Problem 72 - Chemistry The Molecular Nature of Matter

Question

Explain why \(\mathrm{CaO}\) is an anhydrous base and \(\mathrm{SO}_{2}\) is an anhydrous acid.

Step-by-Step Solution

Answer

CaO is an anhydrous base because it reacts with water to form the basic compound Ca(OH)2, while SO2 is an anhydrous acid because it reacts with water to form sulfurous acid H2SO3.

Step 1: Understanding Anhydrous Bases

An anhydrous base is a base that does not contain water (H2O). Calcium oxide (CaO) is an anhydrous base because it can react with water to form calcium hydroxide (Ca(OH)2), which is a base.

Step 2: Reaction of Calcium Oxide with Water

The reaction of calcium oxide with water can be demonstrated through the chemical equation: CaO + H2O -> Ca(OH)2. This shows that CaO reacts with water to form a hydroxide, thus exhibiting basic properties in the absence of water.

Step 3: Understanding Anhydrous Acids

An anhydrous acid is a substance that shows acidic properties in the absence of water. Sulfur dioxide (SO2) is considered an anhydrous acid because it reacts with water to form sulfurous acid (H2SO3), demonstrating acidic properties.

Step 4: Reaction of Sulfur Dioxide with Water

The reaction of sulfur dioxide with water is as follows: SO2 + H2O -> H2SO3. Sulfur dioxide's ability to form an acid upon the addition of water shows that it is an anhydrous acid.

Key Concepts

Calcium Oxide (CaO) as an Anhydrous Base

Calcium oxide, commonly known as quicklime, is a classic example of an anhydrous base. An anhydrous substance is one that does not contain water. To understand why calcium oxide is classified as such, it's important to consider its behavior when brought into contact with water.

Upon the addition of water, calcium oxide undergoes a vigorous reaction that can be depicted by the chemical equation:
\[ \text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 \]
This process transforms the calcium oxide into calcium hydroxide, also recognized as slaked lime, which is a strong base. Remarkably, calcium oxide's ability to accept water and form a hydroxide proves its basic nature even in the absence of water, categorizing it as an anhydrous base. This property is significant when calcium oxide is employed to absorb moisture and remove impurities in various industrial processes, such as the manufacture of steel and cement.

Sulfur Dioxide (SO2) as an Anhydrous Acid

Transitioning from bases to acids, sulfur dioxide (SO2) serves as a prime example of an anhydrous acid. Anhydrous acids, much like their base counterparts, reveal their acidic characteristics in the absence of water. Sulfur dioxide, when exposed to water, engages in a chemical transformation akin to hydration reactions depicted below:

\[ \text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3 \]
This reaction produces sulfurous acid (H2SO3), indicating the acidic disposition of sulfur dioxide. It is pertinent to note that sulfur dioxide itself does not release protons (H+ ions) in its anhydrous form, but upon hydration, it easily forms an acid. This property underscores its role as an acid precursor and explains its wide use in the environmental context, such as in acid rain where it combines with atmospheric moisture.

Acid-Base Reactions

Digging deeper into acidity and basicity brings us to acid-base reactions, which are fundamental to chemistry. Acid-base reactions involve the transfer of protons (H+ ions) from acids to bases. This interaction is central to many chemical processes, both naturally occurring and synthetically designed.

Acid-Base Neutralization

A quintessential acid-base reaction is neutralization, where an acid and a base come together to form salt and water as follows:
\[ \text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{H}_2\text{O} \]
In the context of the substances discussed, if calcium hydroxide (formed from calcium oxide and water) were to react with sulfurous acid (formed from sulfur dioxide and water), a neutralization reaction would occur, ultimately yielding a salt (calcium sulfite) and water, reinforcing the complementary nature of acids and bases.

Understanding pH

The concept of pH also plays a substantial role in acid-base chemistry, measuring the acidity or basicity of a solution, thus indicating the concentration of hydrogen ions present. Acids have a pH less than 7, bases have a pH greater than 7, and a pH of 7 indicates a neutral substance, like pure water.

Through structured learning and exploration of chemical reactions, students can grasp the dynamic interactions between acids and bases, which are central to a myriad of biochemical and industrial processes.