General Chemistry

To compare the boiling points of two hydrocarbons, CH₃CH₂CH₂CH₃ (butane) and CH₃CH₂CH₂CH₂CH₃ (pentane), we need to consider the molecular structure and intermolecular forces present in each compound. The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure surrounding the liquid. The strength of the intermolecular forces determines how much energy is required to overcome these forces and turn the liquid into a gas. Step 1: Identify the Type of Intermolecular Forces Both butane and pentane are nonpolar molecules and exhibit London dispersion forces (also known as van der Waals forces). These are the weakest type of intermolecular forces and arise due to the temporary dipoles that occur when the electron density in a molecule fluctuates. Step 2: Analyze the Impact of Molecular Size The strength of London dispersion forces increases with the size of the molecules because larger molecules have more electrons, which can lead to stronger temporary dipoles. This means that, generally, the larger the molecule, the higher the boiling point, as more energy is required to overcome the stronger intermolecular forces. Step 3: Compare the Molecular Size Butane (C₄H₁₀) has four carbon atoms, while pentane (C₅H₁₂) has five carbon atoms. Pentane is the larger molecule with a greater surface area and more electrons than butane. Step 4: Predict the Boiling Point Based on Molecular Size Since pentane has a larger molecular size than butane, we can predict that pentane will have stronger London dispersion forces and, therefore, a higher boiling point. Step 5: Look Up or Calculate the Boiling Points The boiling point of butane is approximately -0.5°C (31.1°F), and the boiling point of pentane is around 36.1°C (97°F). These values confirm our prediction that pentane has a higher boiling point than butane due to its larger size and stronger intermolecular forces. In conclusion, the boiling point of CH₃CH₂CH₂CH₂CH₃ (pentane) is higher than that of CH₃CH₂CH₂CH₃ (butane) because pentane has a larger molecular size, which leads to stronger London dispersion forces that require more energy to overcome during the phase transition from liquid to gas.

Question

CH3CH2CH2CH3 boiling point vs. CH3CH2CH2CH2CH3 boiling point

Solution

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