Question

The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. How does the length of a hydrocarbon's polymer chains affect its viscosity and volatility? (C)

Short answer

The length of a hydrocarbon's polymer chains affects its viscosity and volatility as follows: An increase in the polymer chain length leads to an increase in viscosity, due to stronger intermolecular forces resulting in greater resistance to flow. Conversely, an increase in the polymer chain length leads to a decrease in the substance's volatility, as larger molecules require more energy to overcome intermolecular forces and evaporate.

Step-by-step solution

Understanding hydrocarbon polymer chains

Hydrocarbon polymers are large molecules made up of smaller hydrocarbon monomers linked together. The length of a polymer chain refers to the number of monomers constituting the chain. The longer the chain, the larger and more complex the molecule.

Understanding viscosity and volatility

Viscosity is a measure of a fluid's resistance to flow, or the internal friction between the fluid's particles. It represents the "thickness" of a fluid, with higher viscosity fluids being thicker and more resistant to flow. Volatility is a measure of a substance's tendency to evaporate or change from a liquid to a gas. A higher volatility implies that a substance evaporates more easily.

Effect of polymer chain length on viscosity

As the length of a hydrocarbon's polymer chains increases, the size and complexity of the molecules also increase. Larger molecules tend to have stronger intermolecular forces, which result in greater internal friction and resistance to flow. Therefore, increasing the polymer chain length of a hydrocarbon generally results in an increased viscosity.

Effect of polymer chain length on volatility

Since larger molecules have stronger intermolecular forces, they require more energy to break these forces and transition from a liquid to a gas phase. Hence, as the length of a hydrocarbon's polymer chains increases, the substance is less likely to evaporate and its volatility decreases.

Summary

The length of a hydrocarbon's polymer chains affects its viscosity and volatility in the following ways: 1. An increase in the polymer chain length leads to an increase in the viscosity of the hydrocarbon, as larger molecules have stronger intermolecular forces causing greater resistance to flow. 2. An increase in the polymer chain length leads to a decrease in the volatility of the hydrocarbon, as larger molecules require more energy to overcome intermolecular forces and evaporate.

Key concepts

Understanding Viscosity

Viscosity is essentially how we describe the 'thickness' of a fluid, which relates directly to how easily it flows. When teaching about viscosity, imagine comparing water to honey. Water flows quickly and with minimal resistance, hence it has low viscosity. Honey, on the other hand, is much thicker and flows slowly, indicating high viscosity.

For hydrocarbon polymers, the viscosity changes depending on the chain length. If you have long polymer chains, they move past each other more slowly due to increased interactions between the molecules. This is similar to trying to walk through a crowd—the more people, the slower you go. Thus, longer chains mean higher viscosity, and the substance will flow less readily. In practical terms, motor oil is a great example; it’s designed to be viscous enough to lubricate engine parts effectively.

Deciphering Volatility

Volatility, simply put, is a substance's willingness to leap into the air as a vapor. Think of alcohol or gasoline; they evaporate very quickly, signaling high volatility. Now, imagine spilling water on your hand; it takes longer to evaporate, indicative of lower volatility.

With hydrocarbon polymers, volatility tends to decrease as the length of the polymer chain increases. This occurs because longer chains have more surface area for intermolecular forces to act upon, which means they'll stick together more tenaciously. To evaporate, molecules need enough energy to overcome these attractions, and longer chains need more energy to do so. That's why heavier, long-chain hydrocarbons, like those in heavy fuel oil, don't evaporate as quickly as lighter ones, such as those found in nail polish remover.

Intermolecular Forces at Play

The strength of intermolecular forces can be a difficult concept to visualize. Intermolecular forces are the forces that hold molecules together, but not the bonds within the molecule itself. These are like the handshakes between molecules. The stronger the handshake, the harder it is to pull your hand away.

In the context of hydrocarbon polymers, the lengthier the polymer chain, the stronger these molecular handshakes become. This strength contributes to the aforementioned increase in viscosity and decrease in volatility. Essentially, molecules with longer chains have more handshakes occurring along their length, leading to stronger overall intermolecular forces.

Impact of Polymer Chain Length

Polymer chain length is akin to the length of a train. A longer train takes more effort to get moving and to stop compared to a short one. Similarly, in polymer chemistry, chain length can have significant effects on physical properties such as melting point, strength, and flexibility.

Longer hydrocarbon polymer chains are bigger and more complex, contributing to the increased intermolecular forces. It's this complexity that increases a polymer's resistance to flow (viscosity) and reduces its eagerness to vaporize (volatility). When dealing with polymers, understanding this concept is critical. It helps explain why certain plastics are more rigid while others are more flexible, and why some substances are better suited for specific applications, like plastic bags versus bulletproof vests.