5. Laboratory 2 Review How does an increase in size and molecular weight alter t

Question

5. Laboratory 2 Review How does an increase in size and molecular weight alter the boiling point of a liquid? How do intermolecular forces assuming similar molecular weight & size affect the bolling point of a liquid? Structural isomers can have very different physical properties. The differences in the boiling points of structural isomers of hydrocarbons discussed in lecture pale in comparison to organic compounds containing oxygen. Rationalize the boiling point trend in the following three structural isomers (C3HgO) ?? ?? boiling pt °C (density, g cm-1) formal name 8 (0.73) methoxyethane ethyl methylether 83 (0.79) 2-propanol iso-propanol 98 (0.81) n-propanol propanol common name

Explanation / Answer

"If no other strong intermolecular interactions are present" the boiling point of a liquid will be proportional to the molecular weight. i.e. larger the molecular weight, higher the boiling point. The larger the molecule, the larger the number of intermolecular interactions such as hydrogen bonding, Van Der Waals interaction etc, so it needs more energy to disturb the intermolecular interactions hence you have to supply more energy in the form of heat, resulting in higher boiling points.

The boiling points of three isomers of hydrocarbons, methoxyethane (8 oC), 2-propanol (83 oC), n-propanol (98 oC) have entirely different boiling points, due the difference in the functional groups present, in 2-propanol, and n-propanol the functional group is an alcohol (-OH) group, which increases the strength of intermolecular interaction as compared to methoxyethane, in which no strong H-bonding is possible (only weak dipole-dipole, and van der waals interaction exists). Comparison between 2-propanol, and n-propanol has higher boiling point, this is due to the fact that as the length of the chain increases the surface area of the compound will also increase, and keeping -OH functional group common in both, the Van der Waals interaction will be more in n-propanol since the molecule is linear and longer, hence more surface area and more Van der Waals interaction and more boiling point (the increase of surface area increases the chances of individual molecules to interact each other).

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