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To define a chemical reaction, consider atomic and energy dynamics. In chemical equations, atoms are rearranged. An energy balance can indicate chemical reactions. Reaction types provide guidance to chemical process dynamics. Catalysts affect reaction energetics. The distinction between chemical and physical processes can be blurry.
Energy Balance
Energetic inventory of a process may leave an amount of energy unaccounted for. That gap could be indicative of energy expended in rearranging atoms between molecules, thereby identifying a chemical process.
Atom Rearrangement
Atoms are not grouped identically. Salt formation written as Na + Cl -> NaCl + (energy), is a chemical reaction. Boiling water: H2O(liquid) -> H2O(gas) is not a chemical reaction, as "H2O" remains unchanged.
Reaction Type
Single-replacement, combination (as in NaCl example above), decomposition, redox and other reaction types categorize chemical processes by common dynamics on a molecular scale.
Response to Catalyst
Reaction rate increases upon catalyst addition. Catalysts work by lowering process activation energy. Reaction rate and energetic balance with varying catalyst levels can give insight into nature and details of a given chemical process.
Ambiguous Cases
Consider hydrous vs. anhydrous salt. Are they chemically different? On one hand, the answer is "yes," because one has water in the molecular matrix, held by weak intermolecular bonds. However, the two species are easily reversible through regular water evaporation/condensation, so the answer also is "no."
Source:
Harvey Mudd College: Theory on Physical and Chemical Reactions
Chemtutor: Reactions-What is a Chemical Reaction?
Department of Chemistry, Oklahoma State University: Chemical Reactions
Resource:
The University of Texas at Dallas: Chemical Reactions
Faculty of Science, University of Waterloo: Features of Chemical Reaction
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