Which among the following has the highest melting point?
A. methanol
B. CH_3F
C. CH_4
D. C_2H_6
The Correct Answer and Explanation is:
To determine which of the following compounds has the highest melting point, we must consider the types of intermolecular forces present in each molecule. The melting point of a substance depends largely on the strength of the intermolecular forces that hold its molecules together. Stronger intermolecular forces result in higher melting points, as more energy is required to overcome these forces.
A. Methanol (CH₃OH)
Methanol is a polar molecule with hydrogen bonding, which is a relatively strong intermolecular force. Hydrogen bonds occur when a hydrogen atom is attached to a highly electronegative atom like oxygen, which is the case with the hydroxyl group (-OH) in methanol. This strong interaction gives methanol a relatively higher melting point compared to nonpolar molecules of similar size.
B. CH₃F (Fluoromethane)
CH₃F is a polar molecule as well, with dipole-dipole interactions due to the electronegativity difference between carbon and fluorine. However, the forces between these molecules (dipole-dipole interactions) are weaker than the hydrogen bonds in methanol. The dipole-dipole interactions are not as strong as hydrogen bonding, so CH₃F will have a lower melting point than methanol.
C. CH₄ (Methane)
Methane is a nonpolar molecule. The intermolecular forces in methane are London dispersion forces, which are relatively weak compared to hydrogen bonding and dipole-dipole interactions. London dispersion forces increase with the size and shape of the molecule, but since methane is a small, simple molecule, it will have a low melting point.
D. C₂H₆ (Ethane)
Ethane is also a nonpolar molecule with London dispersion forces as its primary intermolecular force. Like methane, the dispersion forces in ethane are weak, but ethane has a slightly larger molecular size, meaning it will have slightly stronger dispersion forces than methane. However, its melting point will still be lower than that of molecules with stronger intermolecular forces like hydrogen bonds.
Conclusion:
Among the given options, methanol (CH₃OH) has the highest melting point because it experiences hydrogen bonding, which is stronger than the dipole-dipole interactions in CH₃F and the London dispersion forces in CH₄ and C₂H₆. Therefore, the correct answer is A. methanol.