Which of the following compounds is a gas at room temperature?
A : CH3CH2OH
B : CH3CH2CH2CH2CH2CH2CH3
C : CH3CH2CH3
D : HOCH2CH2OH
The Correct answer and Explanation is:
The correct answer is C: CH₃CH₂CH₃ (propane).
Explanation:
At room temperature (around 20-25°C or 68-77°F), the physical state of a compound depends on the strength of the intermolecular forces between its molecules. Let’s analyze each compound and its intermolecular forces to determine which is a gas at room temperature.
A: CH₃CH₂OH (ethanol)
Ethanol is a small alcohol molecule. It has a hydroxyl group (-OH), which makes it capable of forming hydrogen bonds, a type of strong intermolecular force. These hydrogen bonds hold the ethanol molecules together, requiring more energy (heat) to separate them, so ethanol exists as a liquid at room temperature.
B: CH₃CH₂CH₂CH₂CH₂CH₂CH₃ (heptane)
Heptane is an alkane with seven carbon atoms. As the length of the carbon chain increases, so do the Van der Waals (dispersion) forces between the molecules, making it harder for the molecules to move apart. Heptane, with its larger molecular mass and stronger dispersion forces, exists as a liquid at room temperature.
C: CH₃CH₂CH₃ (propane)
Propane is an alkane with three carbon atoms. Alkanes are nonpolar molecules and exhibit weak Van der Waals forces. Because propane is small and has weaker intermolecular forces compared to larger hydrocarbons, less energy is needed for its molecules to escape into the gas phase. As a result, propane exists as a gas at room temperature.
D: HOCH₂CH₂OH (ethylene glycol)
Ethylene glycol is another alcohol with two hydroxyl groups. These hydroxyl groups enable it to form strong hydrogen bonds between molecules. Due to these strong intermolecular forces, ethylene glycol is a liquid at room temperature.
In summary, propane (C: CH₃CH₂CH₃) is a gas at room temperature due to its relatively small molecular size and weak intermolecular forces.