Problem 48 - Chemistry: The Central Science

Question

What molecular structural features cause high-density polyethylene to be denser than low-density polyethylene?

Step-by-Step Solution

Answer

The primary molecular structural feature causing high-density polyethylene (HDPE) to be denser than low-density polyethylene (LDPE) is their chain organization. HDPE has a linear structure with few or no side chains, allowing its chains to pack more tightly together. In contrast, LDPE has a highly branched structure with many side chains, leading to looser packing and lower density. This difference in packing results in a higher mass per unit volume for HDPE, making it denser than LDPE.

Step 1: 1. Understanding the Structure of Polyethylene

Polyethylene is a polymer made of repeating ethylene (C2H4) units. The difference between high-density polyethylene (HDPE) and low-density polyethylene (LDPE) lies in the organization of these repeating units in the polymer chains.

Step 2: 2. Comparing LDPE and HDPE Molecular Structures

In low-density polyethylene (LDPE), the polymer chains have a highly branched structure, meaning that there are many side chains attached to the main chain. On the other hand, high-density polyethylene (HDPE) has a linear structure, with very few or no side chains branching out from the main chain.

Step 3: 3. Influence of Molecular Structure on Packing

The difference in molecular structure between LDPE and HDPE has a significant impact on how the polymer chains can pack together. In LDPE, the highly branched structure causes polymer chains to pack loosely, creating regions of empty space between the chains. In HDPE, the linear structure allows the polymer chains to pack more tightly together, meaning there is less empty space between the chains.

Step 4: 4. Relating Polymer Packing to Density

Density is defined as the mass per unit volume of a substance. In the case of LDPE and HDPE, the difference in their molecular structures influences how they pack together, which in turn affects their densities. Since HDPE has a more linear structure, its chains can pack tightly together, resulting in a higher mass per unit volume. In contrast, the highly branched structure of LDPE leads to looser packing and a lower mass per unit volume.

Step 5: 5. Conclusion

The molecular structural features that cause high-density polyethylene (HDPE) to be denser than low-density polyethylene (LDPE) are their differences in chain organization. HDPE has a linear structure that allows its chains to pack more tightly together, whereas LDPE has a highly branched structure that leads to looser packing. This difference in packing results in a higher density for HDPE compared to LDPE.