8.8: Arrhenius Acids and Bases: Writing Chemical Formulas and Names of Hydrohalogenated Arrhenius Acids
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As stated previously, an Arrhenius acid must, by definition, contain hydrogen, which ionizes to form a proton, H+1, and a component that ionizes to produce a stable, negatively-charged particle. Furthermore, because hydrogen is present in all Arrhenius acids, the chemical formula and name of a specific Arrhenius acid is dependent on the type of anion that it contains. The patterns for determining the chemical formulas and names of Arrhenius acids that contain monatomic, or single-atom, anions will be presented and applied in the following paragraphs.
Writing Chemical Formulas of Hydrohalogenated (HX) Arrhenius Acids
With the exception of the noble-gases, all of the non-metals that are present on the right side of the periodic table gain electrons to achieve octet configurations and ionize to form monatomic, or single-atom, anions. As discussed previously, while ionization improves the stability of a particle's electron configuration, relative to that of an atom, the charged state of the resultant ion is inherently destabilizing. As a result, many monatomic anions cannot exist independently in solution. However, each of the halogens, which are found in Group 17, or Group 7A, on the periodic table, contains 7 valence electrons, and, therefore, must only gain a single electron to achieve an octet configuration. Consequently, only a slight charge imbalance is present in the halide anions, which are collectively symbolized as "X–1," that are generated through this ionization process, and, as a result, these monatomic ions are relatively stable and can exist independently in solution.
Based on the bonding combinations that were presented in Chapter 3, hydrogen should interact with a halogen to form a covalent molecule, as both of the indicated components are classified as non-metals. However, because an Arrhenius acid generates, by definition, a hydrogen ion, H+1, when dissolved in water, an Arrhenius acid is categorized as an ionic compound. Therefore, the chemical formula of an Arrhenius acid that contains a proton, H+1, and a halide ion, X–1, can be derived by applying the Chapter 3 rules for determining ionic chemical formulas.
Recall that the symbol for the cationic component of an ionic compound is written first in an ionic chemical formula, and that, after removing the "+" and "–" signs from each of the constituent ion symbols, the subscripts in an ionic base formula can be derived using either the "Ratio Method" or the "Criss-Cross Method." Finally, the generated subscripts must be reduced to the lowest-common ratio of whole numbers, if possible, and any explicitly-written "1"s must be removed.
By applying these rules to the cationic, H+1, and anionic, X–1, components of an Arrhenius acid, a formula pattern of "HX" results. Because no subscripts are explicitly-written in this formula pattern, this type of Arrhenius acid, which, due to its chemical composition, is known as a "hydrohalogenated Arrhenius acid," must contain one proton, H+1, and one halide ion, X–1. In order to obtain the chemical formula of a particular hydrohalogenated Arrhenius acid, the "X" portion of the pattern that is shown above can be replaced with the elemental symbol for a specific halogen.
For example, write the chemical formula of the Arrhenius acid that contains a chloride ion.
As stated above, an Arrhenius acid contains a proton, H+1, by definition, and a component that ionizes to produce a stable, negatively-charged particle. Because the indicated anion, the chloride ion, Cl–1, is a halide ion, the corresponding electrolyte is a hydrohalogenated Arrhenius acid, which can be generically-symbolized as "HX." In order to obtain the chemical formula of a particular hydrohalogenated Arrhenius acid, which contains one proton, H+1, and one halide ion, X–1, the "X" portion of the pattern that is shown above can be replaced with the elemental symbol for a specific halogen. Therefore, the chemical formula that corresponds to the Arrhenius acid that contains a chloride ion, Cl–1, is HCl.
Naming Hydrohalogenated (HX) Arrhenius Acids
The name of an ionic compound is based solely on the identities of the ions that it contains. Since the subscripts in an ionic chemical formula are the result of achieving charge-balance between the compound's constituent ions, referencing subscripts in an ionic chemical name is considered redundant. Therefore, the names of ionic compounds do not include any numerical prefixes.
As stated above, because all Arrhenius acids contain hydrogen, the name of a specific Arrhenius acid is derived from the name of its constituent anion. Therefore, because a hydrohalogenated, or "HX," Arrhenius acid contains a halide ion, X–1, the root term that indicates the identity of the halide ion must be incorporated into the name of the corresponding Arrhenius acid. Furthermore, in order to indicate that a molecule is classified as a hydrohalogenated Arrhenius acid, the prefix "hydro-" is written before the halide root word. Finally, because the substance that is being named contains a proton, H+1, in addition to a halide ion, X–1, the "-ide ion" suffix must be replaced by "-ic acid" in the acid name that is being developed.
For example, write the name of the Arrhenius acid that is symbolized as HCl.
Because the given chemical formula contains one proton, H+1, and one halide ion, Cl–1, the corresponding molecule can be classified as a hydrohalogenated, or "HX," Arrhenius acid. In order to name this type of acid, the prefix "hydro-" is written before the halide root term, which corresponds to the identity of the halide ion, and the "-ide ion" suffix is replaced by "-ic acid." Therefore, because the molecule that is symbolized above contains a chloride ion, Cl–1, the root word "chlor" is incorporated into the acid name that is being developed. The full name of the given molecule, hydrochloric acid, is completed by writing the prefix "hydro-" before the "chlor" root term, and replacing the "-ide ion" suffix with "-ic acid."