Problem 71 - General Chemistry: Principles and Modern Applications

Question

For each reaction draw a Lewis structure for each species and indicate which is the acid and which is the base: (a) \(\mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{H}_{2} \mathrm{CO}_{3}\) (b) \(\mathrm{H}_{2} \mathrm{O}+\mathrm{BF}_{3} \longrightarrow \mathrm{H}_{2} \mathrm{OBF}_{3}\) (c) \(\mathrm{O}^{2-}+\mathrm{H}_{2} \mathrm{O} \longrightarrow 2 \mathrm{OH}^{-}\) (d) \(\mathrm{S}^{2-}+\mathrm{SO}_{3} \longrightarrow \mathrm{S}_{2} \mathrm{O}_{3}^{2-}\)

Step-by-Step Solution

Answer

In reaction (a), \(\mathrm{CO}_{2}\) is base and \(\mathrm{H}_{2} \mathrm{O}\) is acid .In reaction (b), \(\mathrm{BF}_{3}\) is the acid and \(\mathrm{H}_{2} \mathrm{O}\) is the base. In reaction (c), \(\mathrm{O}^{2-}\) is the base and \(\mathrm{H}_{2} \mathrm{O}\) is the acid. In reaction (d), \(\mathrm{SO}_{3}\) is the acid and \(\mathrm{S}^{2-}\) is the base.

Step 1: Draw Lewis Structures and Identify Acid and Base for Reaction (a)

Firstly, one has to draw Lewis structures for \(\mathrm{CO}_{2}\) and \(\mathrm{H}_{2} \mathrm{O}\). In \(\mathrm{CO}_{2}\), carbon is in the middle with double bonds to each oxygen. For water, the oxygen has two lone pairs and is bonded to two hydrogen atoms. In the balanced reaction, it is seen that \(\mathrm{CO}_{2}\) gained two hydrogen ions. This means \(\mathrm{H}_{2} \mathrm{O}\) donated hydrogen ions, therefore it is the acid and \(\mathrm{CO}_{2}\) is the base.

Step 2: Draw Lewis Structures and Identify Acid and Base for Reaction (b)

Draw Lewis structures for \(\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{BF}_{3}\). In \(\mathrm{BF}_{3}\), Boron is in the center with three bonding to three fluorine atoms. For water, the oxygen has two lone pairs and is bonded to two hydrogen atoms. In the reaction, \(\mathrm{BF}_{3}\) accepts a lone pair of electrons from \(\mathrm{H}_{2} \mathrm{O}\), making it the acid and \(\mathrm{H}_{2} \mathrm{O}\) the base.

Step 3: Draw Lewis Structures and Identify Acid and Base for Reaction (c)

Draw Lewis structures for \(\mathrm{O}^{2-}\) and \(\mathrm{H}_{2} \mathrm{O}\). The oxide anion has six electrons on the outer shell and accepts two hydrogen ions become water molecules. It is therefore the base and \(\mathrm{H}_{2} \mathrm{O}\) is the acid.

Step 4: Draw Lewis Structures and Identify Acid and Base for Reaction (d)

Draw Lewis structures for \(\mathrm{S}^{2-}\) and \(\mathrm{SO}_{3}\). In this reaction, sulphur ion gains oxygen atoms from \(\mathrm{SO}_{3}\), making \(\mathrm{SO}_{3}\) the acid (donating oxygen) and \(\mathrm{S}^{2-}\) the base (accepting oxygen).

Key Concepts

Acids and Bases

Understanding acids and bases is essential in chemistry, especially when discussing chemical reactions. In simple terms, acids are substances that can donate a proton (H+) or accept an electron pair. Bases, on the other hand, are substances that can accept a proton or donate an electron pair. These concepts explain how substances like water, carbon dioxide, and different ions interact during chemical reactions.

• Reaction (a) features water donating protons to carbon dioxide, acting as an acid, while carbon dioxide is the base.
• In reaction (b), water donates a lone pair of electrons to boron trifluoride, making BF3 an acid and water a base.
• For reaction (c), oxide ions accept protons from water, categorizing oxide ions as a base.
• Finally, in reaction (d), sulfur dioxide acts as an acid by donating oxygen atoms to sulfide ions.

These interactions illustrate the very foundation of acid-base chemistry, enabling substances to change forms or create new compounds.

Chemical Reactions

Chemical reactions involve the transformation of reactants into products. This transformation often involves the breaking and forming of chemical bonds, which can be better understood through Lewis structures. Lewis structures, also known as electron dot structures, show how atoms and electrons are arranged in a molecule. They help in predicting the nature of the chemical reaction.
In the exercises:

• In (a), carbon dioxide ( CO_2 ) reacts with water ( H_2O ) to form carbonic acid ( H_2CO_3 ). Lewis structures reveal the double bonds in CO2 and lone pairs in water.
• For reaction (b), Lewis structures illustrate how a lone pair from water bonds with BF3 to form H_2OBF_3 .
• In (c), the conversion of oxide ion and water to hydroxide ions ( OH^- ) illustrates bases gaining hydrogen ions.
• Lastly, reaction (d) shows sulfur ions combining with sulfur trioxide (via oxygen transfer) to form thiosulfate ions ( S_2O_3^{2-} ).

By observing these Lewis structures, one can easily visualize electron transfers and molecular changes during reactions.

Molecular Structure

Molecular structure significantly impacts how chemical species interact during reactions. The molecular structure is determined by the arrangement of atoms and the bonds between them. Lewis structures are an invaluable tool in representing these arrangements. They help in predicting molecular geometry, bond formation, and reactivity.

• In reaction (a), the linear shape of CO2 and the bent shape of water influence their interaction to form carbonic acid.
• For reaction (b), BF3's trigonal planar structure makes it ideal for accepting electron pairs from water, forming new bonds.
• Reaction (c) involves spherical oxide ions interacting with water to form tetrahedral hydroxide ions.
• While sulfur trioxide ( SO_3 ) has a trigonal planar molecular structure, it helps efficiently transfer oxygen atoms to the tetrahedral sulfide ion shape, forming thiosulfate ions.

These molecular structures not only determine the physical arrangement of atoms but also play a critical role in the chemical behavior during reactions. Understanding these structures from the Lewis diagrams aids in mastering the prediction of how molecules will behave in different scenarios.