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Unit 7: The Energy in Chemical Reactions—Thermodynamics and Enthalpy

Section 9: Bond Enthalpies

During a chemical reaction, old bonds between atoms break and new bonds form. Breaking chemical bonds requires enthalpy; it is an endothermic process. Forming new bonds releases enthalpy; it is exothermic. When a chemical reaction occurs, the balance between these two processes determines if the reaction is endothermic or exothermic overall. A reaction will be endothermic if breaking the old bonds takes more enthalpy than the enthalpy released when the new bonds form. It will be exothermic if the new bonds release more enthalpy than the enthalpy needed to break the old bonds.

Breaking Hydrogen and Chlorine Bonds

Figure 7-13. Breaking Hydrogen and Chlorine Bonds

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Breaking Hydrogen and Chlorine Bonds

Figure 7-13. Breaking Hydrogen and Chlorine Bonds

Breaking the H-H bond and the Cl-Cl bond requires an input of energy. Forming the two H-Cl bonds releases energy.

The amount of energy associated with a bond is called the "bond enthalpy," and it is measured in kilojoules per mole (kJ/mol). The bond that holds oxygen atoms together as an O2 molecule has a bond enthalpy of 495 kJ/mol. In other words, breaking a mole of O2 molecules into individual oxygen atoms requires an input of 495 kJ. Conversely, forming a mole of O2 molecules from oxygen atoms would release 495 kJ.

Chemists have measured and tabulated many bond enthalpy values because they provide yet another way, along with calorimetry and Hess's Law, to find the enthalpy change of a reaction. For example, in the reaction in Figure 7-13, two bonds break. The H-H bond requires 436 kJ/mol to break, and the Cl-Cl bond requires 242 kJ/mol; the total enthalpy input is 678 kJ/mol. When the two H-Cl bonds form, they release 862 kJ/mol (431 from each bond). Overall, the reaction will release 184 kJ/mol; the ΔH for this reaction is -184 kJ/mol.

It takes more enthalpy to break stronger bonds; conversely, forming stronger bonds releases more enthalpy. The strength of one particular bond has posed significant challenges for living things: the triple bond between nitrogen atoms. (See Nitrogen's Triple Bond sidebar)


Bond enthalpy

The amount of enthalpy (or energy) needed to break one mole of a certain chemical bond.


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