Problem 86 - Chemistry The Central Science In Si Units

Question

Write a balanced chemical equation for the formation of a polymer via a condensation reaction from the monomers 1,4-phenylenediamine \(\left(\mathrm{H}_{2} \mathrm{NC}_{6} \mathrm{H}_{4} \mathrm{NH}_{2}\right)\) and terephthalic acid \(\left(\mathrm{HOOCC}_{6} \mathrm{H}_{4} \mathrm{COOH}\right)\)

Step-by-Step Solution

Answer

The balanced equation is: \([\text{H}_2\text{N-C}_6\text{H}_4\text{-NH}_2]_n + n[\text{HOOC-C}_6\text{H}_4\text{-COOH}] \rightarrow [\text{-NH-C}_6\text{H}_4\text{-NH-CO-C}_6\text{H}_4\text{-CO-}]_n + 2n\text{H}_2\text{O} \).

Step 1: Understand Monomers Structure

Recognize the structures of the given monomers: 1,4-phenylenediamine is \( \text{(H}_2\text{N-C}_6\text{H}_4\text{-NH}_2) \) and terephthalic acid is \( \text{(HOOC-C}_6\text{H}_4\text{-COOH)} \). These monomers will react to form a polymer via a condensation reaction.

Step 2: Analyze Condensation Reaction Mechanism

In a condensation reaction, the amino group (–NH₂) of 1,4-phenylenediamine reacts with the carboxylic acid group (–COOH) of terephthalic acid, releasing a water molecule (H₂O) and forming an amide linkage between the monomers.

Step 3: Write the Unbalanced Equation

Set up the unbalanced equation by indicating the transformation: \( \text{H}_2\text{N-C}_6\text{H}_4\text{-NH}_2 + \text{HOOC-C}_6\text{H}_4\text{-COOH} \rightarrow \{\text{Polymer} \} + \text{H}_2\text{O} \).

Step 4: Balance the Chemical Equation

Account for each atom in reactants and products. In this case, to form a polymer of repeating units, balance by adjusting the number of repetitions of each monomer, ensuring one water molecule is produced per linkage. A balanced equation for 'n' repeating units would be: \( [\text{H}_2\text{N-C}_6\text{H}_4\text{-NH}_2]_n +n[\text{HOOC-C}_6\text{H}_4\text{-COOH}]_n \rightarrow [\text{-NH-C}_6\text{H}_4\text{-NH-CO-C}_6\text{H}_4\text{-CO-}]_n + 2n\text{H}_2\text{O} \).

Step 5: Finalize the Balanced Equation

Simplify and confirm the balance of the equation, which shows a typical representation for condensation polymerization. Each amide bond formed between the monomers yields a water molecule, maintaining the stoichiometry of the reaction.

Key Concepts

Balanced Chemical Equation

A balanced chemical equation is crucial because it ensures that the same number of each type of atom appears on both sides of the equation. To accomplish this for condensation polymerization, we need to understand the interaction between monomers during the reaction process.
Condensation reactions involve monomers joining together, typically with the loss of small molecules like water. This process results in the formation of polymers, large molecules composed of repeating structural units.

When writing a balanced chemical equation for polymer formation, consider these steps:

• Identify the reactants: In our case, 1,4-phenylenediamine ext((H_2NC_6H_4NH_2) ext) and terephthalic acid ext((HOOC-C_6H_4-COOH) ext).
• Understand that each amide linkage formed corresponds to the release of one water molecule.
• To form a polymer with 'n' repeating units, simply balance as ext([(H_2NC_6H_4NH_2)]_n + [HOOC-C_6H_4-COOH]_n ightarrow [-NH-C_6H_4-NH-CO-C_6H_4-CO-]_n + 2nH_2O) ext.

Balancing equations in polymer chemistry requires careful attention to the stoichiometry of the reaction.

Amide Linkage

An amide linkage is a type of covalent bond in which an amine group (NH₂) of one monomer reacts with a carboxyl group (COOH) of another monomer.
This results in the formation of an amide (-CONH-) and is central in the creation of polymers through condensation polymerization. The amide linkage gives polymers unique properties like strength and resistance to chemicals, making them valuable for multiple applications.

• In our specific example, the amide linkage forms between the nitrogen on 1,4-phenylenediamine and the carbon from the carboxylic group of terephthalic acid.
• This type of bond is integral to many polymers, including nylons and polyamides.
• The process can repeat in a chain reaction, forming long, durable polymer chains.

The resulting polymer features repeated amide bonds between each former monomer, enhancing stability and versatility.

Water Molecule Release

The release of water molecules is a defining feature of condensation polymerization.
During the reaction to form polymers, the joining of monomers via amide linkages causes the expulsion of water. This is because the carboxyl group (COOH) from one monomer reacts with the amino group (NH₂) from another.

This process is characterized by three important points:

• In every instance where an amide linkage forms, one molecule of water is released.
• The production of water is a result of the removal of a hydrogen atom from the amine group and a hydroxyl group from the carboxyl group.
• For a polymer with 'n' units, the formation typically results in \( 2n \) water molecules being released.

This characteristic is what differentiates condensation polymerization from other types of polymerization, like addition polymerization, where no small molecules are created as by-products.