Bond Enthalpy and Hess Law

Introduction: What is Bond Enthalpy?

Bond enthalpy is the energy required to break a chemical bond in a gaseous state at 298K. It is usually expressed in terms of KJ / mol. In other words, it can be stated as the energy needed to break one mole of the bond into its constituent atoms in a gaseous state. After the cleavage of the bond, the molecules dissociate into its free radicals. Bond enthalpy is also referred to as bond energy.

Breaking of bond- an endothermic process:

Enthalpy is the amount of heat energy contained in a system. The cleavage of the bond also requires some amount of energy. The process which absorbs the heat is called the endothermic process. Similarly, the breaking of bonds is an endothermic process as it requires energy to dissociate and form the constituent radicals. In short, we can say that breaking a bond is an endothermic process, while the formation of bonds is exothermic. To break a bond a positive change in enthalpy is required, while a negative change in enthalpy is accompanied by the formation of a bond.

Factors affecting the bond enthalpy:

The various factors that affect the bond enthalpy are:

• Size of an atom: Bond enthalpy and size of the atom are inversely proportional. Greater the size of an atom, less is the bond enthalpy.
• Electronegativity: These two are directly proportional to each other.Higher the electronegativity difference between the atoms, higher the bond enthalpy.
• Bond length: These are inversely proportional to each other. Longer the bond length, lesser the bond enthalpy.
• Bond order: Bond order and bond enthalpy are directly proportional. Higher the bond order greater is the bond enthalpy.

Hess law

Hess’ law of constant heat summation states is given by a Russian chemist Germain Hess. The Hess law is based on two other popular laws, the first law of thermodynamics and state function character of enthalpy.  It states that “regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes”.  It is the manifestation that enthalpy is a state function.

Mathematical representation:

 The enthalpy change for the overall process is the sum of the enthalpy change of the steps in the process:

Applications:

• It is used to determine the heat formation of substances as it cannot be estimated experimentally.
• It is used to calculate the formation of enthalpies of multi-step and multi different reactions taking place at the same time.
• It is also used in the calculation of bond energy and lattice energy.

Frequently asked questions

1. Define bond enthalpy.

Answer

Bond enthalpy is the energy required to break a chemical bond in a gaseous state at 298K. It is usually expressed in terms of KJ / mol. In other words, it can be stated as the energy needed to break one mole of the bond into its constituent atoms in a gaseous state.

2. Mention the factors affecting bond enthalpy.

Answer

The factors affecting bond enthalpy are:

• Size of an atom
• Electronegativity
• Bond length
• Bond order

3. State Hess law.

Answer

It states that “regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes”.  It is the manifestation that enthalpy is a state function.

4. Given that

2H2(g) + O2(g) ——> H2O(g) , DH = –115.4 kcal the bond energy of H–H and O = O bond respectively is 104 kcal and 119 kcal, then the calculate the  O–H bond energy in water vapour? 

Answer

 We know that heat of reaction

   ΔH = ΣB.E. (reactant) – ΣB.E (product)

   For the  given reaction,

  Given data are:

 ΔH = –115.4 kcal

 B.E. of H–H = 104 kcal

 B.E. of O=O = 119 kcal

Since water  molecule contains two O–H bonds

 –115.4 = (2 × 104) + 119 – 4 (O–H) bond energy

   ∴ 4 (O–H) bond energy = (2 × 104) + 119+115.4

      Hence,  O–H bond energy = (2×104) + 119 + 115.4 / 4 = 110.6 kcal mol^–1