Question

Identify each substance as an acid or a base and write a chemical equation showing how it is an acid or a base in aqueous solution according to the Arrhenius definition. a. \(\mathrm{NaOH}(a q)\) b. \(\mathrm{H}_{2} \mathrm{SO}_{4}(a q)\) c. \(\operatorname{HBr}(a q)\) d. \(\operatorname{Sr}(\mathrm{OH})_{2}(a q)\)

Short answer

\(\mathrm{NaOH}(aq)\) and \(\mathrm{Sr}(\mathrm{OH})_2(aq)\) are bases; \(\mathrm{H}_2\mathrm{SO}_4(aq)\) and \(\mathrm{HBr}(aq)\) are acids. Equations are: \(\mathrm{NaOH}(aq) \rightarrow \mathrm{Na}^+(aq) + \mathrm{OH}^-(aq)\), \(\mathrm{H}_2\mathrm{SO}_4(aq) \rightarrow 2\mathrm{H}^+(aq) + \mathrm{SO}_4^{2-}(aq)\), \(\mathrm{HBr}(aq) \rightarrow \mathrm{H}^+(aq) + \mathrm{Br}^-(aq)\), and \(\mathrm{Sr}(\mathrm{OH})_2(aq) \rightarrow \mathrm{Sr}^{2+}(aq) + 2\mathrm{OH}^-(aq)\).

Step-by-step solution

Identify Substance A as Acid or Base

Identify \(\mathrm{NaOH}(aq)\) as an acid or a base. According to the Arrhenius definition, a base is a substance that increases the concentration of \(\mathrm{OH}^-\) ions in aqueous solution. Since \(\mathrm{NaOH}\) dissociates to produce \(\mathrm{OH}^-\) ions in water, it is an Arrhenius base.

Write Equation for Substance A

Write the chemical equation for \(\mathrm{NaOH}(aq)\) acting as a base in aqueous solution:\[\mathrm{NaOH}(aq) \rightarrow \mathrm{Na}^+(aq) + \mathrm{OH}^-(aq)\]

Identify Substance B as Acid or Base

Identify \(\mathrm{H}_2\mathrm{SO}_4(aq)\) as an acid or a base. According to the Arrhenius definition, an acid is a substance that increases the concentration of \(\mathrm{H}^+\) ions in aqueous solution. As \(\mathrm{H}_2\mathrm{SO}_4\) dissociates to produce \(\mathrm{H}^+\) ions in water, it is an Arrhenius acid.

Write Equation for Substance B

Write the chemical equation for \(\mathrm{H}_2\mathrm{SO}_4(aq)\) acting as an acid in aqueous solution:\[\mathrm{H}_2\mathrm{SO}_4(aq) \rightarrow 2\mathrm{H}^+(aq) + \mathrm{SO}_4^{2-}(aq)\]

Identify Substance C as Acid or Base

Identify \(\mathrm{HBr}(aq)\) as an acid or a base. As \(\mathrm{HBr}\) dissociates in water to produce \(\mathrm{H}^+\) ions, it follows the Arrhenius definition of an acid.

Write Equation for Substance C

Write the chemical equation for \(\mathrm{HBr}(aq)\) acting as an acid in aqueous solution:\[\mathrm{HBr}(aq) \rightarrow \mathrm{H}^+(aq) + \mathrm{Br}^-(aq)\]

Identify Substance D as Acid or Base

Identify \(\mathrm{Sr}(\mathrm{OH})_2(aq)\) as an acid or a base. Since it dissociates in water to produce \(\mathrm{OH}^-\) ions, it behaves as an Arrhenius base.

Write Equation for Substance D

Write the chemical equation for \(\mathrm{Sr}(\mathrm{OH})_2(aq)\) acting as a base in aqueous solution: \[^+_2\mathrm{Sr}(\mathrm{OH})_2(aq) \rightarrow \mathrm{Sr}^{2+}(aq) + 2\mathrm{OH}^-(aq)\]

Key concepts

Acid-Base Reactions

Understanding acid-base reactions is fundamental to grasping the concepts of chemistry, especially in aqueous solutions where these reactions commonly occur. An acid-base reaction involves the transfer of a proton (a hydrogen ion, \( H^+ \)) from an acid to a base. According to the Arrhenius definition, an acid is a substance that increases the concentration of hydrogen ions \( H^+ \) in an aqueous solution, while a base increases the concentration of hydroxide ions \( OH^- \).

When \( NaOH \) (sodium hydroxide) is dissolved in water, it dissociates into \( Na^+ \) and \( OH^- \) ions. Being a strong base, it readily donates \( OH^- \) ions to the solution, increasing its basicity. Conversely, \( H_2SO_4 \) (sulfuric acid) and \( HBr \) (hydrobromic acid) are strong acids that dissociate in water to release \( H^+ \) ions, increasing the solution's acidity.

Such reactions are essential in many chemical and biological processes, including metabolism and environmental science. They are also the basis for titration techniques used to determine the concentration of an acidic or basic solution.

Aqueous Solution Chemistry

Aqueous solution chemistry is the study of substances dissolved in water, which is a common solvent due to its polarity and ability to dissolve a wide range of compounds. When substances dissolve in water, they may dissociate into ions (an ionic compound such as \( NaOH \) or produce \( H^+ \) or \( OH^- \) ions if they are acids or bases according to the Arrhenius theory.

For instance, when \( Sr(OH)_2 \) (strontium hydroxide) is in aqueous solution, it dissociates into \( Sr^{2+} \) and \( OH^- \). This ability to produce \( OH^- \) ions characterizes it as an Arrhenius base. The degree of dissociation depends on the compound's nature and the temperature of the solution. Strong electrolytes like \( NaOH \) and \( H_2SO_4 \) dissociate completely in solution, while weak electrolytes do not.

Understanding the behavior of substances in water is crucial for fields such as biochemistry, environmental chemistry, and pharmacology, as it affects reactions, bioavailability, and the properties of natural and waste waters.

Chemical Equations

Chemical equations are symbolic representations of chemical reactions, showing the reactants, products, and their states of matter. They provide valuable information about the quantitative and qualitative aspects of the chemical reaction.

For an acid or a base in an aqueous solution, the chemical equation is used to represent the dissociation into ions. For example, the dissociation of \( NaOH \) in water can be written as:
\[\mathrm{NaOH}(aq) \rightarrow \mathrm{Na}^+(aq) + \mathrm{OH}^-(aq)\]
Sulfuric acid's dissociation, on the other hand, appears as:
\[\mathrm{H}_2\mathrm{SO}_4(aq) \rightarrow 2\mathrm{H}^+(aq) + \mathrm{SO}_4^{2-}(aq)\]
Each chemical equation must be balanced, meaning the number of atoms for each element, and the total charge must be the same on both sides of the equation. Writing and balancing chemical equations is a crucial skill in chemistry that enables one to predict the outcomes of reactions and calculate the amounts of reactants and products involved.