Equation, Chemical
Chemical equations represent, and are a summary of, chemical reactions.
Chemical equations are powerful and useful notations that can be used to quickly and concisely convey large amounts of information regarding chemical reactions.
Chemical reactions are composed of reactants (the initial substances that enter into the reaction) and products (the final substances that are present at the end of the reaction). Chemical equations are written with the reactants on the left side of the equation and with the reaction products on the right side of the equation. An arrow separates the reactants and products.
The arrow in a chemical equation represents the process of the chemical reaction. Because the chemical reaction can be simple or complex, the arrow can stand for a simple process or a complex reaction that involves many steps. Chemists often use other symbols in association with the arrow (above or below the arrow) to specify the conditions under which a chemical reaction takes place or to identify chemicals that are required for a reaction to take place but that do not directly enter into the reaction to become reactants or products (e.g., catalysts).
In chemical equations, the arrow always points in the direction of the products, that is, from the reactants to the products. The arrow is usually read as the word yield or produce. In some cases the arrow points in both directions. A two-headed (bi-directional) arrow ([lrarr2]) pointing both from reactants to products and from products to reactants signifies that a reaction can also run in reverse (i.e., with products of the original reaction becoming new reactants for a reverse reaction that reforms the original reactants).
In theory, all chemical reactions are reversible. In some reactions there are simultaneous reactions that form products and that revert products back to reactants. In these reactions, the formation of products represents the net chemical reaction. The direction of a reaction can usually be controlled by altering the conditions of the reaction. In theory then, all chemical reactions should contain bi-directional arrows. In practice, however, conditions are usually selected that ensure the maximum conversion of reactants into products. Although it is not always feasible to control conditions that ensure complete irreversibility, reactions that take place under conditions that minimize the reversibility of the reaction are appropriately designated as chemical equations using a single headed (directional) arrow pointing from reactants to products.
Reactants and products are separated by addition symbols (+). The addition signs represent the interaction of the reactants and are used to separate and list the products formed. The chemical equations for some reactions may have a lone reactant or a single product.
The subscript numbers associated with the chemical formula designating individual reactants and products represent the number of atoms of each element that are in in each molecule (for covalently bonded substances) or formula unit (for ionicly associated substances) of reactants or products.
In a balanced reaction, all of the matter (i.e., atoms or molecules) that enters into a reaction must be accounted for in the products of a reaction. Accordingly, associated with the symbols for the reactants and products are numbers (stoichiometry coefficients) to the left of the reactants and products that represent the number of molecules, formula units, or moles of a particular reactant involved in the reaction, or the number of molecules, formula units, or moles that result from a balanced chemical reaction.
Balanced chemical equations reflect the law of conservation of mass. For a chemical reaction to be balanced, all of the atoms present in molecules or formula units or moles of reactants to the left of the equation arrow must be present in the molecules, formula units and moles of product to the right of the equation arrow. The combinations of the atoms may change (indeed, this is what chemical reactions do) but the number of atoms present in reactants must equal the number of atoms present in products.
Charge is also conserved in balanced chemical reactions. Accordingly, chemical equations must reflect the conservation of electrical charge between reactants and products.
Chemical equations are usually concerned only with reactants and products. As a result chemical equations often do not show intermediate steps in a reaction. Some reactions, however are written as multi-step reactions where the products of one reaction become the reactants for the next step in the reaction sequence. When equations are written with more than one step, each step is represented by an arrow. In multi-step equations, products not written to the right of the last arrow in the multi-step equation are termed intermediate products (or intermediary products). It is these intermediate products that become new reactants for the next step in the reaction sequence.
Chemical equations that illustrate intermediate steps in a reaction may be listed as a continuous equation or may be broken up into intermediate steps with each step represented on a new line where the products of the previous step become the reactants for the next step in the reaction.
Chemical equations also convey information regarding the states of matter of the reactants and products of a chemical reaction. Solids, liquids and gases are designated by the respective subscripts (s), (l), and (g). Products and/or reactants dissolved in water are designated by the symbol for aqueous (aq). Subscripts designating states of matter are always enclosed in parentheses. When chemical equations lack phase notations the aqueous phase is understood to be assumed.
Reaction catalysts are usually written above or below the reaction arrow. In some cases, the energy changes associated with a particular reaction (e.g., heat given off by an exothermic reaction) are also designated.
Chemical equations are sometimes written in a shorthand type form where chemicals that are present both as reactants and products are not included in what is termed the overall chemical equation). These reaction schemes replace the formal chemical equation. The use of reaction schemes is widespread in scientific literature to summarize chemical reactions.
To be an accurate representation of a chemical reaction, a chemical equation must be entirely consistent with experimental data regarding the chemical reaction it describes. It must specifically and accurately state the reactants used up and the products formed in the chemical reaction it describes. Chemical equations accurately reflect various types of chemical reactions.
Combustion reactions are those where oxygen combines with another compound to form water and carbon dioxide. The equations for these reactions usually designate that the reaction is exothermic (heat producing). Equations for synthesis reactions demonstrate that two or more simple compounds combine to form a more complicated compound. Chemical equations for decomposition reactions reflect the reversal of synthesis reactions (e.g., reactions where complex molecules are broken down into simpler molecules). The electrolysis of water to make oxygen and hydrogen is an excellent example of a decomposition reaction.
Equations for single displacement reactions, double displacement, and acid-base reactions reflect the appropriate reallocation of atoms in the products.
Regardless of the type of chemical reaction the chemical equation represents, it is important to first identify the reactants and the products. When attempting to balance chemical equations, it is critical that the chemical formulas for the reactants and the products remain unchanged. Balancing should only be accomplished through manipulation of the stoichiometry coefficients associated with reactants and products in accord with experimental observation.
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