# 8.15: Arrhenius Acids and Bases: Reactions of Arrhenius Acids and Arrhenius Bases

Learning Objectives
• Predict the products that are generated when Arrhenius acids and Arrhenius bases undergo double replacement, or neutralization, reactions.

Because the protons, H+1, and hydroxide ions, OH–1, that are formed as a result of the dissociation of Arrhenius acids and Arrhenius bases, respectively, are analyzed as independent chemical entities, the reactivity of an Arrhenius chemical is limited, but predictable.  The previous section of this chapter discussed the reaction that occurs between an Arrhenius acid and an elemental metal, in which a salt and molecular hydrogen, H2, are generated as products.  The following paragraphs will describe and symbolically-represent a second Arrhenius reaction, in which an Arrhenius acid reacts with an Arrhenius base.  The generic reactivity patterns that will be developed can be applied to predict the products that are generated during a specific chemical reaction.

Recall that, because the hydrohalogenated, "HX," and polyatomic, "HNPoly," Arrhenius acids both contain a cationic, H+1, and an anionic, X–1 or Poly–N, respectively, component, both of these types of acids are classified as ionic compounds.  An Arrhenius base, which is comprised of a monatomic metal cation, M+Y, and one or more anionic hydroxide ions, OH–1, is also an ionic compound that can be generically-symbolized as "M(OH)Y."  Consequently, the reaction that occurs between an Arrhenius base and an Arrhenius acid can be categorized as a double replacement reaction.  During this type of chemical change, either the cationic or the anionic portions of two compounds exchange their relative positions.  Therefore, in a double replacement reaction involving an Arrhenius base and an Arrhenius acid, the cationic portions of these chemicals, a metal cation, M+Y, and a proton, H+1, respectively, replace one another.  As a result, two new ionic compounds are produced by bonding the M+Y and H+1 cations with the remaining anionic portions of the Arrhenius acid, X–1 or Poly–N, and Arrhenius base, OH–1, respectively.  The reaction patterns that are shown below symbolize the double replacement reactions that occur between an Arrhenius base, "M(OH)Y," and a hydrohalogenated, "HX,"

___ $$\ce{M(OH)_{Y}}$$ + ___ $$\ce{HX}$$ $$\rightarrow$$ ___ $$\ce{MX_{Y}} \; +$$ ___ $$\ce{HOH}$$

and a polyatomic, "HNPoly,"

___ $$\ce{M(OH)_{Y}}$$ + ___ $$\ce{H_{N}Poly}$$ $$\rightarrow$$ ___ $$\ce{M_{N}(Poly)_{Y}} \; +$$ ___ $$\ce{HOH}$$

Arrhenius acid, respectively.  The order in which the reactants are given does not impact the classification of the reaction or the identities of the products that are generated.  Similarly, the formulas for the products, which must be separated by a plus sign, can be written in any order.  Furthermore, recall that all halide ions, X–1, bear –1 charges, that "Y" corresponds to the numerical charge of the cation that results from the ionization of a metal, "M," and that "N" represents the charge of the polyatomic ion, "Poly–N" that is present in a polyatomic, "HNPoly," Arrhenius acid.  Because, as stated above, a double replacement reaction generates an ionic compound by bonding a metal cation, M+Y, with the anionic portion, X–1 or Poly–N, of an Arrhenius acid, the "MXY" and "MN(Poly)Y" formulas that are written in these reaction patterns are derived by applying the symbolic information in the previous sentence to the Chapter 3 rules for determining ionic chemical formulas.  Since the "MXY" and "MN(Poly)Y" chemical formulas do not contain protons, H+1, or hydroxide ions, OH–1, the corresponding ionic compounds are classified as salts, which were also produced in the reactions that occur between Arrhenius acids and elemental metals.

Furthermore, a double replacement reaction generates a second compound, HOH, by bonding a proton, H+1, from the Arrhenius acid with the anionic portion, OH–1, of the Arrhenius base.  This chemical formula, which contains two hydrogen atoms and one oxygen atom, is an alternative representation of water.  Therefore, each of the "HOH" symbolisms in the reaction patterns that are shown above could be replaced with the standard chemical formula for water, H2O.  The reaction that occurs between an Arrhenius acid and an Arrhenius base is also known as a neutralization reaction, because the product solution is neither acidic nor basic and, instead, has a neutral pH value of exactly 7.00.  The relationship between pH and the acidic, basic, or neutral character of a solution will be explained in a later section of this chapter.  Finally, after determining the chemical formula of the ionic compound that is produced, the resultant chemical equation must be balanced by incorporating coefficients, as necessary, in order to uphold the Law of Conservation of Matter.

For example, complete the following chemical equation by determining the chemical formula(s) of the product(s) that are predicted to form.  Balance the resultant equation by writing coefficients in the "blanks," as necessary.  (States of matter are not required.)

___ $$\ce{AgOH}$$ + ___ $$\ce{HBr}$$ $$\rightarrow$$

Because the second chemical formula in this equation contains a proton, H+1, and a halide ion, Br–1, the bromide ion, the corresponding molecule, hydrobromic acid, HBr, can be classified as a hydrohalogenated, "HX," Arrhenius acid.  The first reactant, silver hydroxide, AgOH, is an Arrhenius base that contains a monatomic metal cation, Ag+1, and one hydroxide ion, OH–1.  Silver, the metal that is present in the given base, is one of the few transition metals that achieves a stable electron configuration through a single ionization pathway.  The charge of the silver ion, Ag+1, could be determined using the "Reverse Ratio Method" that was described in Section 3.10.  Alternatively, recall that the "Y" subscript that is present in the Arrhenius base formula pattern, "M(OH)Y," corresponds to the numerical charge of the cation that is contained in a particular base.  Therefore, because the numerical value of "Y" in the given Arrhenius base, AgOH, is an unwritten "1," the silver ion that is present in the this base bears a +1 charge.

These chemicals will undergo a double replacement reaction in which the cationic or the anionic portions of the given compounds exchange their relative positions.  Therefore, based on the first reaction pattern that is shown above, a new ionic compound, AgBr, forms when the metal cation from the Arrhenius base, Ag+1, bonds with the anionic portion, Br–1, of the Arrhenius acid.  Furthermore, this double replacement reaction generates a second compound, HOH, which can also be written as "H2O," by bonding a proton, H+1, from the Arrhenius acid with the anionic portion, OH–1, of the Arrhenius base.  The information that is described in this paragraph is reflected in the reaction equation that is shown below.

___ $$\ce{AgOH}$$ + ___ $$\ce{HBr}$$ $$\rightarrow$$ ___ $$\ce{AgBr}$$ + ___ $$\ce{H_2O}$$

Because all of the components in this reaction equation are balanced, the equation that is shown above is the chemically-correct representation of the double replacement reaction that occurs between silver hydroxide, AgOH, an Arrhenius base, and hydrobromic acid, HBr, an Arrhenius acid.

Exercise $$\PageIndex{1}$$

Complete the following chemical equation by determining the chemical formula(s) of the product(s) that are predicted to form.  Balance the resultant equation by writing coefficients in the "blanks," as necessary.  (States of matter are not required.)

___ $$\ce{Ca(OH)_2}$$ + ___ $$\ce{H_3PO_3}$$ $$\rightarrow$$

___ $$\ce{Ca(OH)_2}$$ + ___ $$\ce{H_3PO_3}$$ $$\rightarrow$$ ___ $$\ce{Ca_3(PO_3)_2}$$ + ___ $$\ce{H_2O}$$
3 $$\ce{Ca(OH)_2}$$ + 2 $$\ce{H_3PO_3}$$ $$\rightarrow$$ ___ $$\ce{Ca_3(PO_3)_2}$$ + ___ $$\ce{H_2O}$$
3 $$\ce{Ca(OH)_2}$$ + 2 $$\ce{H_3PO_3}$$ $$\rightarrow$$ ___ $$\ce{Ca_3(PO_3)_2}$$ + 6 $$\ce{H_2O}$$