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8.18: Brønsted-Lowry Acids and Bases: Writing Acid/Base Equations that Represent Aqueous Proton Transfers

  • Page ID
    227314
  • Learning Objectives
    • Define acid/base equation.
    • Develop a Brønsted-Lowry acid/base equation that represents the absorption of a proton by an aqueous solvent.
    • Write the chemical formula of the hydronium ion.

    The remaining sections of this chapter will explore the applications of Brønsted-Lowry acid/base reactivity.  Recall that a Brønsted-Lowry acid is defined as a proton, H+1, donor in solution, and a Brønsted-Lowry base is defined as a proton, H+1, acceptor in solution.  During their investigations of solutes that dissociate to form H+1 ions in solution, Brønsted and Lowry discovered that the protons that were generated upon the solvation of an acid did not exist independently in solution.  As stated in Section 3.6, a charged particle cannot typically exist alone in nature and, instead, usually combines with an oppositely-charged ion to form an ionic compound.  As an H+1 ion consists, by definition, of only a single proton, the hydrogen ion is extremely unstable, as it does not contain any electrons that would typically off-set some of the charge imbalance that inherently exists in all ions.  Therefore, when a Brønsted-Lowry acid dissociates, the hydrogen ion that is produced is immediately absorbed by a nearby solvent molecule.  Brønsted and Lowry regarded this phenomenon as chemically-significant and, as a result, classified these solvent molecules as bases.  Finally, these chemists defined a Brønsted-Lowry acid/base reaction as the transfer of a proton, H+1, from a solute particle, which is categorized as a Brønsted-Lowry acid, to a solvent molecule, which is classified as a Brønsted-Lowry base.

    The synergistic relationship that exists between a Brønsted-Lowry acid and a Brønsted-Lowry base can be symbolically-represented in a Brønsted-Lowry acid/base equation, which, in turn, can be derived by restructuring a solution equation.  Recall that the solution equations that were presented in Chapter 7 are, by definition, symbolic representations of the dissociative behaviors of solutes, such as Arrhenius and Brønsted-Lowry acids.  Additionally, while an Arrhenius chemical must be dissolved in water, Brønsted and Lowry did not explicitly identify the solvent in which their acids must be dissolved.  Therefore, because water is a specific solvent, the Arrhenius system is a subcategory of the Brønsted-Lowry definition, and all Arrhenius acids can also be categorized as Brønsted-Lowry acids.  As a result, the solution equations that were written in Section 8.12, which were used to verify that molecules that were symbolized according to previously-developed "HX" and "HNPoly" patterns were correctly classified as Arrhenius acids, can be used as the basis for developing Brønsted-Lowry acid/base equations.  The following paragraphs will present and apply the process for transforming a solution equation into a Brønsted-Lowry acid/base equation.

    For example, develop a Brønsted-Lowry acid/base equation by restructuring the following balanced solution equation.  Balance the final Brønsted-Lowry equation by writing coefficients, as necessary.  (States of matter are not required.)

    \(\ce{HNO_3}\) \(\overset{\ce{H_2O}}{\longrightarrow}\) \(\ce{H^{+1}}\) + \(\ce{NO_3^{–1}}\)

    As stated previously, when a Brønsted-Lowry acid dissociates, the hydrogen ion, H+1, that is produced is immediately absorbed by a solvent molecule, which is classified as a Brønsted-Lowry base.  In order for this proton, H+1, transfer to occur, a solute particle must exist in close physical proximity to a solvent molecule.  Because a solution is, by definition, a homogenous mixture, solute and solvent molecules are evenly-distributed, relative to one another, when present in the same solution.  However, because the chemical formula of the solute, nitric acid, HNO3, is written on the left side of the arrow in the given solution equation, and the formula of the solvent, water, H2O, is written above this arrow, the format of the solution equation that is shown above does not reflect that  these  solute and solvent molecules exist with one another in solution.  Therefore, in order to indicate that solute molecules are uniformly-dispersed among solvent molecules in a homogeneous mixture, the chemical formula of the solvent, H2O, must be moved from its current position in the given solution equation to the left side of the arrow.  A plus sign must be used to separate the formulas of these chemicals, as shown below.

    \(\ce{HNO_3}\) + \(\ce{H_2O}\) \(\longrightarrow\) \(\ce{H^{+1}}\) + \(\ce{NO_3^{–1}}\)

    While the equation that is shown above accurately reflects that solute and solvent molecules exist in the same chemical environment in a solution, this equation is not balanced.  Because a new chemical formula was introduced on the left side of the solution equation arrow, the relative quantity of atoms and ions on the left and right sides of this equation cannot be balanced through the incorporation of coefficients.  However, recall that, by definition, the solvent is the chemical that is present in the greatest amount in a given solution.  Consequently, after the appropriate quantity of solute particles have dissociated, and the protonsH+1, that are produced have been absorbed by an equal number of solvent molecules, excess solvent molecules are still present in the resultant solution.  Therefore, the chemical formula of the solvent, H2O, should also be written on the right side of the arrow.  Again, a plus sign must be used to separate this formula from those that were already present, as shown below.  Because the chemical formula of the solvent is now written on both sides of the arrow, the resultant equation is balanced. 

    \(\ce{HNO_3}\) + \(\ce{H_2O}\) \(\longrightarrow\) \(\ce{H^{+1}}\) + \(\ce{NO_3^{–1}}\) + \(\ce{H_2O}\) 

    Finally, as stated above, the purpose of a Brønsted-Lowry acid/base equation is to symbolically-represent the transfer of a proton, H+1, from a solute particle to a solvent molecule.  Therefore, in order to represent the absorption of a hydrogen ion, H+1, by a nearby solvent molecule, the formulas of these chemicals must be combined.  While the chemical formula of the solvent, H2O, is written on both sides of the arrow in the equation that is shown above, the hydrogen ion, H+1, is only symbolized on the right side of this equation.  As explained above, in order to indicate that two chemicals exist in the same environment and, consequently, can interact with one another, the formulas of those substances must be written on the same side of the arrow in an equation.  Therefore, the ion symbol of the proton, H+1, is combined with the solvent formula, H2O, on the right side of the equation, as shown below.

    \(\ce{HNO_3}\) + \(\ce{H_2O}\) \(\longrightarrow\) \(\ce{H_3O^{+1}}\) + \(\ce{NO_3^{–1}}\)

    The formula of the chemical that is generated by combining these substances is derived by adding both the subscripts that are associated with each constituent element and the overall charges of the particles.  Therefore, because the solvent in the given solution is water, H2O, combining this chemical with a hydrogen ion, H+1, generates a particle that contains three hydrogens, H, one oxygen, O, and a net +1 charge and, therefore, is symbolized as H3O+1.  The name of this ion, the hydronium ion, is derived by combining a prefix, "hydro-," that indicates the identity of the solvent, water, with a suffix, "-ium ion," that indicates that this particle bears a net positive charge.  Because the hydronium ion is always generated in aqueous solutions that contain acidic solutes, this particle will be further discussed in the remaining sections of this chapter.

    Because all of the components in the final equation that is shown above are balanced, this equation is the chemically-correct representation of the Brønsted-Lowry acid/base reaction that occurs between nitric acid, HNO3, which, as the solute, is a Brønsted-Lowry acid, and water, H2O, which is the solvent, and, therefore, the Brønsted-Lowry base, in the corresponding solution.  A Brønsted-Lowry acid/base equation will always be balanced, as-written, if the restructuring process that is described above is correctly applied to the chemicals that are present in a given solution equation.  Additionally, the molecules on the left and right sides of the reaction arrow can be written in any order, as long as their positions relative to the arrow remain constant.  Finally, recall that a "forward," or left-to-right, arrow is incorporated into a solution equation to indicate that a strong electrolyte has completely dissociated and that an equilibrium arrow represents the "forward" dissociation and "reverse" recombination processes that occur simultaneously during the solvation of a weak electrolyte.  Therefore, because only the relative locations of the chemical formulas in a solution equation, not the relative strengths of the associated molecules, are changed during the development of a Brønsted-Lowry acid/base equation, the type of arrow that is written in the final Brønsted-Lowry acid/base equation must be identical to the arrow that was given in the initial solution equation.

    Exercise \(\PageIndex{1}\)

    Develop a Brønsted-Lowry acid/base equation by restructuring the following balanced solution equation.  Balance the final Brønsted-Lowry equation by writing coefficients, as necessary.  (States of matter are not required.)

    \(\ce{HF}\) \(\overset{\ce{H_2O}}{\longrightleftharpoons}\) \(\ce{H^{+1}}\) + \(\ce{F^{–1}}\)

    Answer
    As stated previously, when a Brønsted-Lowry acid dissociates, the hydrogen ion, H+1, that is produced is immediately absorbed by a solvent molecule, which is classified as a Brønsted-Lowry base.  In order for this proton, H+1, transfer to occur, a solute particle must exist in close physical proximity to a solvent molecule.  However, because the chemical formula of the solute, hydrofluoric acid, HF, is written on the left side of the arrow in the given solution equation, and the formula of the solvent, water, H2O, is written above this arrow, the format of the solution equation that is shown above does not reflect that  these  solute and solvent molecules exist with one another in solution.  Therefore, the chemical formula of the solvent, H2O, must be moved from its current position in the given solution equation to the left side of the arrow, and a plus sign must be used to separate the formulas of these chemicals, as shown below.

    \(\ce{HF}\) + \(\ce{H_2O}\) \(\longrightleftharpoons\) \(\ce{H^{+1}}\) + \(\ce{F^{–1}}\)

    While the equation that is shown above accurately reflects that solute and solvent molecules exist in the same chemical environment in a solution, this equation is not balanced.  Recall that, by definition, the solvent is the chemical that is present in the greatest amount in a given solution.  Consequently, after the appropriate quantity of solute particles have dissociated, and the protonsH+1, that are produced have been absorbed by an equal number of solvent molecules, excess solvent molecules are still present in the resultant solution.  Therefore, the chemical formula of the solvent, H2O, should also be written on the right side of the arrow.  Again, a plus sign must be used to separate this formula from those that were already present, as shown below.  Because the chemical formula of the solvent is now written on both sides of the arrow, the resultant equation is balanced.

    \(\ce{HF}\) + \(\ce{H_2O}\) \(\longrightleftharpoons\) \(\ce{H^{+1}}\) + \(\ce{F^{–1}}\) + \(\ce{H_2O}\) 

    Finally, as stated above, the purpose of a Brønsted-Lowry acid/base equation is to symbolically-represent the transfer of a proton, H+1, from a solute particle to a solvent molecule.  Therefore, in order to represent the absorption of a hydrogen ion, H+1, by a nearby solvent molecule, the formulas of these chemicals must be combined.  As explained above, the formulas of two chemicals that exist in the same environment and, consequently, interact with one another, must be written on the same side of the arrow in an equation.  Therefore, the ion symbol of the proton, H+1, is combined with the solvent formula, H2O, on the right side of the equation that is shown above.  The formula of the chemical that is generated by combining these substances is derived by adding both the subscripts that are associated with each constituent element and the overall charges of the particles.  Therefore, because the solvent in the given solution is water, H2O, combining this chemical with a hydrogen ion, H+1, generates a hydronium ion, which contains three hydrogens, H, one oxygen, O, and a net +1 charge and, therefore, is symbolized as H3O+1, as shown below.

    \(\ce{HF}\) + \(\ce{H_2O}\) \(\longrightleftharpoons\) \(\ce{H_3O^{+1}}\) + \(\ce{F^{–1}}\)

    Because all of the components in the final equation that is shown above are balanced, this equation is the chemically-correct representation of the Brønsted-Lowry acid/base reaction that occurs between hydrofluoric acid, HF, which, as the solute, is a Brønsted-Lowry acid, and water, H2O, which is the solvent, and, therefore, the Brønsted-Lowry base, in the corresponding solution.  The molecules on the left and right sides of the reaction arrow can be written in any order, as long as their positions relative to the arrow remain constant.  Finally, because only the relative locations of the chemical formulas in a solution equation, not the relative strengths of the associated molecules, are changed during the development of a Brønsted-Lowry acid/base equation, the type of arrow that is written in the final Brønsted-Lowry acid/base equation must be identical to the arrow that was given in the initial solution equation.
    Exercise \(\PageIndex{2}\)

    Develop a Brønsted-Lowry acid/base equation by restructuring the following balanced solution equation.  Balance the final Brønsted-Lowry equation by writing coefficients, as necessary.  (States of matter are not required.)

    \(\ce{H_3PO_4}\) \(\overset{\ce{H_2O}}{\longrightleftharpoons}\) \(\ce{H^{+1}}\) + \(\ce{H_2PO_4^{–1}}\)

    Answer
    As stated previously, when a Brønsted-Lowry acid dissociates, the hydrogen ion, H+1, that is produced is immediately absorbed by a solvent molecule, which is classified as a Brønsted-Lowry base.  In order for this proton, H+1, transfer to occur, a solute particle must exist in close physical proximity to a solvent molecule.  However, because the chemical formula of the solute, phosphoric acid, H3PO4, is written on the left side of the arrow in the given solution equation, and the formula of the solvent, water, H2O, is written above this arrow, the format of the solution equation that is shown above does not reflect that  these  solute and solvent molecules exist with one another in solution.  Therefore, the chemical formula of the solvent, H2O, must be moved from its current position in the given solution equation to the left side of the arrow, and a plus sign must be used to separate the formulas of these chemicals, as shown below.

    \(\ce{H_3PO_4}\) + \(\ce{H_2O}\) \(\longrightleftharpoons\) \(\ce{H^{+1}}\) + \(\ce{H_2PO_4^{–1}}\)

    While the equation that is shown above accurately reflects that solute and solvent molecules exist in the same chemical environment in a solution, this equation is not balanced.  Recall that, by definition, the solvent is the chemical that is present in the greatest amount in a given solution.  Consequently, after the appropriate quantity of solute particles have dissociated, and the protonsH+1, that are produced have been absorbed by an equal number of solvent molecules, excess solvent molecules are still present in the resultant solution.  Therefore, the chemical formula of the solvent, H2O, should also be written on the right side of the arrow.  Again, a plus sign must be used to separate this formula from those that were already present, as shown below.  Because the chemical formula of the solvent is now written on both sides of the arrow, the resultant equation is balanced.

    \(\ce{H_3PO_4}\) + \(\ce{H_2O}\) \(\longrightleftharpoons\) \(\ce{H^{+1}}\) + \(\ce{H_2PO_4^{–1}}\) + \(\ce{H_2O}\) 

    Finally, as stated above, the purpose of a Brønsted-Lowry acid/base equation is to symbolically-represent the transfer of a proton, H+1, from a solute particle to a solvent molecule.  Therefore, in order to represent the absorption of a hydrogen ion, H+1, by a nearby solvent molecule, the formulas of these chemicals must be combined.  As explained above, the formulas of two chemicals that exist in the same environment and, consequently, interact with one another, must be written on the same side of the arrow in an equation.  Therefore, the ion symbol of the proton, H+1, is combined with the solvent formula, H2O, on the right side of the equation that is shown above.  The formula of the chemical that is generated by combining the these substances is derived by adding both the subscripts that are associated with each constituent element and the overall charges of the particles.  Therefore, because the solvent in the given solution is water, H2O, combining this chemical with a hydrogen ion, H+1, generates a hydronium ion, which contains three hydrogens, H, one oxygen, O, and a net +1 charge and, therefore, is symbolized as H3O+1, as shown below.

    \(\ce{H_3PO_4}\) + \(\ce{H_2O}\) \(\longrightleftharpoons\) \(\ce{H_3O^{+1}}\) + \(\ce{H_2PO_4^{–1}}\)

    Because all of the components in the final equation that is shown above are balanced, this equation is the chemically-correct representation of the Brønsted-Lowry acid/base reaction that occurs between phosphoric acid, H3PO4, which, as the solute, is a Brønsted-Lowry acid, and water, H2O, which is the solvent, and, therefore, the Brønsted-Lowry base, in the corresponding solution.  The molecules on the left and right sides of the reaction arrow can be written in any order, as long as their positions relative to the arrow remain constant.  Finally, because only the relative locations of the chemical formulas in a solution equation, not the relative strengths of the associated molecules, are changed during the development of a Brønsted-Lowry acid/base equation, the type of arrow that is written in the final Brønsted-Lowry acid/base equation must be identical to the arrow that was given in the initial solution equation.