Dear student,

For any particular chemical bond, say the covalent bond between hydrogen and oxygen, the amount of energy it takes to break that bond is exactly the same as the amount of energy released when the bond is formed. This value is called the bond energy.

There are many forms of energy:

• electrical
• mechanical
• chemical

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Askiitians Expert

Sagar Singh

B.Tech, IIT Delhi

bond energy (E) is a measure of bond strength in a chemical bond. It is the heat required to break Avogadro's number of molecules into their individual atoms. For example, the carbon-hydrogen bond energy in methane E(C–H) is the enthalpy change involved with breaking up one molecule of methane into a carbon atom and 4 hydrogen radicals divided by 0.5. Bond energy (E) should not be confused with bond dissociation energy.

bond dissociation energy (BDE) or D₀, is one measure of the bond strenght in a chemical bond. It is defined as the standard enthalpy change when a bond is cleaved by homolysis, with reactants and products of the homolysis reaction at 0 K (absolute zero). For instance, the bond dissociation energy for one of the C-H bonds in ethane (C₂H₆) is defined by the process:

CH₃CH₂-H → CH₃CH₂· + H·

D₀ = ΔH = 101.1 kcal/mol (423.0 kJ/mol)

The standard enthalpy of combustion is the enthalpy change when one mole of a substance completely reacts under standard thermodynamic conditions

The enthalpy of neutralization is the change in enthalpy that occurs when an acid and base undergo a neutralization reaction to form water and a salt. It is a special case of the enthalpy of reaction.

When a reaction is carried out under standard conditions at the temperature of 298 K and 1 atm of pressure, it is called the standard enthalpy of neutralization.

The heat (Q) released during a reaction is

Q = m c ΔT

where m is the mass of the solution, c is the specific heat capacity of the solution, and ?T is the temperature change observed during the reaction. From this, the standard enthalpy change (?H) is obtained by division with the amount of substance (in moles) involved, usually the amount of H⁺ ions:

?H = - Q/n

The standard enthalpy change of neutralization for a strong acid and base is -57.3 kJ/mol.

The standard enthalpy of neutralization for mineral acids is far greater than that of organic acids, because strong acids ionize completely while organic acids are usually weak and ionize only partially.