Bond energies

In chemistry, when a chemical reaction occurs, some molecular bonds are broken giving way to other bonds to make some different molecules. For example, the bonds between the 2 water molecules are broken to produce hydrogen and oxygen.

2H2O→2H2+O2(1)(1)2H2O→2H2+O2

Bond energy (BE) is frequently used in chemistry as the formation of chemical compounds requires some bonds. Bond energy is also called the mean bond enthalpy or average bond enthalpy that measures the strength in the bonds of a molecule. Bond energy is a 2-way process because energy is always required to break a bond whereas the same is released when a bond formation occurs. Bonding plays a vital role in atomic energy because joining atoms attain lower energies while they acquire more energy when they are individual.

Bond energy

According to  IUPAC  “bond energy is the average value of the gas-phase Bond-dissociation energy at a temperature of 298.15 K  for all bonds of the same type within the same chemical species”. When hydrogen atoms combine to form a molecule, a lot of energy is given out in the form of heat and this implies that the product formed is more stable than its reactants. The covalent bond in the hydrogen molecule is so strong that it needs approximately 435kJ of energy to dissociate one mole of the hydrogen molecules to hydrogen atoms.  Bond dissociation energy refers to the amount of energy required to break the bond between two covalently bonded atoms. A carbon-carbon single covalent bond has a bond dissociation of about 347Kj. The ability of carbon to form strong C-C bonds helps explain the stability of carbon compounds. Compounds with only C-C and C-H have single covalent bonds, for example,  methane. 

Bond dissociation

Bond Dissociation energy is defined as the standard enthalpy change of the following fission: R - X → R + X. The BDE, denoted by Dº(R - X), is usually derived by the thermochemical equation. Therefore, it is the energy required to disrupt or dissociate bonds in a chemical reaction. It is otherwise known as bond disruption energy or binding energy.

The energy released during dissociation

Bond energy is the basis of explanation for how strong or weaker the bond strength is. Stronger the ionic bond, greater the amount of energy released when the bonds dissociate. Ionic bonds are crystalline and very tough in their structure, this is because of the nature of bonds that they form. When a bond is so strong, it implies that it needs a lot of energy to break the bond. However, almost all the ionic bonds can be broken or dissolved despite the fact that they have high melting points. Surprisingly, most of the ionic solids dissolve readily in water, and they are very good conductors of heat and electricity. The tables below give information about bond energies and bond dissociation energies of common atoms

 

Bond enthalpies or bond energy table

Single Bonds

ΔH°*

H–H

104.2

C–C

83

N–N

38.4

O–O

35

F–F

36.6

Si–Si

52

P–P

50

S–S

54

Cl–Cl

58

Br–Br

46

I–I

36.

H–C

99

H–N

93

H–O

111

H–F

135

H–Cl

103

H–Br

87.5

H–I

71

H–B

90

H–S

81

H–Si

75

H–P

B–F

B–O

125

C–N

73

N–CO

86

C–O

85.5

O–CO

110

C–S

65

C–F

116

C–Cl

81

C–Br

68

C–I

51

C–B

90

C–Si

76

C–P

70

N–O

55

S–O

87

Si–F

135

Si–Cl

90

Si–O

110

P–Cl

79

P–Br

65

P–O

90          

 

Multiple Bonds

ΔH°*

C=C

146

N=N

109

O=O

119

C=N

147

C=O (CO2)

192

C=O (aldehyde)

177

C=O (ketone)

178

C=O (ester)

179

C=O (amide)

179

C=O (halide)

177

C=S (CS2)

138

N=O (HONO)

143

P=O (POCl3)

110

P=S (PSCl3)

70

S=O (SO2)

128

S=O (DMSO)

93

P=P

84

P≡P

117

C≡O

258

C≡C

200

N≡N

226

C≡N

213

Factors affecting the ionic bond energy

The electronegativity of the 2 atoms bonding together affects ionic bond energy. There is a strong bond when the electronegativity of 2 atoms are farther away. The Strongest polar covalent bond is found in the Carbon-Fluorine bond. And mostly, ionic bonds are stronger than covalent bonds. By checking at melting points, ionic compounds have high melting points and covalent compounds have low melting points.

Read More

  1. Covalent bond

  2. Ionic bonds and ionic compounds

  3. Bond Polarity and Inter-molecular interactions

Check your understanding

  1. What is the difference between bond energy and the bond-dissociation energy?

  2. Explain the factors affecting ionic bond energy.

  3. Define bond energy.

  4. Describe the factors affecting the ionic bond energy

  5. How much energy is released when we dissociate H-H bond?



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