Problem 81 - Chemistry The Central Science In Si Units

Question

Use the normal boiling points propane \(\left(\mathrm{C}_{3} \mathrm{H}_{8}\right) \quad-42.1^{\circ} \mathrm{C}\) \(\begin{array}{lc}\text { propane }\left(\mathrm{C}_{3} \mathrm{H}_{8}\right) & -42.1^{\circ} \mathrm{C} \\ \text { butane }\left(\mathrm{C}_{4} \mathrm{H}_{10}\right) & -0.5^{\circ} \mathrm{C} \\ \text { pentane }\left(\mathrm{C}_{5} \mathrm{H}_{12}\right) & 36.1^{\circ} \mathrm{C} \\\ \text { hexane }\left(\mathrm{C}_{6} \mathrm{H}_{14}\right) & 68.7^{\circ} \mathrm{C}\end{array}\) heptane \(\left(\mathrm{C}_{7} \mathrm{H}_{16}\right) \quad 98.4{ }^{\circ} \mathrm{C}\) to estimate the normal boiling point of octane \(\left(\mathrm{C}_{8} \mathrm{H}_{18}\right)\). Explain the trend in the boiling points.

Step-by-Step Solution

Answer

The estimated boiling point of octane is around 120 ^{8} C, as boiling points increase with more carbon atoms.

Step 1: Review Given Boiling Points

Look at the given normal boiling points: Propane (C_{3} H_{8}) at -42.1^{8} C, Butane (C_{4} H_{10}) at -0.5^{8} C, Pentane (C_{5} H_{12}) at 36.1^{8} C, Hexane (C_{6} H_{14}) at 68.7^{8} C, and Heptane (C_{7} H_{16}) at 98.4^{8} C.

Step 2: Identify the Trend

Notice the trend where the boiling points increase as the number of carbon atoms increases. This is due to the increase in molecular weight and surface area, leading to stronger van der Waals forces between the molecules.

Step 3: Estimate Octane's Boiling Point

Using the trend, estimate the boiling point of Octane (C_{8} H_{18}). Increasing from C7 to C8, the boiling point should be higher than 98.4^{8} C. A reasonable estimate is above 100 ^{8} C, likely close to 120 ^{8} C based on the pattern.

Step 4: Confirm with Contextual Knowledge

This estimate aligns with the known boiling point range for such hydrocarbons, as larger alkanes typically have higher boiling points due to increased intermolecular interactions.

Key Concepts

Alkanes

Alkanes are simple hydrocarbons consisting solely of carbon and hydrogen. They are saturated compounds, meaning all carbon atoms are connected by single bonds. This linear or branched structure determines their physical properties, including boiling points. As you move from smaller alkanes, like propane (\(\mathrm{C}_{3}\mathrm{H}_{8}\)), to larger ones, such as octane (\(\mathrm{C}_{8}\mathrm{H}_{18}\)), the boiling point tends to increase. This is primarily influenced by the increasing number of carbon atoms. More carbon atoms mean a larger molecular size and higher molecular weight.
This results in a tendency for the alkanes to have stronger intermolecular interactions.
Understanding this correlation between structure and boiling point is crucial when studying the properties of alkanes.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles. In hydrocarbons like alkanes, these are primarily van der Waals forces, also known as London dispersion forces. These forces increase with molecular size.
So, as you consider alkanes from propane to octane, the surface area of the molecules increases, enhancing these van der Waals forces.
This elevation in intermolecular forces is why the boiling point increases as the alkane chain lengthens. In general, larger molecules have more electrons and a larger area over which these forces can act, making them stronger.
Understanding intermolecular forces offers insight into why physical properties, such as boiling points, vary among similar molecules with different sizes.

Hydrocarbons

Hydrocarbons are organic compounds composed entirely of hydrogen and carbon atoms, and they vary in structure from linear chains to complex rings and branches. Alkanes are among the simplest form of hydrocarbons, showcasing a direct relationship between molecular weight and boiling points. The varieties of hydrocarbons, which include alkanes, alkenes, and alkynes, have different boiling points due to distinct intermolecular forces.

• Alkanes, being saturated hydrocarbons, follow a straightforward increase in boiling point trend as molecular size increases.
• In contrast, unsaturated hydrocarbons like alkenes and alkynes might showcase different trends due to the presence of double or triple bonds.

Understanding how hydrocarbons are classified and their resulting physical properties helps predict behaviors in processes like refining and usage as fuels.