|MadSci Network: Chemistry|
An arrow rather than an equal sign is used in a regular chemical equation because such equations account only for the material balance of the particular reaction, and not for the energetic change associated with this reaction.
If the energetic change has been taken into account, then an equal sign can be used. Such equations are called thermochemical.
Consider this example:
2 H2 + O2 ---> 2 H2O (arrow, energy change not included) 2 H2 + O2 = 2 H2O + 285 kJ/mol (equal sign, energy change included)
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