Chemical Energy

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\(\mathrm{H_2O_{\large{(l)}} \rightarrow H_{2\large{(g)}} + \dfrac{1}{2} O_2}, \hspace{20px} dH = \mathrm{+285.8\: kJ/mol}\)

A Chemical Energy Level Diagram
------------H₂(g) + ¹/₂O₂ \| \| \| 286 kJ \| \| -286 kJ \| \| \| ¯ ------------ H₂O

We can also use an energy level diagram to show the relative content of energy. The energy content of \(\mathrm{H_{2\large{(g)}} + 0.5\, O_2}\) is 285.8 kJ higher than a mole of water, \(\ce{H2O}\).

Oil, gas, and food are often called energy by the news media, but more precisely they are sources of (chemical) energy -- energy stored in chemicals with a potential to be released in a chemical reaction. The released energy performs work or causes physical and chemical changes.

It is obvious that the amount of energy released in a chemical reaction is related to the amount of reactants. For example, when the amount is doubled, so is the amount of energy released.

\(\mathrm{2 H_{2\large{(g)}} + O_2 \rightarrow 2 H_2O_{\large{(l)}}}, \hspace{20px} dH = \mathrm{-571.6\: kJ/mol}\)

Example 1 shows the calculation when the amount of reactants is only a fraction of a mole.

The sudden release of energy causes an explosion.

Contributors and Attributions

Chung (Peter) Chieh (Professor Emeritus, Chemistry @ University of Waterloo)