Problem 86 - Chemistry: The Central Science in SI Units, Global Edition

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 chemical equation for the formation of a polymer via a condensation reaction between 1,4-phenylenediamine $\left(\mathrm{H}_{2} \mathrm{NC}_{6} \mathrm{H}_{4} \mathrm{NH}_{2}\right)$ and terephthalic acid $\left(\mathrm{HOOCC}_{6} \mathrm{H}_{4} \mathrm{COOH}\right)$ is: \( \mathrm{H}_{2} \mathrm{NC}_{6} \mathrm{H}_{4} \mathrm{NH}_{2} + \mathrm{HOOCC}_{6} \mathrm{H}_{4} \mathrm{COOH \to [H}_{2} \mathrm{NC}_{6} \mathrm{H}_{4} \mathrm{NH - CO - C}_{6} \mathrm{H}_{4} \mathrm{CO]_n + \mathrm{H}_{2} \mathrm{O} \)

Step 1: Identify the reacting groups of the monomers

In the monomers, 1,4-phenylenediamine has two amino groups (-NH_2), and terephthalic acid has two carboxylic acid groups (-COOH). These groups will react to form an amide bond (-CONH-) and release a water molecule.

Step 2: Write the chemical equation for the reaction

The condensation reaction between 1,4-phenylenediamine and terephthalic acid can be represented as: H_2NC_6H_4NH_2 + HOOC - C_6H_4 - COOH → [H_2NC_6H_4NH - CO - C_6H_4 - CO]n + H_2O Here, [H_2NC_6H_4NH - CO - C_6H_4 - CO]n represents the repeating unit of the polymer, and n is the degree of polymerization.

Step 3: Balance the chemical equation

In this case, the chemical equation is already balanced as each side of the equation contains the same number of atoms for each element (2 H, 1 N, 4 C for the 1,4-phenylenediamine monomer and 8 H, 2 O, and 8 C for the terephthalic acid monomer). Therefore, the balanced chemical equation for the formation of the polymer via this condensation reaction is: H_2NC_6H_4NH_2 + HOOC - C_6H_4 - COOH → [H_2NC_6H_4NH - CON(C_6H_4)]n + H_2O

Key Concepts

Polymer Formation

Polymer formation through a condensation reaction involves combining monomers to create long-chain polymer molecules. In the case of 1,4-phenylenediamine and terephthalic acid, these monomers join by establishing amide bonds through eliminating a molecule of water. This kind of reaction not only extends the chain length but also builds stronger polymer structures. The newly formed amide linkages increase the chain's rigidity and strength. As monomers combine, their functional groups react; here, the amino groups (-NH₂) of 1,4-phenylenediamine react with the carboxylic acid groups (-COOH) of terephthalic acid to form the polyamide chain. The repeating unit of the polymer acts as the foundation and dictates the overall properties of the resulting material.

Balanced Chemical Equation

A balanced chemical equation accurately portrays the reactants and products involved in a chemical reaction. Ensuring that the number of atoms for each element is equal on both sides of the equation is vital. In our polymerization example, the equation provided: H₂NC₆H₄NH₂ + HOOC-C₆H₄-COOH → [H₂NC₆H₄NH-CO-C₆H₄-CO]ₙ + H₂O This equation captures the major reactants and the resulting polyamide formed. Each atom count is maintained across the equation, indicating a sound balance. To write a balanced equation, follow these steps:

• Identify the functional groups and reacting portions of each monomer.
• Calculate the atoms on both sides of the equation.
• Confirm that all elements balance, resulting in no excess atoms or molecules left out.

Balancing equations is fundamental in illustrating conservation of mass and showing that the reaction metrics adhere to empirical laws.

Amide Bond

Amide bonds are crucial connections within polymer chains formed through condensation reactions. In the synthesis of repeating polyamide units, an amide bond (-CONH-) connects monomers. How do these bonds form? When primary amines (-NH₂) react with carboxylic acids (-COOH), the loss of a water molecule leads to the covalent linkage called an amide bond:

• The amino group's nitrogen atom attaches to the carbon atom in the carboxyl group, while a hydroxyl group (-OH) and a hydrogen (-H) are removed as water.
• This new amide linkage acts as a sturdy chain in polymers, contributing to the toughness and heat resistance typical of many polyamide materials such as nylon.

Understanding amide bonds provides insights into polymer durability and performance, which makes them desirable for many industrial applications.

Degree of Polymerization

The degree of polymerization (DP) refers to the total number of repeating units within a polymer molecule. It provides insight into the length and, therefore, the properties of the polymer. For any polymerization reaction described, the value of "n" in the equation [H₂NC₆H₄NH-CO-C₆H₄-CO]ₙ indicates this degree. The larger the "n",

• The longer the polymer chain, resulting in increased material strength and durability.
• Increased viscosity and melting point.
• Altered mechanical properties which may include elasticity, rigidity, or toughness depending on the polymer type.

Understanding DP helps us tailor the polymer synthesis to meet specific engineering and commercial requirements, creating materials with precisely controlled attributes.