Question

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

Short answer

\(H_2SO_4\) and \(HBr\) are acids; they dissociate in water forming \(H^+\) ions. \(Sr(OH)_2\) and \(NaOH\) are bases; they dissociate in water forming \(OH^-\) ions.

Step-by-step solution

Identify the Acids

An Arrhenius acid is a substance which, when dissolved in water, increases the concentration of hydrogen ion, \(H^+\)). Look for substances that can release \(H^+\)) ions into solution. \(H_2SO_4\)) and \(HBr\)) are such substances, so they are Arrhenius acids.

Write Acid Equations

Write the dissociation equations for the acids: \(H_2SO_4(aq) \rightarrow 2H^+(aq) + SO_4^{2-}(aq)\)) and \(HBr(aq) \rightarrow H^+(aq) + Br^-(aq)\)).

Identify the Bases

An Arrhenius base is a substance which, when dissolved in water, increases the concentration of hydroxide ion \(\(OH^-\)\)). Look for substances that can release \(\(OH^-\)\)) ions into solution. \(Sr(OH)_2\)) and \(NaOH\)) are such substances, so they are Arrhenius bases.

Write Base Equations

Write the dissociation equations for the bases: \(Sr(OH)_2(aq) \rightarrow Sr^{2+}(aq) + 2OH^-(aq)\)) and \(NaOH(aq) \rightarrow Na^+(aq) + OH^-(aq)\)).

Key concepts

Acid-Base Chemistry

Acid-base chemistry is a fundamental concept in chemistry that deals with the properties and reactions of acids and bases. According to the Arrhenius definition, an acid is a substance that, when dissolved in water, increases the concentration of hydrogen ions \(H^+\), and a base is a substance that, when dissolved in water, increases the concentration of hydroxide ions \(OH^-\). Understanding the nature of acids and bases allows us to predict their behavior in various chemical reactions, including neutralization, where an acid reacts with a base to produce water and a salt. An easy way to remember this is that Arrhenius acids produce 'H' for hydrogen ions, and Arrhenius bases produce 'OH' for hydroxide ions when dissolved in water.

Substances like sulfuric acid \(H_2SO_4\) and hydrobromic acid \(HBr\) are typical Arrhenius acids because they donate \(H^+\) ions to the solution. Similarly, strontium hydroxide \(Sr(OH)_2\) and sodium hydroxide \(NaOH\) are Arrhenius bases as they provide \(OH^-\) ions when dissolved. The behavior of these substances in water is crucial for predicting the outcome of acid-base reactions.

Chemical Dissociation Equations

Chemical dissociation equations represent how acids and bases break down into ions in water, a process that is essential to understanding the Arrhenius concept. For instance, sulfuric acid \(H_2SO_4\) dissociates into two hydrogen ions \(H^+\) and one sulfate ion \(SO_4^{2-}\) as shown: \[H_2SO_4(aq) \rightarrow 2H^+(aq) + SO_4^{2-}(aq)\]. Similarly, strontium hydroxide \(Sr(OH)_2\) dissociates into \(Sr^{2+}\) and hydroxide ions \(OH^-\): \[Sr(OH)_2(aq) \rightarrow Sr^{2+}(aq) + 2OH^-(aq)\]. Understanding these equations is critical as they demonstrate the actual change that occurs when an acid or base is dissolved in water, which in turn, affects the chemical properties of the solution, such as its pH level, reactivity, and electrical conductivity.

The ability to write these dissociation equations correctly is a key skill in acid-base chemistry because it allows students to visualize the ionic species present in solution, which is fundamental for further chemical analysis and predictions.

Hydrogen Ion Concentration

Hydrogen ion concentration is a measure of the amount of hydrogen ions \(H^+\) present in a solution. It's a crucial concept in acid-base chemistry because it directly influences the acidity of a solution. The more hydrogen ions present, the more acidic the solution and, consequently, the lower the pH. For instance, when sulfuric acid \(H_2SO_4\) dissociates in water, it releases hydrogen ions, thereby increasing the hydrogen ion concentration: \[H_2SO_4(aq) \rightarrow 2H^+(aq) + SO_4^{2-}(aq)\].

Measuring and understanding the hydrogen ion concentration in a solution is vital for several reasons. It can affect reaction rates, the solubility of compounds, and biological processes. For students, it is essential to relate the concentration of hydrogen ions to the strength of an acid; strong acids like \(H_2SO_4\) and \(HBr\) release more \(H^+\) ions into solution, making them more potent compared to weak acids.

Hydroxide Ion Concentration

Hydroxide ion concentration, in contrast to hydrogen ion concentration, determines the basicity of a solution. A higher concentration of hydroxide ions \(OH^-\) means a more basic (or alkaline) solution, and thus a higher pH. Bases like sodium hydroxide \(NaOH\) dissolve in water and increase the hydroxide ion concentration: \[NaOH(aq) \rightarrow Na^+(aq) + OH^-(aq)\].

The level of hydroxide ions is a key factor for various chemical processes, such as titrations and precipitations. For students, understanding the relationship between the hydroxide ion concentration and the base strength is critical. Strong bases like \(Sr(OH)_2\) and \(NaOH\) dissociate fully in water, contributing a larger amount of \(OH^-\) ions and thus, are more powerful in their basic properties compared to weak bases. Learning to calculate and interpret the hydroxide ion concentration helps students to grasp the concept of pH and its relationship to chemical solutions.