Standard Enthalpy of Formation & Combustion-Bond Dissociation Enthalpy

Best JEE & NEET Physics, Chemistry, Biology, and Math Study Material – Anand Classes by Neeraj Anand (Published by Anand Technical Publishers)

If you are preparing for JEE Mains, JEE Advanced, or NEET, having the right study material can make all the difference. Anand Classes, authored by Neeraj Anand and published by Anand Technical Publishers, provides one of the most comprehensive, structured, and exam-oriented study materials for Physics, Chemistry, Biology, and Mathematics Subjects according to latest trends of JEE & NEET Entrance Exams.

Why Choose Anand Classes Study Material for JEE & NEET Preparation?

The study materials by Neeraj Anand are designed to simplify complex concepts, provide in-depth explanations, and offer ample practice questions to help students achieve top scores in competitive exams.

Comprehensive Coverage – Detailed explanations of Physics, Chemistry, Biology, and Mathematics concepts.
JEE & NEET Focused – Designed specifically for competitive exam success.
Solved Examples & Practice Questions – Strengthen your understanding with concept-based problems.
Short Tricks & Formulas – Easy-to-remember techniques for quick problem-solving.
NCERT & Advanced Level Questions – Covers both board exams and entrance exam syllabus.

Subjects Covered in Anand Classes Study Material:

📘 Physics – Mechanics, Thermodynamics, Optics, Modern Physics, and more.
📕 Chemistry – Organic, Inorganic, and Physical Chemistry with problem-solving techniques.
📗 Biology – Genetics, Human Physiology, Ecology, and more for NEET aspirants.
📙 Mathematics – Algebra, Calculus, Trigonometry, Probability, and Coordinate Geometry.

📥 Download PDF & Buy Now!

Get the Best JEE & NEET Study Material by Anand Classes today! Strengthen your concepts, practice with important questions, and boost your exam preparation.

🔗 Visit Anand Technical Publishers for more details!


This post is optimized with SEO keywords such as:

  • Best JEE NEET Study Material
  • Physics Chemistry Biology Math Study Notes
  • Anand Classes by Neeraj Anand
  • JEE Mains & NEET Preparation Books PDF
  • Best Books for JEE & NEET 2025

As we know all reactions result in the formation of products from the reactants. Of all the reactions that take place, some absorb energy while other results in the evolution of energy.

Hence, we always experience a change in enthalpy whenever a reaction takes place. This enthalpy change is described as the enthalpy of reaction.

Here, we are going to deal with a few other enthalpy changes like enthalpy of formation, enthalpy of bond dissociation and enthalpy of combustion.

Standard Enthalpy of Formation

Standard enthalpy of formation is defined as the enthalpy change when one mole of a compound is formed from its elements in their most stable state of aggregation (stable state of aggregation at temperature: 298.15 K, pressure: 1 atm).  For example formation of methane from carbon and hydrogen:

\(\begin{array}{l}C (graphite, s) +2H_2 (g) \rightarrow CH_4 (g)\end{array} \)

\(\begin{array}{l}Δ_fH° = -74.81kJmol^{−1}\end{array} \)

Enthalpy of formation is basically a special case of standard enthalpy of reaction where two or more reactants combine to form one mole of the product. Let us take an example of the formation of hydrogen bromide from hydrogen and bromine.

\(\begin{array}{l}H_2 (g) + Br_2 (l) \rightarrow 2HBr (g)\end{array} \)

\(\begin{array}{l}Δ_rH° =  -72.81kJmol^{−1}\end{array} \)

As we can see in this case two moles of hydrogen bromide are produced. Hence, enthalpy of reaction cannot be taken as enthalpy of formation of hydrogen bromide rather we can say:

\(\begin{array}{l}Δ_r H° = 2Δ_fH° \end{array} \)

\(\begin{array}{l}Δ_r H° = enthalpy~ of ~reaction\end{array} \)

\(\begin{array}{l}Δ_fH° = enthalpy ~of~ formation\end{array} \)

Standard Enthalpy of Formation is Zero for an element

For an element: the form in which the element is most stable under 1 bar of pressure.

One exception is phosphorus, for which the most stable form at 1 bar is black phosphorus, but white phosphorus is chosen as the standard reference state for zero enthalpy of formation.

All elements in their standard states (oxygen gas, solid carbon in the form of graphite, etc.) have a standard enthalpy of formation of zero, as there is no change involved in their formation.

Bond Dissociation Enthalpy

Bond dissociation enthalpy is defined as the enthalpy change when one mole of covalent bonds of a gaseous covalent compound is broken to form products in the gaseous phase. Generally, enthalpy of bond dissociation values differs from bond enthalpy values which is the average of some of all the bond dissociation energy in a molecule except, in the case of diatomic molecules. For example:

\(\begin{array}{l} Cl_2(g) \rightarrow 2Cl(g)\end{array} \)

\(\begin{array}{l}Δ_{Cl–Cl}H^0 = 242 kJmol^{-1}\end{array} \)

Standard Enthalpy of Combustion

Standard enthalpy of combustion is defined as the enthalpy change when one mole of a compound is completely burnt in oxygen with all the reactants and products in their standard state under standard conditions (298K and 1 bar pressure). For example:

\(\begin{array}{l}H_2 (g) + \frac{1}{2} O_2 (g) \rightarrow H_2O (l); Δ_cH° = -286 kJmol^{-1}\end{array} \)

\(\begin{array}{l}C_4 H_{10} (g) + \frac{13}{2} O_2 (g) \rightarrow 4CO_2 (g) + 5H_2O (l)\end{array} \)

\(\begin{array}{l}Δ_cH° = -2658 kJmol{-1}\end{array} \)

Frequently Asked Questions – FAQs

Q1

What do you mean by the enthalpy of formation?

Formation enthalpy is the normal reaction enthalpy for the formation of the compound from its elements (atoms or molecules) at the chosen temperature (298.15 K) and at 1 bar pressure in their most stable reference states.

Q2

What is the equation of enthalpy?

In the symbols, the enthalpy, H, is equivalent to the sum of the internal energy, E, and the pressure, P, and volume, V, of the system: H = E + PV, respectively. Under the law of conservation of energy, the shift of internal energy is proportional to the heat transmitted to the device, minus the work performed by it.

Q3

Why is the enthalpy of formation important?

In the estimation of reaction enthalpies, enthalpies (or heats) of formation are incredibly useful. This is because it is possible to imagine any reaction as happening along a path through which all reactant compounds are first converted to elements and then all elements are converted into compounds of the substance.

Q4

What is enthalpy in simple terms?

Enthalpy is the measurement of energy in a thermodynamic system. The quantity of enthalpy equals the total content of heat of a system which is equivalent to the system’s internal energy plus the product of volume and pressure.

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.