Problem 76 - Chemistry The Central Science

Question

In dichloromethane, \(\mathrm{CH}_{2} \mathrm{Cl}_{2}(\mu=1.60 \mathrm{D})\), the dispersion force contribution to the intermolecular attractive forces is about five times larger than the dipole-dipole contribution. Compared to \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\), would you expect the relative importance of the dipole-dipole contribution to increase or decrease (a) in dibromomethane \((\mu=1.43 \mathrm{D}),(\mathbf{b})\) in difluoromethane \((\mu=1.93 \mathrm{D}) ?\) Explain.

Step-by-Step Solution

Answer

In conclusion, the relative importance of the dipole-dipole contribution to intermolecular attractive forces: (a) decreases for dibromomethane (\(\mathrm{CH}_{2}\mathrm{Br}_{2}\)) compared to dichloromethane (\(\mathrm{CH}_{2} \mathrm{Cl}_{2}\)) (b) increases for difluoromethane (\(\mathrm{CH}_{2}\mathrm{F}_{2}\)) compared to dichloromethane (\(\mathrm{CH}_{2} \mathrm{Cl}_{2}\)).

Step 1: Dispersion Force and Dipole-Dipole Force Ratio for CH2Cl2

First, let's analyze the given values for CH2Cl2 to understand the question's context. We are given that the dispersion force contribution is about five times larger than the dipole-dipole contribution for dichloromethane (CH2Cl2). Thus, the ratio of dispersion force to dipole-dipole force for CH2Cl2 is: Dispersion Force / Dipole-Dipole Force = 5 Keep this in mind as we continue to analyze the other two compounds.

Step 2: Analyzing the Dipole Moment for CH2Br2 and CH2F2

We are given the dipole moments for dibromomethane (CH2Br2) and difluoromethane (CH2F2) as 1.43 D and 1.93 D, respectively. Dipole moment for CH2Br2: \(\mu = 1.43 \,\mathrm{D}\) Dipole moment for CH2F2: \(\mu = 1.93 \,\mathrm{D}\) Higher dipole moments typically correlate with stronger dipole-dipole interactions. To determine which compound has a larger relative importance of the dipole-dipole contribution, we need to compare the ratios of the dispersion forces to the dipole-dipole contributions for each compound.

Step 3: Comparing the Dispersion Force to Dipole-Dipole Force Ratios

Let's analyze the dipole-dipole contribution of each compound in comparison to CH2Cl2. For CH2Br2: We can see that the dipole moment of CH2Br2 (1.43 D) is slightly lower than that of CH2Cl2 (1.60 D), which indicates that the strength of the dipole-dipole force contribution for CH2Br2 is expected to be weaker. As a result, it is likely that the dispersion force to dipole-dipole force ratio would be higher (more significant dispersion force contribution) for CH2Br2. Thus, the relative importance of dipole-dipole contribution for CH2Br2 decreases. For CH2F2: The dipole moment of CH2F2 (1.93 D) is higher than that of CH2Cl2 (1.60 D), indicating that the strength of the dipole-dipole force contribution for CH2F2 is likely to be stronger. Thus, the dispersion force to dipole-dipole force ratio would be lower (less significant dispersion force contribution) for CH2F2. So, the relative importance of dipole-dipole contribution for CH2F2 increases.

Step 4: Conclusion

In conclusion, the relative importance of the dipole-dipole contribution to intermolecular attractive forces: (a) decreases for dibromomethane (CH2Br2) compared to dichloromethane (CH2Cl2) (b) increases for difluoromethane (CH2F2) compared to dichloromethane (CH2Cl2)

Key Concepts

Dipole Moment

The dipole moment is an important property in chemistry, as it measures the separation of positive and negative charges in a molecule. Imagine a molecule as a collection of atoms bonded together. Each atom has its electrons that move around. When electrons in the molecule are unequally distributed, it creates a dipole moment.

This distribution makes one part of the molecule slightly positive and another part slightly negative. The dipole moment is like a tiny arrow pointing from the positive charge to the negative charge.

The larger the dipole moment, the more polar the molecule. This means it has a stronger tendency to interact with other molecules through dipole-dipole interactions. A common unit for measuring the dipole moment is the Debye (D).

• Higher dipole moments indicate stronger attractions between molecules.
• Molecules with similar dipole moments can mix better, like water and alcohol.

When comparing the dipole moments in our example, CH₂Cl₂, CH₂Br₂, and CH₂F₂, we see different values that hint at how these molecules might interact.

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are a type of weak intermolecular force. They occur even between nonpolar molecules or noble gases. Picture a room full of balloons that sometimes just pull closer to each other for a moment; that's similar to how dispersion forces work.

These forces arise due to momentary fluctuations in electron density. This means that at any given instance, the electron cloud around an atom or molecule can be unevenly spread. This fluctuation can induce a temporary dipole in a neighboring atom or molecule.

• Dispersion forces increase with a bigger electron cloud since there’s more room for fluctuation.
• Larger molecules typically exhibit stronger dispersion forces.

In the example provided, the dispersion forces in CH₂Cl₂ were said to be five times that of dipole-dipole forces, showcasing their prevalence in certain situations. Compared to dipole-dipole forces, dispersion forces are weak individually, but collectively, they can significantly impact physical properties like boiling points and state of matter.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between molecules that possess a permanent dipole moment, meaning a constant uneven distribution of charge. Think of these interactions like magnets sticking to a fridge; the positive end of one dipole attracts the negative end of another.

These interactions are stronger than dispersion forces because the charge separation makes the attraction consistent, like magnets that always line up a certain way. However, they are generally weaker than hydrogen bonds, which are a special type of dipole-dipole interaction.

• These interactions are crucial in determining boiling and melting points.
• Molecular polarity and a higher dipole moment promote strong dipole-dipole interactions.

In comparing CH₂Cl₂ with CH₂Br₂ and CH₂F₂, this concept explains why CH₂F₂'s higher dipole moment increases its dipole-dipole interaction importance compared to the others. Thus, understanding how molecules interact helps predict their physical and chemical behaviors.