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Chemical Equation

Conventions And Symbols

In general, the reactants (A and B) are always placed on the left-hand side of the equation, and the products (C and D) are shown on the right. The symbol "→" indicates the direction in which the reaction proceeds. If the reaction is reversible, the symbol " &NA; " should be used to show that the reaction can proceed in both the forward and reverse directions. Δ means that heat is added during the reaction, and not equal implies that D escapes while produced. Sometimes, Δ is replaced by "light" (to initiate reactions) or "flame" (for combustion reactions.) Instead of showing the symbol Δ, at the same place we may just indicate the operating temperature or what enzymes and catalysts are need to speed the reaction.

Each chemical species involved in an equation is represented by chemical formula associated with stoichiometric coefficients. For instance, a, b, c, and d are the stoichiometric coefficients for A, B, C, and D, respectively. Stoichiometric coefficients can be integers, vulgar fractions, (e.g. 3/4) or decimal fractions (e.g. 0.5). They define the mole ratio (not mass ratio) that permits us to calculate the moles of one substance as related to the moles of another substance in the chemical equation. In the present case, we know that a moles of A react with b moles of B to form c moles of C and d moles of D.

The chemical equation needs to be balanced, that is, the same number of atoms of each "element" (not compounds) must be shown on the right-hand side as on the left-hand side. If the equation is based on an oxidationreduction reaction which involves electron transfer, the charges should also be balanced. In other words, the oxidizing agent gains the same number of electrons as are lost by the reducing agent. For this reason, we must know the oxidation numbers for elements and ions in chemical compounds. An element can also have more than one oxidation number, for instance, Fe2+ and Fe3+ for iron.

Under certain conditions, the information on phase, temperature, and pressure should be included in the equation. For instance, H2O can exist as solid, liquid, and vapor (gas) that can be represented by H2O(s), H2O(l), and H2O(g), respectively. If we have an infinitely dilute solution, say HCl, it can be denoted as HCl(aq). For solubility problems, A underlined (A) means that A is a solid or precipitated phase. In many cases, the heat of reaction, Δ H, is also given; a positive number implies an endothermic reaction (where heat is absorbed), and a negative number implies an exothermic reaction (where heat is given off). Unless otherwise specified, the heat of reaction is normally obtained for all the chemical species involved in the reaction at the standard state of 77°F (25°C) and 1 atmosphere total pressure, the socalled "standard heat of reaction" and denoted by ΔH°.


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Science EncyclopediaScience & Philosophy: Ephemeris to Evolution - Historical BackgroundChemical Equation - Conventions And Symbols, A Few Examples - Applications