6.3.9: The Solvent System Acid Base Concept
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The proton-centered nature of Arrhenius and Brønsted-Lowry definitions of acids and bases limits the utility of the acid-base formalism to reactions in protic solvents. A more generalizable theory is the solvent-system acid/base definition, which can be used to describe acid/base chemistry in non-protic solutions.
The solvent system acid-base concept generalizes the Arrhenius acid-base concept
Remember that the Brønsted-Lowry concept seeks to generalize Arrhenius acidity in ways that allow all hydrogen ion transfers to be thought of as acid-base reactions.
\[\ce{H_2O(l) + H_2O(l) → H_3O^{+}(aq) + OH^{-}(aq)} \nonumber \]
Like the Brønsted-Lowry acid-base concept, the solvent system acid-base concept is a way to generalize the Arrhenius acid-base concept by focusing on cations and anions generated in any solvent that autoionizes. These include Brønsted-Lowry type autoionizations:
\[\begin{align*} \ce{2NH_3(l)} &\ce{⇌ NH_4^{+} + NH_2^{-}} \\[4pt] \ce{2H_2SO_4(l)} &\ce{⇌ H_3SO_4^{+} + HSO_4^{-}} \end{align*} \nonumber \]
However, the solvent system definition also allows for autoionizations that involve the transfer of an ion other than hydrogen. For example,
\[\begin{align*}\ce{2SeOCl_2(l) }&\ce{⇌ SeOCl^{+} + SeOCl_3^{-}} \\[4pt] \ce{2BrF_3(l)} &\ce{ ⇌ BrF_2^{+} + BrF_4^{-}} \end{align*} \nonumber \]
However, the solvent system concept does not define acidity in terms of ion transfer. Rather, like the Arrhenius concept, it defines acids and bases in terms of the impact those acids and bases have on the concentrations of cations and anions in solution (Figure \(\PageIndex{1}\)).
This means that under the solvent system definition:
- an acid is the solvent cation or any substance that increases the concentration of the solvent cations normally produced by solvent autoionization.
- a base is the solvent anion or any substance that increases the concentration of the solvent anions normally produced by solvent autoionization.
Note that in the solvent system concept, salts of the solvent cation are acids and salts of the solvent anion are bases. For instance, if a salt of \(\ce{BrF_4^{-}}\) such as sodium tetrafluorobromate (\(\ce{NaBrF_4}\)) is added to \(\ce{BrF_3(l)}\), the concentration of \(\ce{BrF_4^{-}}\) increases.
The Solvent System Concept
The solvent system definition collapses to the Arrhenius definition for Brønsted acids in water. For instance, \(\ce{HCl}\) is an acid in water since it increases the concentration of \(\ce{H_3^{+}O}\) when it dissociates:
\[\ce{HCl(aq) + H_2O(l) → H_3O^{+}(aq) + Cl^{-}(aq)}\nonumber \]
The solvent system broadens the Arrhenius definition by allowing for similar reactions in a variety of solvents. For instance, \(\ce{HCl}\) also acts as an acid in liquid ammonia since it gives an \(\ce{NH_4^{+}}\) ion on dissociation.
\[ \underset{\textcolor{red}{acid}}{\ce{HCl}} + \ce{NH_3(l)} → \ce{NH_4^{+}} + \ce{Cl^{-}} \nonumber \]
Antimony pentafluoride acts as an acid in liquid \(\ce{BrF3}\) since it abstracts a fluoride to give \(\ce{BrF_2^{+}}\).
\[ \underset{\textcolor{red}{acid}}{\ce{SbF_5}} + \ce{BrF_3(l)} ⇌ \ce{SbF_6^{-}} + \ce{BrF_2^{+}} \nonumber \]
In contrast, the fluoride ion of potassium fluoride acts as a base, since it adds to \(\ce{BrF_3}\) to give \(\ce{BrF_4^{-}}\).
\[ \underset{\textcolor{blue}{base}}{\ce{KF}} + \ce{BrF_3(l)} ⇌ \ce{K^{+}} + \ce{BrF_4^{-}} \nonumber \]
The solvent system definition also allows for acid-base neutralization reactions. An example would be the reaction between \(\ce{KF}\) and \(\ce{SbF_5}\) in \(\ce{BrF_3}\).
\[ \underset{\textcolor{blue}{base}}{\ce{KF}}+ \underset{\textcolor{red}{acid}}{\ce{SbF_5}} → \ce{KSbF_6}\nonumber \]
although in this case it may be easier to see what is happening by writing the complete ionic form of the neutralization reaction equation.
\[\ce{K^{+}} + \underset{\textcolor{blue}{base}}{\ce{F^{-}}} + \underset{\textcolor{red}{acid}}{\ce{SbF_5}} → \ce{KSbF_6}\nonumber \]
The solvent system concept allows for successive ionizations just as the Brønsted-Lowry system does. Acids like sulfuric acid can be described well under the Arrhenius, Brønsted-Lowry, and solvent system definitions.
\[\begin{align*} \ce{2H_2SO_4} &⇌ \ce{H_3SO_4^{+} + HSO_4^{-}} \\[4pt] \ce{HSO_4^{-} + H_2SO_4} &⇌ \ce{H_3SO_4^+ + SO_4^{2-}} \end{align*} \nonumber \]
The solvent system definition also describes the autoionization of nonprotic solvents like thionyl chloride, \(\ce{SOCl_2}\):
\[\begin{align*} \ce{SOCl_2(l)} &⇌ \ce{ SOCl^{+} + SOCl_3^{-}} \\[4pt] \ce{SOCl^{+} + SOCl_2(l)} &⇌ \ce{SO^{2+} + SOCl_3^{-}} \end{align*} \nonumber \]
Successive equilibria are often useful in applying the solvent system to neutralization reactions, as may be seen in Example \(\PageIndex{1}\).
The reaction between sodium sulfite, \(\ce{Na_2SO_3}\), and thionyl chloride, \(\ce{SOCl_2}\), is a neutralization reaction according to the solvent system acid-base concept.
\[\ce{Na_2SO_3(s) + SOCl_2(l) → 2NaCl(s) + 2SO_2(g)} \nonumber \]
Write out the relevant equilibria and use them to explain the reaction in acid-base terms.
Solution
In the reaction shown the \(\ce{Na^+}\) just acts as a counterion. Consequently the reaction that should be considered is
\[\ce{2SO_3^{2-} + SOCl_2 → 2Cl^{-} + 2SO_2}\nonumber \]
A good place to start is to consider each of the species involved under the solvent-system acid-base concept by seeing if it is possible to write out ionization (or autoionization) reactions for them.
For \(\ce{SO_3^{2-}}\) these are
\[\begin{align*} \ce{2SO_3^{2-}} &⇌ \underset{\textcolor{red}{acid}}{\ce{SO_2}} + \underset{\textcolor{blue}{base}}{\ce{SO_4^{4-}}} \\[4pt] \underset{\textcolor{red}{acid}}{\ce{SO_2}} + \underset{\textcolor{blue}{base}}{\ce{2SO_3^{2-}}} &⇌ \underset{\textcolor{red}{acid}}{\ce{SO^{2+}}} + \underset{\textcolor{blue}{base}}{\ce{SO_4^{4-}}}\end{align*} \nonumber \]
These equations reveal that
- \(\ce{SO_3^{2-}}\) and \(\ce{SO_2}\) are amphoteric since they can act as either an acid or a base
- \(\ce{SO_4^{4-}}\) acts only as a base since it is the solvent anion
- \(\ce{SO^{2+}}\) acts only as an acid since it is the solvent cation
Since \(\ce{SO_3^{2-}}\) is amphoteric, this does not resolve the issue of whether it is acting as an acid or a base. However, notice that the product of the reaction between \(\ce{SO_3^{2-}}\) and \(\ce{SOCl_2}\) is \(\ce{SO_2}\). This means that \(\ce{SO3^{2-}}\) is acting as if it is following the pathway:
\[\ce{2SO_3^{2-} ⇌ SO_2 + SO_4^{4-}} \nonumber \]
The ionizations of \(\ce{SOCl_2}\) were already given in the main text but are also reproduced here for convenience:
\[\begin{align*} \ce{SOCl_2(l)} &\ce{<=> } \ce{SOCl^{+}} + \ce{SOCl_3^{-}} \\[4pt] \ce{SOCl^{+}} + \ce{SOCl_2(l)} &\ce{<=>} \ce{SO^{2+}} + \ce{SOCl_3^{-}}\end{align*} \nonumber \]
We could write an expression involving ionization of \(\ce{2SO^{2+}}\) to \(\ce{S^{4+}}\) and \(\ce{SO_2}\) but that is unnecessary for solving the present problem.
The equations we have written reveal that
- \(\ce{SOCl_2}\) and \(\ce{SOCl^{+}}\) are amphoteric since they can act as either an acid or a base
- \(\ce{SOCl_3^{-}}\) acts only as a base since it is the solvent anion
- \(\ce{SO^{2+}}\) acts only as an acid since it is the solvent cation
Again, since \(\ce{SOCl_2}\) is amphoteric this does not resolve the issue of whether it is acting as an acid or a base. However, notice that the product of the reaction between \(\ce{SO_3^{2-}}\) and \(\ce{SOCl_2}\) is \(\ce{SO_2}\). The only species in a reaction pathway involving \(\ce{SOCl_2}\) that can react to form \(\ce{SO_2}\) is \(\ce{SO^{2+}}\). This means that \(\ce{SOCl_2}\) is acting as if it is following the pathway:
\[\begin{align*} \ce{2SOCl_2(l) &⇌ SOCl^+ + SOCl_3^{-}} \\[4pt] \ce{SOCl^{+} + SOCl_2(l) &⇌ SO^{2+} + SOCl_3^{-}} \\[4pt] \ce{SO^{2+} + base &⇌ SO_2} + \text{the base's conjugate acid} \end{align*} \nonumber \]
But what sort of base could react with \(\ce{SO^{2+}}\) to give \(\ce{SO_2}\)? The candidates in our list of bases are \(\ce{SO_4^{4-}}\), \(\ce{SO_3^{2-}}\), and \(\ce{SO^{2+}}\) It must be one that donates an oxide ion, such as \(\ce{SO_4^{4-}}\). So in other words the \(\ce{SOCl_2}\) is following the pathway:
\[\begin{align*} \ce{2SOCl_2(l) &⇌ SOCl^+ + SOCl_3^{-}} \\[4pt] \ce{SOCl^{+} + SOCl_2(l) &⇌ SO^{2+} + SOCl_3^{-}} \\[4pt] \ce{SO^{2+} + SO_4^{4-} &⇌ SO_2 + SO_3^{2-}} \end{align*} \nonumber \]
and \(\ce{SO_3^{2-}}\) the pathway
\[\ce{2SO_3^{2-} ⇌ SO_2 + SO_4^{4-}} \nonumber \]
Adding these together gives the following net reaction:
\[\ce{3SOCl_2(l) + SO_3^{2-} ⇌ SO_2 + 2SOCl_3^{-}} \nonumber \]
At first glance this seems like it is not the correct equation. However, there is one acid-base reaction we are neglecting. The \(\ce{Cl^{-}}\) product of the reaction between sodium sulfite and thionyl chloride acts as a base in thionyl chloride:
\[\ce{SOCl_2(l) + Cl^- ⇌ SOCl_3^{-}} \nonumber \]
So the net reaction above simply corresponds to the reaction between sodium sulfite and thionyl chloride with additional reactions between the product \(\ce{Cl^{-}}\) and thionyl chloride added in. However, if a 1:1 ratio of thionyl chloride and sodium sulfate is used in the reaction, then the thionyl chloride will be consumed and the equilibrium between thionyl chloride and chloride ion shifted towards chloride, giving the desired net equation:
\[\ce{2SO_3^{2-} + SOCl_2 → 2Cl^- + 2SO_2} \nonumber \]
As Example \(\PageIndex{1}\) illustrates, the application of the solvent system concept to the understanding of acid-base reactions in solution can be quite involved but also serves to allow chemists to apply a detailed understanding of solution chemistry to a wider variety of reactions. However, it is usually much simpler to think about solvent system neutralization reactions using the Lewis acid-base concept.