Problem 59 - Introductory Chemistry Atoms First

Question

Which is more likely to be a gas at room temperature, \(\mathrm{CH}_{4}\) or \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) ? Justify your answer.

Step-by-Step Solution

Answer

CH4 is more likely to be a gas at room temperature than CH2Cl2 because it has weaker intermolecular forces (only dispersion forces) and a lower boiling point (\(-164^\circ\text{C}\)). In contrast, CH2Cl2 has stronger intermolecular forces (dispersion forces and dipole-dipole interactions) and a higher boiling point (\(39.8^\circ\text{C}\)).

Step 1: Identify the types of intermolecular forces present in each substance

For CH4, the molecule is nonpolar and the only intermolecular forces present are dispersion forces, also known as London dispersion forces or van der Waals forces. For CH2Cl2, the molecule is polar and has both dispersion forces and dipole-dipole forces due to the polarity of the C-Cl bond.

Step 2: Compare the strength of intermolecular forces

Dispersion forces are generally weaker than dipole-dipole interactions. Since CH4 only has dispersion forces and CH2Cl2 has both dispersion forces and dipole-dipole interactions, we can infer that the intermolecular forces in CH2Cl2 are stronger than those in CH4.

Step 3: Compare the boiling points of the two substances

A substance with stronger intermolecular forces will generally have a higher boiling point, as more energy is required to overcome the forces holding the molecules together in the liquid phase. The boiling point of CH4 is \(-164^\circ\text{C}\), while the boiling point of CH2Cl2 is \(39.8^\circ\text{C}\). Since room temperature is around \(25^\circ\text{C}\), CH4 has a lower boiling point, and so it is more likely to be a gas at room temperature.

Step 4: Conclusion

Based on our analysis of the intermolecular forces in CH4 and CH2Cl2, we determined that CH4 has weaker intermolecular forces and a lower boiling point. Therefore, CH4 is more likely to be a gas at room temperature than CH2Cl2.

Key Concepts

Intermolecular Forces

When we think about why substances exist in different states (solid, liquid, or gas), one of the key reasons is intermolecular forces. These are the forces that exist between molecules, influencing how they interact with each other.

There are several types of intermolecular forces, but the main ones include dispersion forces, dipole-dipole interactions, and hydrogen bonds. Dispersion forces, also known as London dispersion forces, are the weakest but present in all molecules. These are caused by the temporary uneven distribution of electrons leading to temporary dipoles.

On the other hand, dipole-dipole interactions occur in molecules where there is a permanent dipole moment, meaning there’s a consistent difference in electronegativity between bonded atoms in a molecule, like in the case with CH₂Cl₂. Hydrogen bonding is a special, stronger type of dipole-dipole interaction occurring in molecules containing an H atom bonded to a highly electronegative atom (like N, O, or F).

In summary, the stronger the intermolecular forces, the more energy is required for molecules to move past each other, influencing the physical state at a given temperature.

Boiling Points

The concept of boiling points is directly impacted by intermolecular forces. A boiling point is the temperature at which a substance transitions from a liquid to a gas. This occurs when the kinetic energy of the molecules is enough to overcome the attraction from intermolecular forces.

Substances with stronger intermolecular forces have higher boiling points since it takes more energy to separate the molecules. Conversely, substances with weaker intermolecular forces have lower boiling points. Understanding this concept helps to explain why methane (CH₄), with its weaker dispersion forces, boils at a much lower temperature (-164°C) than dichloromethane (CH₂Cl₂), which also has dipole-dipole interactions and boils at 39.8°C.

Therefore, when comparing substances, we can predict their state at a given temperature based on their boiling points, which are a reflection of the strength of the intermolecular forces present.

Molecular Polarity

The molecular polarity of a compound is an intrinsic property defined by its molecular geometry and the electronegativity differences between its atoms. A molecule is considered polar if it has an uneven distribution of electron density, resulting in a molecular dipole moment.

Polar molecules, like CH₂Cl₂, have areas of partial positive and partial negative charges due to the electronegative chlorine atoms drawing electrons away from the carbon and hydrogen atoms. This polarity leads to dipole-dipole interactions, which are stronger than dispersion forces found in nonpolar molecules like CH₄.

In essence, polar molecules tend to stick together more robustly due to their charge differences, leading to higher boiling points compared to their nonpolar counterparts. This property is crucial when predicting the physical state of a substance under specific conditions, such as room temperature.