Enthalpy

When atoms combine with other atoms, the bond created will have a certain strength depending on the types of atoms involved. The strength of this bond is related to the energy that was released in forming the bond, and is characteristic of the atoms involved.

A term called bond enthalpy relates the amount of energy associated with different types of chemical bonds. Since each type of bond has a characteristic energy value, we can find these values in published data tables that have been compiled by scientists. The short table below gives you some bond enthalpy values for several common chemical bonds. Do not worry about memorizing the values in the table. Bond enthalpy values will be given to you any time you need them in this course.

This table shows you, for example, that forming a bond between two hydrogen atoms (H-H) releases 436 kilojoules of energy as heat. Forming a bond between a hydrogen and a chlorine atom (H-Cl) releases 433 kilojoules of heat energy and so forth.

If forming a H-H bond releases 436 kilojoules of heat, what can you say about breaking a H-H bond?

Breaking a H-H would absorb 436 kilojoules of heat energy. Remember that forming bonds releases energy and that breaking bonds requires energy. Therefore, breaking a bond means that energy will be absorbed from the surroundings.

How much energy is absorbed from the surroundings when a C-H bond is broken? What happens when a C-H bond is formed instead?

413 kJ of energy are absorbed when a C-H bond is broken. On the other hand, if a C-H bond is formed, 413 kJ of energy is released to the surroundings.