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Standard Enthalpy Of Formation, Combustion And Bond Dissociation

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.

Table of Contents

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: 298.15k, pressure: 1 atm).  For example formation of methane from carbon and hydrogen:

\(\begin{array}{l}C (graphite, s) +2H_2 (g) \rightarrow CH_4 (g)\end{array} \)
\(\begin{array}{l}Δ_fH°\end{array} \)
\(\begin{array}{l}-74.81kJmol^{−1}\end{array} \)

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 formation of hydrogen bromide from hydrogen and bromine.

\(\begin{array}{l}H_2 (g) + Br_2 (l) \rightarrow 2HBr (g)\end{array} \)
\(\begin{array}{l}Δ_rH°\end{array} \)
\(\begin{array}{l} -72.81kJmol^{−1}\end{array} \)

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

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

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

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

Standard Enthalpy of Formation is Zero for

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.

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Bond Dissociation Enthalpy

Enthalpy of bond dissociation 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 differ from bond enthalpy values which is the average of some of all the bond dissociation energy in a molecule except, in case of diatomic molecules. For example:

\(\begin{array}{l} Cl_2(g) \rightarrow 2Cl(g)\end{array} \)
\(\begin{array}{l}Δ_{Cl–Cl}H^0\end{array} \)
\(\begin{array}{l}242 kJmol^{-1}\end{array} \)

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:

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

\(\begin{array}{l}C_4 H_{10} (g) + \frac{13}{2} O_2 (g) \rightarrow 4CO_2 (g) + 5H_2O (l)\end{array} \)
\(\begin{array}{l}Δ_cH°\end{array} \)
\(\begin{array}{l}-2658 kJmol{-1}\end{array} \)

Frequently Asked Questions – FAQs

What do you mean by 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 (298.15K) and at 1 bar pressure in their most stable reference states.

What is the equation of enthalpy?

In the symbols, the enthalpy, H, is equivalent to the sum of the internal energy, E, and the strain, 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.

Why is 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.

What is the difference between standard enthalpy of reaction and standard enthalpy of formation?

The normal formation enthalpy is the transition in enthalpy that accompanies from its elements the formation of one mole of the compound. The normal reaction enthalpy happens in a system where a chemical reaction converts one mole of matter.

What is enthalpy in simple terms?

Enthalpy is a term used in science and engineering where it is required to quantify heat and function. At constant strain, as a material varies, enthalpy informs how much heat and effort has been applied or extracted from the substance. Enthalpy is energy-like, but not the same.

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