Standard Enthalpy of Formation: Explanation and Bond Dissociation Enthalpy

The standard enthalpy of formation or standard heat of formation of a compound is referred to as the change of enthalpy during the formation of one mole of the substance from its constituent elements.

• A pure element in its standard state possesses a standard enthalpy of formation of zero.
• Standard states are specified for different types of substances.
• For gas, it is the hypothetical state that it would assume if it followed the ideal gas equation at 1 bar of pressure.
• For a gaseous or solid solute in a diluted ideal solution, the standard state is the hypothetical solute concentration of exactly one mole per liter (1 M) at 1 bar pressure extrapolated from infinite dilution.
• The standard state of a pure substance or solvent in a condensed state (a liquid or a solid) is the pure liquid or solid at 1 bar of pressure.

Key Terms: Enthalpy, Bond Enthalpy, Standard formation of enthalpy, Pressure, Pure element, Thermodynamics, Internal energy, Volume, Temperature, Enthalpy change, Chemical reaction

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What is Enthalpy?

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The term “enthalpy” is specifically used in science and engineering in order to quantify heat and function. At a constant strain, the enthalpy notifies how much heat and effort has been applied or extracted from the substance.

• Enthalpy is the sum of internal energy and the product of pressure and volume in a thermodynamic system.
• Enthalpy is an energy-like characteristic or state function.
• It has the dimensions of energy and is thus measured in joules or ergs.
• Its value is determined only by the system's temperature, pressure, and composition, not history.
• In symbols, the enthalpy, H, is equal to the sum of the internal energy, E, and the product of the system's pressure, P, and volume, V: H = E + PV.

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Standard Enthalpy of Formation

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The typical enthalpy of the composition or the normal heat of the composition is the combination of the enthalpy while forming a single mole of the object from its objects, and all the elements in their common states.

• In other words, the standard enthalpy is the enthalpy change reaction where all the factors involved are in standard measurements.
• The enthalpy change of a chemical reaction is denoted by the symbol Δ_rH.

A stable integration condition is considered when the temperature is 298.15 K, and the atmospheric pressure is 1 atm. Now, let us understand this reaction by referring to some of the examples -

1. Formation of methane from hydrogen and oxygen

C (graphite, s) + 2H₂ (g) → CH₄ (g)

Δ_fH^o = -74.81kJmol^-1

2. Formation of hydrogen bromide from bromine and hydrogen

H₂ (g) + Br₂(l) → 2HBr(g)

Δ_rH^o = -72.81kJmol^-1

Here, we can say that by combining two or more reactants we can create a new mole of product. Also, a typical enthalpy of combustion of any object in its state is at zero levels by definition. Moreover, with the help of Hess's Law, we can calculate the enthalpy change of any chemical reaction using a small set of tabulated data.

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Standard Enthalpy of Combustion

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Enthalpy combustion of an object is an enthalpy change produced when a single mole molecule is completely burned in the air or oxygen at a given temperature.

• It can be used to compare which oils or substances produce the most energy when burned.
• They can be calculated using a calorimeter of a bomb.

Some examples are as follows:

H₂(g) + 1/2 O₂(g) → H₂O(l) ; Δ_cH^o = - 286 kJmol^-1

C₄H₁₀ (g) + 13/2 O₂(g) → 4CO2(g) + 5H₂O(l) ; Δ_cH^o = -2658 kJmol^-1

Significantly, combustion reactions are exothermic so the value for the enthalpy change (is ΔH) is always negative.

• Although, in the conventional process, the molar enthalpy of combustion is considered a positive value.
• Therefore, the enthalpy combustion of an object is defined as the heat dissipated when a single mole of an object completely burns in oxygen.

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Bond Dissociation Enthalpy

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Cl₂(g) → 2Cl(g);

Δ_Cl–ClH⁰ = 242 kJ mol^-1

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Things to Remember

• The standard enthalpy of formation is the change of enthalpy during the formation of one mole of the substance from its constituent elements.
• Enthalpy is the sum of internal energy and the product of pressure and volume in a thermodynamic system.
• It has the dimensions of energy and is thus measured in joules or ergs.
• The standard enthalpy is the enthalpy change reaction where all the factors involved are in standard measurements.
• Enthalpy combustion is an enthalpy change produced when a single mole molecule is completely burned in the air or oxygen at a given temperature.

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