A formation reaction is simply a synthesis reaction that starts with only elements and ends with one mole of a product. For example, this is a formation reaction:
3/2 N2 + 6 H2 + 1/4 P4 + 2 O2 → (NH4)3PO4
The following reaction is not a formation reaction because although it makes one mole of product, not everything on the left is an element:
H2O + 1/2 O2 → H2O2
The next reaction is not a formation reaction because although everything on the left is an element, it produces more than one mole of product:
2 H2 + 2 C + 3 O2 → 2 H2CO3
The heat for a formation reaction is called the heat of formation and is signified by ΔHf.
Formation reactions matter because when we put them into a Hess' Law problem some very distinctive and useful patterns appear. Here is an example.
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