Question

The Nature and Definitions of Acids and Base 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. $$\begin{array}{l}{\text { a. } \mathrm{HNO}_{3}(a q)} \\ {\text { b. } \mathrm{NH}_{4}^{+}(a q)} \\ {\text { c. } \mathrm{KOH}(a q)} \\ {\text { d. } \mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}(a q)}\end{array}$$

Short answer

a. Acid: HNO3(aq) -> H+(aq) + NO3-(aq), b. Acid: NH4+(aq) + H2O(l) -> NH3(aq) + H3O+(aq), c. Base: KOH(aq) -> K+(aq) + OH-(aq), d. Acid: HC2H3O2(aq) -> H+(aq) + C2H3O2-(aq).

Step-by-step solution

Identify the Nature of HNO3

Since HNO3 dissociates in water to produce H+ ions, it is an acid according to the Arrhenius definition. The chemical equation for its dissociation is: HNO3(aq) -> H+(aq) + NO3-(aq).

Identify the Nature of NH4+

The NH4+ ion reacts with water to produce H+ ions, therefore according to the Arrhenius definition, it is an acid. The chemical equation is: NH4+(aq) + H2O(l) -> NH3(aq) + H3O+(aq).

Identify the Nature of KOH

KOH dissociates in water to produce OH- ions, so it is a base according to the Arrhenius definition. The chemical equation is: KOH(aq) -> K+(aq) + OH-(aq).

Identify the Nature of HC2H3O2

HC2H3O2, also known as acetic acid, dissociates in water to produce H+ ions, which makes it an acid according to the Arrhenius definition. The chemical equation is: HC2H3O2(aq) -> H+(aq) + C2H3O2-(aq).

Key concepts

Chemical Equations

Understanding chemical equations is fundamental to grasping how substances interact during chemical reactions. A chemical equation is a symbolic representation showing how reactants are transformed into products. It can be thought of as a recipe that outlines which ingredients (reactants) combine and what they make (products).

For instance, when we write the chemical equation for the dissociation of hydrochloric acid (HCl), it looks like this:
HCl(aq) → H+(aq) + Cl-(aq).
This equation shows that when HCl is dissolved in water (indicated by the 'aq' for aqueous), it separates into hydrogen ions (H+) and chloride ions (Cl-). This specific language of chemistry is precise, allowing scientists and students alike to communicate complex processes in a simple, standardized way. Every chemical equation must be balanced, meaning the number of atoms for each element in the reactants must equal the number in the products, reflecting the conservation of mass.

Acid-Base Reactions

An acid-base reaction is a type of chemical reaction that occurs between an acid and a base. Acid-base reactions are essential in both chemistry and biology. The Arrhenius definition, one of the earliest concepts taught, specifies that acids produce hydrogen ions (H+) when dissolved in water, and bases produce hydroxide ions (OH-) when dissolved in water.

The reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH) is a classic example:
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l).
In this reaction, the H+ from the acid (HCl) reacts with the OH- from the base (NaOH) to form water (H2O) and an ionic compound, sodium chloride (NaCl), which remains dissolved in the water. This is also known as neutralization because the acid and base neutralize each other's properties, resulting in a salt and water.

Dissociation of Acids and Bases

The dissociation of acids and bases refers to their separation into ions when dissolved in water. For acids, this means releasing hydrogen ions (H+) into the solution, while for bases, it involves releasing hydroxide ions (OH-).

The strength of an acid or base is determined by its ability to dissociate in water. A strong acid or base dissociates completely, while a weak acid or base only partially dissociates. For example, hydrochloric acid (HCl), a strong acid, dissociates completely as follows:
HCl(aq) → H+(aq) + Cl-(aq).
Meanwhile, acetic acid (HC2H3O2), a weak acid, only partially dissociates, creating fewer hydrogen ions:
HC2H3O2(aq) ↔ H+(aq) + C2H3O2-(aq).
The double arrow indicates the reaction can go both ways and the acid doesn't fully dissociate in water. Understanding acid and base dissociation is essential for predicting the outcome of many chemical reactions, including those that govern biological systems and environmental processes.