Problem 145 - Chemistry

Question

Give an example of (a) a weak acid that contains oxygen atoms, (b) a weak acid that does not contain oxygen atoms, (c) a neutral molecule that acts as a Lewis acid, (d) a neutral molecule that acts as a Lewis base, (e) a weak acid that contains two ionizable \(\mathrm{H}\) atoms, (f) a conjugate acid-base pair, both of which react with \(\mathrm{HCl}\) to give carbon dioxide gas.

Step-by-Step Solution

Answer

(a) Acetic acid; (b) Hydrocyanic acid; (c) Boron trifluoride; (d) Ammonia; (e) Carbonic acid; (f) HCO3- and H2CO3.

Step 1: Identifying a Weak Acid with Oxygen

One example of a weak acid that contains oxygen atoms is acetic acid, \(\text{CH}_3\text{COOH}.\) Acetic acid contains both carbon and oxygen atoms and is found in vinegar. It does not completely dissociate in water, which makes it a weak acid.

Step 2: Identifying a Weak Acid without Oxygen

An example of a weak acid that does not contain oxygen atoms is hydrocyanic acid, \(\text{HCN}.\) Hydrocyanic acid is a weak acid because it only partially ionizes in water, and it contains hydrogen, carbon, and nitrogen, but no oxygen.

Step 3: Identifying a Neutral Lewis Acid

A neutral molecule that acts as a Lewis acid is boron trifluoride, \(\text{BF}_3.\) Boron trifluoride can accept electron pairs into its empty p-orbital, making it a Lewis acid.

Step 4: Identifying a Neutral Lewis Base

A neutral molecule that acts as a Lewis base is ammonia, \(\text{NH}_3.\) Ammonia has a lone pair of electrons, allowing it to donate an electron pair and act as a Lewis base.

Step 5: Identifying a Weak Acid with Two Ionizable Hydrogens

Carbonic acid, \(\text{H}_2\text{CO}_3,\) is a weak acid that contains two ionizable hydrogen atoms. It can donate two protons, \(\text{H}^+,\) in solution.

Step 6: Finding a Conjugate Acid-Base Pair with CO2 Formation

The bicarbonate ion, \(\text{HCO}_3^-\) (base), and carbonic acid, \(\text{H}_2\text{CO}_3\) (acid), form a conjugate acid-base pair. When they react with \(\text{HCl},\) they produce carbon dioxide gas, as shown in the reactions: \(\text{HCO}_3^- + \text{HCl} \rightarrow \text{CO}_2 + \text{Cl}^- + \text{H}_2\text{O}\) and \(\text{H}_2\text{CO}_3 + \text{HCl} \rightarrow \text{CO}_2 + \text{Cl}^- + \text{H}_2\text{O}.\)

Key Concepts

Oxygen-containing acids

Oxygen-containing acids, also known as oxoacids, are acids that have oxygen atoms in their molecular structure. A common example is acetic acid, represented as \( \text{CH}_3\text{COOH} \). In such molecules, the presence of oxygen is significant because it often forms strong bonds with hydrogen, helping to stabilize the conjugate base. These acids typically exist in equilibrium in solution, only partially dissociating to form a small number of \( \text{H}^+ \) ions.

• Presence of oxygen contributes to the acid's properties.
• Oxygen's electronegativity enables stronger hydrogen bonding.
• They usually result in a well-stabilized conjugate base.

While the dissociation is not complete, the presence of oxygen still impacts their overall chemical behavior.

Lewis acids

Lewis acids are species that can accept a pair of electrons. They are characterized by having an empty orbital capable of holding an electron pair. Boron trifluoride, \(\text{BF}_3\), is a classic example. In this molecule, the boron atom has an incomplete octet and readily accepts electron pairs from donors.

• They often have an electron-deficient center.
• Commonly involve elements from groups 13 and 14 in the periodic table.
• Electron pair acceptance forms strong coordination bonds.

Understanding the electron-accepting nature of Lewis acids is critical in many chemical reactions and catalysis processes.

Lewis bases

Lewis bases are the counterparts to Lewis acids; they donate an electron pair. A good example is ammonia, \(\text{NH}_3\). Ammonia contains a lone pair of electrons that it can donate to form a bond with a Lewis acid.

• They have a non-bonding pair of electrons available for donation.
• They are often compounds that contain nitrogen, oxygen, or sulfur atoms.
• Their basicity is influenced by the availability and stability of the electron pair.

This electron donation ability plays a crucial role in forming complexes and many organic reactions, such as nucleophilic substitutions.

Ionizable hydrogen atoms

Ionizable hydrogen atoms in acids are those hydrogen atoms that can be released as \( \text{H}^+ \) ions. A molecule like carbonic acid, \(\text{H}_2\text{CO}_3\), has two such ionizable hydrogen atoms.

• Each hydrogen atom can dissociate under specific conditions.
• Determines the acidity and functionality of the acid.
• Often related to the hydrogens bonded to electronegative elements.

The ability to release more than one \( \text{H}^+ \) ion gives rise to polyprotic acids, which undergo multiple dissociation steps, each with its specific \(pK_a\) value.

Conjugate acid-base pairs

Conjugate acid-base pairs consist of two species that transform into each other by the gain or loss of a proton \( \text{H}^+ \). An example is the bicarbonate ion \(\text{HCO}_3^-\) and carbonic acid \(\text{H}_2\text{CO}_3\). In reactions, these pairs are crucial as they help maintain equilibrium in acid-base chemistry.

• One species donates a proton while the other accepts it.
• They are fundamental to buffer solutions.
• Can transform in the presence of strong acids or bases.

The interaction between these pairs and stronger acids, such as \( \text{HCl} \), can release gases like carbon dioxide, demonstrating their dynamic nature in chemical equilibria.