Predict which compound will have the higher boiling point and explain how you made your choice by including intermolecular forces.
1. H2O or CH2O
2. CS2 or CH4
1. We would expect H2O to have the higher boiling point due to hydrogen bonding.
2. We would CS2 to have the higher boiling point due to dispersion forces.
To predict which compound has a higher boiling point, we need to consider the intermolecular forces present in each compound. The strength of these forces determines the boiling point because it affects the energy required to overcome these forces and change the substance from a liquid to a gas.
1. H2O or CH2O:
In H2O (water), the oxygen atom is highly electronegative, giving it a partial negative charge, while the hydrogen atoms have partial positive charges. This creates a polar molecule with a bent geometry. The polar nature of water allows for hydrogen bonding, which is a strong intermolecular force. Hydrogen bonding occurs when the partially positive hydrogen atom of one molecule is attracted to the partially negative oxygen atom of another molecule.
On the other hand, CH2O (formaldehyde) has a trigonal planar geometry, and although it has a polar C=O bond, it does not have the necessary hydrogen atoms to form extensive hydrogen bonding. It mostly experiences weaker dipole-dipole forces.
Since hydrogen bonding is stronger than dipole-dipole forces, H2O (water) will have a higher boiling point than CH2O (formaldehyde).
2. CS2 or CH4:
CS2 (carbon disulfide) consists of nonpolar covalent bonds between carbon and sulfur atoms. Since the molecule is symmetric and lacking an electronegativity difference, it is nonpolar. Nonpolar molecules do not possess permanent dipole moments and only experience weak dispersion forces. Additionally, CS2 is a larger molecule than CH4.
CH4 (methane) has nonpolar covalent bonds between carbon and hydrogen atoms, resulting in a symmetrical tetrahedral shape. Like CS2, CH4 only experiences weak dispersion forces.
In this case, since both CS2 and CH4 experience only weak dispersion forces, the molecular size becomes the deciding factor. Larger molecules have stronger dispersion forces due to increased surface area. Since CS2 is larger than CH4, it will have stronger dispersion forces and, therefore, a higher boiling point.