΔH = ΔU + RT Δn
Requirements for ΔH to be equal to ΔU
There are two cases when ΔH and ΔU become equal, which are as follows:
When the reaction is conducted inside a closed container it prevents the alteration of the volume of the system (ΔV = 0). Then change in enthalpy will change as ΔH = ΔU.
When there are only solids or liquids involved in the reactions then we can neglect ΔV as the change in them due to the pressure is significant. So, ΔH = ΔU.
There reaction in which the moles of gaseous products and reactants are the same (i.e. n2 = n1). So, ΔH =ΔU
Difference between Entropy and Enthalpy
Let’s understand the differences between Enthalpy and Entropy in the table below:
Enthalpy
Entropy
The total heat associated with a system is called the enthalpy of the system.
The measure of degree of randomness of the molecule is defined as the entropy of the molecule.
Enthalpy is measured in regular conditions.
Entropy is measured under controlled conditions.
Enthalpy of any reaction is measured in Joule per Mole.
Entropy of any reaction is measured in Joule per Kelvin.
Enthalpy change of any reaction must be minimum in any process.
Entropy change of the system must be maximum in any process.
Endothermic and Exothermic Reactions
A reaction is a process in which two or more two reactants react to form some products we can have a reaction in which we are required to give some energy on the other hand some redactions can give energy to the products. So on this basis, we can have two types of reactions that include
Endothermic Reactions
Exothermic Reactions
Endothermic Reactions
If in any chemical reaction heat is given to the reaction for it to proceed this reaction is called the endothermic reaction. Thus, an endothermic reaction takes heat from the surrounding and makes the surroundings cooler.
Exothermic Reactions
If in any chemical reaction heat is produced as the result of the reaction then it is called the exothermic reaction. Thus, an exothermic reaction gives heat to the surrounding and makes the surroundings warmer.
Work done in Chemical Reactions
The work done at constant pressure and temperature by a system is given by
W = – Pex × ΔV
Assume Pex = P, then the equation becomes
W = -P( V2 – V1)
⇒ W = PV1 – PV2
Using the Ideal gas equation,
⇒ W = n1RT – n2RT
⇒ W = -RT (n2 – n1)
⇒ W= – RT Δn
Sample Problems on Enthalpy Formula
Problem 1: For a reaction, the system absorbs 10 kJ of heat and does 3 kJ of work on its surroundings. What are the changes in the Internal energy and Enthalpy of the system?
Solution:
According to the First law of thermodynamics,
ΔU = Q + W
Q = +10 kJ and W = -3 kJ
(W = -3 kJ because the work is done on the surrounding by the system so the system has lose that energy)
ΔU = 10 kJ – 3 kJ
∴ ΔU = +7 kJ
and, Qp = ΔH
∴ Qp = +10kJ
Thus, the Internal energy increases by 7 kJ and Enthalpy by 10 kJ.
Problem 2: An Ideal gas expands from a volume of 5 dm3 to 15 dm3 against a constant external pressure of 3.036 x 105Nm-2. Find ΔH if ΔU is 400 J.
Problem 3: Calculate the work done in the following reaction when 2 moles of HCl are used at Constant pressure at 420 K.
4HCl (g) + O2 (g) → 2Cl2 (g) + 2H2O (g)
State, whether the work done, is by the system or on the system.
Solution:
According to the Formula to calculate the work done in chemical reactions,.
W = – Δn RT
⇒ W = – RT ( n2 – n1 )
2 moles of HCl react with 0.5 mole of O2 to give 1 mole of Cl2 and 1 mole of H2O
Hence, n1 = 2.5, n2 = 2, R = 8.314 JK-1 mol-1 , T = 420 K
Substituting the values in the equation,
W = – 8.314 J K-1 mol-1 × 420 K × (2 – 2.5) mol
⇒ W = -8.314 × 420 × (-0.5) J
⇒ W = 1745.94 J
Problem 4: Calculate the change in enthalpy (ΔH) for the combustion of methane (CH4) if the standard enthalpy of formation of methane is -74.8 kJ/mol.
The combustion reaction of methane is:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
The standard enthalpy of the formation of CO2(g) is -393.5 kJ/mol and the standard enthalpy of the formation of H2O(l) is -285.8 kJ/mol.
Solution:
To calculate the ΔH for the combustion of methane, we need to use the standard enthalpies of formation of the reactants and products. The ΔH can be calculated using the formula:
ΔH = ΣnΔHf(products) – ΣnΔHf(reactants)
where n is the stoichiometric coefficient of each species in the balanced chemical equation.
Therefore, the change in enthalpy for the combustion of methane is -727.4 kJ/mol.
Enthalpy – FAQs
What does Enthalpy mean?
Enthalpy is a thermodynamic property representing the total heat content of a system.
What is Internal Energy?
Every substance is associated with a definite amount of energy. This energy is stored in a substance (System) is called its internal energy and is denoted by U. The internal energy is the sum of kinetic energies of all the molecules, ions and atoms of the system, the potential energies associated with the forces between the particles, the kinetic and potential energies of nuclei and electrons in the particles and the energy associated with existence of mass of the system.
How to Calculate Change in Enthalpy?
The change in enthalpy ( ΔH) can be obtained by
ΔH = ΔU + RT Δn
What is Isobaric Process?
Most chemical reactions are run in open containers under constant pressure. In such reactions the volume of the system is allowed to change, such kind of processes are called as Isobaric processes.
Examples:
Boiling of Water and its Conversion into Steam
Freezing of Water into Ice
What is Relation between Enthalpy Change and Internal Energy Change?
Relation between enthalpy change and internal energy change is as follows:
ΔH = ΔU + Δ(PV)
where,
ΔU is the Change in Internal Energy
Δ(PV) is the Change in Product of Pressure and Volume
What is First Law of Thermodynamics?
First Law of Thermodynamics is simply the law of conversation of energy. According to this law the total energy of a system and its surroundings remain constant when the system changes from initial state to final state. The law is stated in different ways but the meaning is the same that the energy is conserved in all the changes.
Mathematical expression for the First Law of Thermodynamics,
ΔU = Q + W
What is SI Unit of Enthalpy?
The SI unit of enthalpy is the joule (J). Enthalpy is measured in joules per mole (J/mol) or simply joules (J), depending on the context.
What is the difference between Enthalpy and Entropy?
Enthalpy is the measure of the overall amount of energy in the system whereas Entropy is the measure of the randomness in the system.
How to Find Enthalpy of a Reaction?
Enthalpy of a reaction is calculated using the formula, ∆H = m x s x ∆T.
What is Enthalpy of Fusion?
Enthalpy of Fusion is also called Latent Heat of Fusion, is the amount of Energy supplied to a solid substance to convert its state to the liquid state.
What is Enthalpy of Vaporization?
Enthalpy of Vaporization is also called Latent Heat of Vaporization, is the amount of Energy supplied to a liquid substance to convert its state to the vapor state.
What is Enthalpy of Atomization?
Enthalpy of atomization, often denoted as ΔHᵥ, is the amount of energy required to break apart one mole of a compound’s molecules into individual atoms in their gaseous state.
What is Enthalpy of Formation?
Enthalpy of formation, often denoted as ΔHᵒf, is the change in enthalpy that occurs when one mole of a compound is formed from its constituent elements in their standard states.
What is Enthalpy of Solution?
Enthalpy of solution, often denoted as ΔHsol ,is a thermodynamic quantity that represents the heat change associated with the dissolution of a substance in a solvent.
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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".
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CBSE Class 11 Chemistry Syllabus is a vast which needs a clear understanding of the concepts and topics. Knowing CBSE Class 11 Chemistry syllabus helps students to understand the course structure of Chemistry.
Unit-wise CBSE Class 11 Syllabus for Chemistry
Below is a list of detailed information on each unit for Class 11 Students.
UNIT I – Some Basic Concepts of Chemistry
General Introduction: Importance and scope of Chemistry.
Nature of matter, laws of chemical combination, Dalton’s atomic theory: concept of elements, atoms and molecules.
Atomic and molecular masses, mole concept and molar mass, percentage composition, empirical and molecular formula, chemical reactions, stoichiometry and calculations based on stoichiometry.
UNIT II – Structure of Atom
Discovery of Electron, Proton and Neutron, atomic number, isotopes and isobars. Thomson’s model and its limitations. Rutherford’s model and its limitations, Bohr’s model and its limitations, concept of shells and subshells, dual nature of matter and light, de Broglie’s relationship, Heisenberg uncertainty principle, concept of orbitals, quantum numbers, shapes of s, p and d orbitals, rules for filling electrons in orbitals – Aufbau principle, Pauli’s exclusion principle and Hund’s rule, electronic configuration of atoms, stability of half-filled and completely filled orbitals.
UNIT III – Classification of Elements and Periodicity in Properties
Significance of classification, brief history of the development of periodic table, modern periodic law and the present form of periodic table, periodic trends in properties of elements -atomic radii, ionic radii, inert gas radii, Ionization enthalpy, electron gain enthalpy, electronegativity, valency. Nomenclature of elements with atomic number greater than 100.
UNIT IV – Chemical Bonding and Molecular Structure
Valence electrons, ionic bond, covalent bond, bond parameters, Lewis structure, polar character of covalent bond, covalent character of ionic bond, valence bond theory, resonance, geometry of covalent molecules, VSEPR theory, concept of hybridization, involving s, p and d orbitals and shapes of some simple molecules, molecular orbital theory of homonuclear diatomic molecules(qualitative idea only), Hydrogen bond.
UNIT V – Chemical Thermodynamics
Concepts of System and types of systems, surroundings, work, heat, energy, extensive and intensive properties, state functions. First law of thermodynamics – internal energy and enthalpy, measurement of U and H, Hess’s law of constant heat summation, enthalpy of bond dissociation, combustion, formation, atomization, sublimation, phase transition, ionization, solution and dilution. Second law of Thermodynamics (brief introduction) Introduction of entropy as a state function, Gibb’s energy change for spontaneous and nonspontaneous processes. Third law of thermodynamics (brief introduction).
UNIT VI – Equilibrium
Equilibrium in physical and chemical processes, dynamic nature of equilibrium, law of mass action, equilibrium constant, factors affecting equilibrium – Le Chatelier’s principle, ionic equilibrium- ionization of acids and bases, strong and weak electrolytes, degree of ionization, ionization of poly basic acids, acid strength, concept of pH, hydrolysis of salts (elementary idea), buffer solution, Henderson Equation, solubility product, common ion effect (with illustrative examples).
UNIT VII – Redox Reactions
Concept of oxidation and reduction, redox reactions, oxidation number, balancing redox reactions, in terms of loss and gain of electrons and change in oxidation number, applications of redox reactions.
UNIT VIII – Organic Chemistry: Some basic Principles and Techniques
General introduction, classification and IUPAC nomenclature of organic compounds. Electronic displacements in a covalent bond: inductive effect, electromeric effect, resonance and hyper conjugation. Homolytic and heterolytic fission of a covalent bond: free radicals, carbocations, carbanions, electrophiles and nucleophiles, types of organic reactions.
UNIT IX – Hydrocarbons
Classification of Hydrocarbons Aliphatic Hydrocarbons: Alkanes – Nomenclature, isomerism, conformation (ethane only), physical properties, chemical reactions. Alkenes – Nomenclature, structure of double bond (ethene), geometrical isomerism, physical properties, methods of preparation, chemical reactions: addition of hydrogen, halogen, water, hydrogen halides (Markovnikov’s addition and peroxide effect), ozonolysis, oxidation, mechanism of electrophilic addition. Alkynes – Nomenclature, structure of triple bond (ethyne), physical properties, methods of preparation, chemical reactions: acidic character of alkynes, addition reaction of – hydrogen, halogens, hydrogen halides and water.
Aromatic Hydrocarbons:
Introduction, IUPAC nomenclature, benzene: resonance, aromaticity, chemical properties: mechanism of electrophilic substitution. Nitration, sulphonation, halogenation, Friedel Craft’s alkylation and acylation, directive influence of functional group in monosubstituted benzene. Carcinogenicity and toxicity.
To know the CBSE Syllabus for all the classes from 1 to 12, visit the Syllabus page of CBSE. Meanwhile, to get the Practical Syllabus of Class 11 Chemistry, read on to find out more about the syllabus and related information in this page.
CBSE Class 11 Chemistry Practical Syllabus with Marking Scheme
In Chemistry subject, practical also plays a vital role in improving their academic scores in the subject. The overall weightage of Chemistry practical mentioned in the CBSE Class 11 Chemistry syllabus is 30 marks. So, students must try their best to score well in practicals along with theory. It will help in increasing their overall academic score.
CBSE Class 11 Chemistry Practical Syllabus
The experiments will be conducted under the supervision of subject teacher. CBSE Chemistry Practicals is for 30 marks. This contribute to the overall practical marks for the subject.
The table below consists of evaluation scheme of practical exams.
Evaluation Scheme
Marks
Volumetric Analysis
08
Salt Analysis
08
Content Based Experiment
06
Project Work
04
Class record and viva
04
Total
30
CBSE Syllabus for Class 11 Chemistry Practical
Micro-chemical methods are available for several of the practical experiments. Wherever possible such techniques should be used.
A. Basic Laboratory Techniques 1. Cutting glass tube and glass rod 2. Bending a glass tube 3. Drawing out a glass jet 4. Boring a cork
B. Characterization and Purification of Chemical Substances 1. Determination of melting point of an organic compound. 2. Determination of boiling point of an organic compound. 3. Crystallization of impure sample of any one of the following: Alum, Copper Sulphate, Benzoic Acid.
C. Experiments based on pH
1. Any one of the following experiments:
Determination of pH of some solutions obtained from fruit juices, solution of known and varied concentrations of acids, bases and salts using pH paper or universal indicator.
Comparing the pH of solutions of strong and weak acids of same concentration.
Study the pH change in the titration of a strong base using universal indicator.
2. Study the pH change by common-ion in case of weak acids and weak bases.
D. Chemical Equilibrium One of the following experiments:
1. Study the shift in equilibrium between ferric ions and thiocyanate ions by increasing/decreasing the concentration of either of the ions. 2. Study the shift in equilibrium between [Co(H2O)6] 2+ and chloride ions by changing the concentration of either of the ions.
E. Quantitative Estimation i. Using a mechanical balance/electronic balance. ii. Preparation of standard solution of Oxalic acid. iii. Determination of strength of a given solution of Sodium hydroxide by titrating it against standard solution of Oxalic acid. iv. Preparation of standard solution of Sodium carbonate. v. Determination of strength of a given solution of hydrochloric acid by titrating it against standard Sodium Carbonatesolution.
F. Qualitative Analysis 1) Determination of one anion and one cation in a given salt Cations‐ Pb2+, Cu2+, As3+, Al3+, Fe3+, Mn2+, Ni2+, Zn2+, Co2+, Ca2+, Sr2+, Ba2+, Mg2+, NH4+ Anions – (CO3)2‐ , S2‐, NO2‐ , SO32‐, SO2‐ , NO ‐ , Cl‐ , Br‐, I‐, PO43‐ , C2O2‐ ,CH3COO‐ (Note: Insoluble salts excluded)
2) Detection of ‐ Nitrogen, Sulphur, Chlorine in organic compounds.
G) PROJECTS Scientific investigations involving laboratory testing and collecting information from other sources.
A few suggested projects are as follows:
Checking the bacterial contamination in drinking water by testing sulphide ion
Study of the methods of purification of water.
Testing the hardness, presence of Iron, Fluoride, Chloride, etc., depending upon the regional variation in drinking water and study of causes of presence of these ions above permissible limit (if any).
Investigation of the foaming capacity of different washing soaps and the effect of addition of Sodium carbonate on it.
Study the acidity of different samples of tea leaves.
Determination of the rate of evaporation of different liquids Study the effect of acids and bases on the tensile strength of fibres.
Study of acidity of fruit and vegetable juices.
Note: Any other investigatory project, which involves about 10 periods of work, can be chosen with theapproval of the teacher.
Practical Examination for Visually Impaired Students of Class 11
Below is a list of practicals for the visually impaired students.
A. List of apparatus for identification for assessment in practicals (All experiments) Beaker, tripod stand, wire gauze, glass rod, funnel, filter paper, Bunsen burner, test tube, test tube stand, dropper, test tube holder, ignition tube, china dish, tongs, standard flask, pipette, burette, conical flask, clamp stand, dropper, wash bottle • Odour detection in qualitative analysis • Procedure/Setup of the apparatus
B. List of Experiments A. Characterization and Purification of Chemical Substances 1. Crystallization of an impure sample of any one of the following: copper sulphate, benzoic acid B. Experiments based on pH 1. Determination of pH of some solutions obtained from fruit juices, solutions of known and varied concentrations of acids, bases and salts using pH paper 2. Comparing the pH of solutions of strong and weak acids of same concentration.
C. Chemical Equilibrium 1. Study the shift in equilibrium between ferric ions and thiocyanate ions by increasing/decreasing the concentration of eitherions. 2. Study the shift in equilibrium between [Co(H2O)6]2+ and chloride ions by changing the concentration of either of the ions.
D. Quantitative estimation 1. Preparation of standard solution of oxalic acid. 2. Determination of molarity of a given solution of sodium hydroxide by titrating it against standard solution of oxalic acid.
E. Qualitative Analysis 1. Determination of one anion and one cation in a given salt 2. Cations – NH+4 Anions – (CO3)2-, S2-, (SO3)2-, Cl-, CH3COO- (Note: insoluble salts excluded) 3. Detection of Nitrogen in the given organic compound. 4. Detection of Halogen in the given organic compound.
Note: The above practicals may be carried out in an experiential manner rather than recording observations.
We hope students must have found this information on CBSE Syllabus useful for their studying Chemistry. Learn Maths & Science in interactive and fun loving ways with ANAND CLASSES (A School Of Competitions) App/Tablet.
Frequently Asked Questions on CBSE Class 11 Chemistry Syllabus
Q1
How many units are in the CBSE Class 11 Chemistry Syllabus?
There are 9 units in the CBSE Class 11 Chemistry Syllabus. Students can access various study materials for the chapters mentioned in this article for free at ANAND CLASSES (A School Of Competitions).
Q2
What is the total marks for practicals examination as per the CBSE Class 11 Chemistry Syllabus?
The total marks for the practicals as per the CBSE Class 11 Chemistry Syllabus is 30. It includes volumetric analysis, content-based experiment, salt analysis, class record, project work and viva.
Q3
Which chapter carries more weightage as per the CBSE Syllabus for Class 11 Chemistry?
The organic chemistry chapter carries more weightage as per the CBSE Syllabus for Class 11 Chemistry.
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