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 all Arrhenius acids contain hydrogen, the chemical formula and name of a specific Arrhenius acid is dependent on the type of anion that it contains. The previous section of this chapter presented and applied the patterns for determining the chemical formulas and names of the hydrohalogenated, or "HX," Arrhenius acids, which contain monatomic, or single-atom, anions. The following paragraphs will present and apply the patterns for determining the chemical formulas and names of Arrhenius acids that contain polyatomic, or multi-atom, anions.
Writing Chemical Formulas of Polyatomic (HNPoly) Arrhenius Acids
As stated in Section 3.11, polyatomic, or multi-atom, ions have defined formulas, names, and charges. Furthermore, these types of ions are inherently highly stable and, therefore, can exist independently in solution as indivisible charged entities. However, only the polyatomic anions that are named using "-ate ion" or "-ite ion" suffixes can successfully bond with protons to form Arrhenius acids.
Based on the bonding combinations that were presented in Chapter 3, non-metals, such as hydrogen, and polyatomic anions are electronically-incompatible and, therefore, should not bond with one another. However, an Arrhenius acid generates, by definition, a hydrogen ion, H+1, when dissolved in water. Because a proton is a positively-charged cation, an Arrhenius acid that also contains a polyatomic anion is categorized as an ionic compound. Therefore, the chemical formula of an Arrhenius acid that contains protons and a polyatomic anion can be derived by applying the Chapter 3 rules for determining ionic chemical formulas. In order to develop a formula pattern that can be used to represent these types of acids, which are known as "polyatomic Arrhenius acids," a polyatomic anion will be generically symbolized as "Poly–N" in the following paragraphs. In this notation, the elemental symbols and subscripts that are present in a polyatomic ion are represented as "Poly," and "N" corresponds to the numerical portion of the charge of that ion.
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." Because a polyatomic ion is an indivisible unit, its chemical formula must be enclosed inside of two parentheses when incorporated into an ionic chemical formula, and the subscript that specifies how many of that ion are present within the corresponding compound must be written after the closing parenthesis. The subscripts in the base formula must be reduced to the lowest-common ratio of whole numbers, if possible, and any explicitly-written "1"s must be removed. Finally, if no subscript is present after the closing parentheses in the resultant chemical formula, the parentheses surrounding the polyatomic ion should be removed.
By applying these rules to the cationic, H+1, and anionic, Poly–N, components of a polyatomic Arrhenius acid, a formula pattern of "HNPoly". Since the original cation, H+1, bears a +1 charge, the subscript that is written after the parentheses that enclose the chemical formula of the polyatomic ion has a value of "1," which should not be explicitly-written in the chemical formula that is being developed. As a result, the parentheses surrounding the chemical formula of the polyatomic ion should also be eliminated, and the corresponding polyatomic Arrhenius acid is understood to contain one polyatomic anion, Poly–N. However, because polyatomic anions that are named using "-ate ion" or "-ite ion" suffixes, which can successfully bond with protons to form Arrhenius acids, can bear –1, –2, or –3 charges, the corresponding acids must contain 1, 2, or 3 protons, respectively, in order to achieve charge-balance in the final compound. Finally, to obtain the chemical formula of a particular polyatomic Arrhenius acid, the "Poly" and "N" portions of the pattern that is shown above can be replaced with the chemical formula of a specific polyatomic anion and the absolute value of the charge of that ion, respectively.
For example, write the chemical formula of the Arrhenius acid that contains a sulfate 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 sulfate ion, SO4–2, is a polyatomic anion, the corresponding electrolyte is a polyatomic Arrhenius acid, which can be generically-symbolized as "HNPoly." In order to obtain the chemical formula of a particular polyatomic Arrhenius acid, which contains one polyatomic anion, Poly–N, and one or more protons, H+1, the "Poly" and "N" portions of the pattern that is shown above can be replaced with the chemical formula of a specific polyatomic anion and the absolute value of the charge of that ion, respectively. Because the given polyatomic anion bears a –2 charge, the corresponding polyatomic Arrhenius acid must contain 2 protons, in order to achieve charge-balance in the final compound. Therefore, the chemical formula of the Arrhenius acid that contains a sulfate ion, SO4–2, is H2SO4.
Naming Polyatomic (HNPoly) 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 polyatomic, or "HNPoly," Arrheniusacid contains a polyatomic anion, Poly–N, the root term that indicates the identity of the polyatomic ion must be incorporated into the name of the corresponding Arrheniusacid. Additionally, because the names of the Arrheniusacids that contain sulfur- and phosphorus-based polyatomic anions do not have fluid pronunciations, an additional "ur" or "or" syllable, respectively, must be incorporated into the root terms in the corresponding acid names. Furthermore, because the substance that is being named contains one or more protons, H+1, in addition to a polyatomic ion, Poly–N, the suffix of the polyatomic ion must be replaced with an acid suffix. As stated above, polyatomic anions that are named using "-ate ion" and "-ite ion" suffixescan successfully bond with protons to form Arrheniusacids. As has been repeatedly emphasized in several sections of this text, no two chemical formulas should share a common chemical name. Therefore, the acid suffixes that correspond to the "-ate ion" and "-ite ion" polyatomic anions must be unique. If a polyatomic anion that is named using an "-ate ion" suffix is present in an Arrhenius acid, that acid should be named using an "-ic acid" suffix, and an "-ite ion" polyatomic suffix should be replaced an "-ous acid" suffix when naming a polyatomic Arrhenius acid. Finally, both the "-ate ion" polyatomic Arrheniusacids and the hydrohalogenated Arrhenius acids are named using "-ic acid" suffixes. Therefore, in order to distinguish the names of these acids, which have distinctive chemical compositions, and consequently, should have unique chemical names, the prefix "hydro-" is not incorporated into the names of the "-ate ion" or the "-ite ion" polyatomic Arrheniusacids.
For example, write the name of the Arrhenius acid that is symbolized as H2SO4.
Because the given chemical formula contains two protons, H+1, and a polyatomic ion, SO4–2, the corresponding molecule can be classified as a polyatomic, or "HNPoly," Arrhenius acid. In order to name this type of acid, the suffix that is written after the root term that indicates the identity of the polyatomic ion must be replaced with an acid suffix. Finally, because the prefix "hydro-" is only incorporated into the names of the hydrohalogenated Arrhenius acids, this word should not be written in the name of a polyatomic Arrheniusacid. Therefore, because the molecule that is symbolized above contains a sulfate ion, SO4–2, the root word "sulf" is incorporated into the acid name that is being developed. Additionally, because the name of an Arrhenius acid that contains a sulfur-based polyatomic anion does not have a fluid pronunciation, an additional "ur" syllable is incorporated into the "sulf" root term. The full name of the given molecule, sulfuric acid, is completed by replacing the "-ate ion" polyatomic ion suffixwith "-ic acid."
