The surface tension of water is 0.072 N/m, its boiling point is 100 degrees Celsius, and its melting point is 0 degrees Celsius. Does water fit the pattern in your table of bulk properties for simple hydrocarbons? If so, describe where in the ordering from weakest to strongest it should fit. If not, explain what you think might be some of the causes for this difference. (Hint: Consider how molecular shape, molecular weight, and other factors can affect bulk properties.)

Question

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Water does not fit the pattern. Its surface tension and melting points are higher than all the hydrocarbons listed, but its boiling point is between Heptane and Octane. Reasons for this difference might be because of the different structure and composition of water molecules. All the hydrocarbons had very similar structures to each other.

Answer 1

Water (H₂O) does not fit the pattern typically observed in a table of bulk properties for simple hydrocarbons. Specifically, water exhibits unique properties that distinguish it from hydrocarbons like pentane, hexane, heptane, and octane.

Here are the key comparisons:

1. Surface Tension: Water's surface tension of 0.072 N/m is significantly higher than that of hydrocarbons. Hydrocarbons tend to have lower surface tensions due to their nonpolar nature, while water's hydrogen bonding leads to much stronger intermolecular attractions.

2. Melting and Boiling Points: Water has a melting point of 0 degrees Celsius and a boiling point of 100 degrees Celsius. In contrast, hydrocarbons usually have much lower melting points and boiling points that increase with molecular weight, but are still relatively low compared to water. For example, the melting points of simple hydrocarbons can range from around -160 degrees Celsius (for methane) to slightly above room temperature (for larger hydrocarbons), while their boiling points also increase, but are generally still below that of water.

3. Molecular Structure: Water is a polar molecule with a bent shape due to the two hydrogen atoms bonded to oxygen at an angle. This unique structure allows for hydrogen bonding, which is absent in hydrocarbons that are primarily nonpolar and linear or branched in structure. The presence of strong hydrogen bonds significantly influences water's bulk properties, such as high surface tension, high boiling and melting points, and high specific heat capacity.

4. Molecular Weight: The molecular weight of water is relatively low (18 g/mol), while the simple hydrocarbons included in the comparison have higher molecular weights. In hydrocarbons, increasing molecular weight typically corresponds to increased boiling points and melting points due to larger van der Waals forces; however, this trend is not seen with water due to the significant impacts of hydrogen bonding.

In summary, while water and simple hydrocarbons may share some commonalities as liquids, the unique hydrogen bonding and polar nature of water lead to much stronger intermolecular interactions, resulting in higher surface tension and boiling/melting points than what would be expected based on molecular weight alone. Therefore, water would be positioned well above simple hydrocarbons in terms of surface tension and melting/boiling points due to these differences in molecular structure and intermolecular forces.