Which one of the following should have the lowest boiling point?
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Which one of the following should have the lowest boiling point?
A. SiH4
B. HCl
C. PH3
D. H2S
E. H2O
The boiling point of a substance is influenced by its intermolecular forces. Generally, substances with weaker intermolecular forces tend to have lower boiling points. Let's analyze the given substances:
A. SiH4 (Silane): Silane is a nonpolar molecule, and its primary intermolecular force is London dispersion forces. It has relatively weak intermolecular forces.
B. HCl (Hydrochloric Acid): HCl is a polar molecule, and it exhibits dipole-dipole interactions. These are stronger than London dispersion forces.
C. PH3 (Phosphine): PH3 is a polar molecule, and it also exhibits dipole-dipole interactions, which are stronger than London dispersion forces.
D. H2S (Hydrogen Sulfide): H2S is a polar molecule, and it exhibits dipole-dipole interactions, which are stronger than London dispersion forces.
E. H2O (Water): H2O is a highly polar molecule, and it exhibits strong hydrogen bonding in addition to dipole-dipole interactions.
Based on the strength of intermolecular forces:
1. H2O (H-bonding) > HCl (dipole-dipole) > PH3 (dipole-dipole) > H2S (dipole-dipole) > SiH4 (London dispersion)
So, SiH4 should have the lowest boiling point because it only has London dispersion forces, which are the weakest intermolecular forces among the options.
How to compare the strength of intermolecular forces
Comparing the strength of intermolecular forces involves understanding the different types of intermolecular forces and their relative strengths. The strength of intermolecular forces can be ranked from weakest to strongest as follows:
1. London Dispersion Forces (Van der Waals Forces):
- Occur in all molecules and atoms.
- Result from temporary fluctuations in electron distribution, creating temporary dipoles.
- Strength increases with the size and shape of the molecule or atom (more electrons).
- Generally, the weakest of all intermolecular forces.
2. Dipole-Dipole Interactions:
- Occur in polar molecules where there is an uneven distribution of charge (a permanent dipole moment).
- Strength is influenced by the magnitude of the dipole moment.
- Stronger than London dispersion forces but weaker than hydrogen bonding.
3. Hydrogen Bonding:
- A special type of dipole-dipole interaction.
- Occurs when a hydrogen atom is bonded to a highly electronegative atom (usually nitrogen, oxygen, or fluorine) and is attracted to another electronegative atom with a lone pair of electrons.
- Significantly stronger than regular dipole-dipole interactions.
To compare the strength of intermolecular forces between substances:
1. Identify the Types of Intermolecular Forces: Determine which types of intermolecular forces are present in each substance. Consider factors like molecular polarity, hydrogen bonding, and dipole moments.
2. Rank the Strengths: Use the hierarchy mentioned above to rank the strengths of the intermolecular forces present in each substance. London dispersion forces are the weakest, followed by dipole-dipole interactions, and hydrogen bonding is the strongest among these three.
3. Consider Molecular Factors: Keep in mind that the strength of intermolecular forces can also be influenced by molecular size, shape, and electronegativity of the atoms involved. Larger molecules with more electrons tend to have stronger London dispersion forces. Polar molecules have dipole-dipole interactions, and those with hydrogen bonded to highly electronegative atoms exhibit hydrogen bonding.
4. Compare Based on Rankings: When comparing substances, consider the intermolecular forces involved and their relative strengths. Substances with stronger intermolecular forces will generally have higher boiling points, lower vapor pressures, and greater cohesion between molecules.
Understanding the types and strengths of intermolecular forces is essential in predicting physical properties of substances and explaining various phenomena in chemistry and physics.
How to compare the strength of d-d forces?
Comparing the strength of dipole-dipole (d-d) forces between different molecules involves evaluating the magnitude of the permanent dipole moments in each molecule. The strength of d-d forces is directly related to the degree of polarity within a molecule. Here's how you can compare the strength of d-d forces:
1. **Identify Polar Molecules:** Determine which molecules in your comparison are polar. Polar molecules have an uneven distribution of charge due to differences in electronegativity between their atoms. This uneven distribution creates a permanent dipole moment.
2. **Analyze the Dipole Moments:** The strength of dipole-dipole forces is directly proportional to the magnitude of the dipole moment. The larger the dipole moment, the stronger the d-d forces.
3. **Consider Electronegativity:** The electronegativity difference between bonded atoms within a molecule is a key factor in determining the magnitude of the dipole moment. Generally, a greater electronegativity difference results in a larger dipole moment and stronger d-d forces.
4. **Evaluate Molecular Geometry:** The shape or geometry of a molecule can affect the direction of the dipole moment. For example, in linear molecules, the dipole moment points directly along the molecular axis, whereas in molecules with bent or angular shapes, the dipole moment may not be aligned with the molecular axis. This can influence the strength of d-d forces.
5. **Compare Dipole Moments:** To compare the strength of d-d forces, compare the dipole moments of the polar molecules in question. You can find dipole moment values in reference materials or experimental data.
6. **Rank by Dipole Moment:** Rank the molecules based on their dipole moments. A molecule with a larger dipole moment will have stronger d-d forces compared to a molecule with a smaller dipole moment.
7. **Consider Molecular Size:** In some cases, molecules with similar dipole moments may exhibit different d-d forces due to differences in size and shape. Larger molecules with more atoms may have stronger d-d forces compared to smaller molecules with the same dipole moment.
It's important to note that dipole-dipole forces are relatively weaker compared to hydrogen bonding, which is a special type of dipole-dipole interaction involving hydrogen atoms bonded to highly electronegative atoms (such as nitrogen, oxygen, or fluorine). When comparing d-d forces, keep in mind that hydrogen bonding, if present, will typically result in stronger intermolecular forces than regular dipole-dipole interactions.
In summary, the key to comparing the strength of dipole-dipole forces is to assess the magnitude of the dipole moments in polar molecules, which depends on factors like electronegativity differences and molecular geometry.
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比较不同分子之间的偶极-偶极(d-d)力的强度需要评估每个分子中永久偶极矩的大小。d-d力的强度与分子内的极性程度直接相关。以下是你如何比较d-d力的强度:
1. **识别极性分子:**确定比较中哪些分子是极性的。极性分子由于其原子间的电负性不同而具有不均匀的电荷分布。这种不均匀的分布产生了一个永久的偶极矩。
2. **分析偶极矩:**偶极-偶极力的强度与偶极矩的大小成正比。偶极矩越大，d-d力越强。
3.**考虑电负性:**分子内键合原子之间的电负性差异是决定偶极矩大小的关键因素。一般来说，较大的电负性差导致较大的偶极矩和较强的d-d力。
4. **评估分子几何:**分子的形状或几何可以影响偶极矩的方向。例如，在线性分子中，偶极矩直接指向分子轴，而在弯曲或有角形状的分子中，偶极矩可能不与分子轴对齐。这可以影响d-d力的强度。
5. **按偶极矩排序:**根据偶极矩对分子进行排序。与偶极矩较小的分子相比，偶极矩较大的分子具有更强的d-d力。
6. **考虑分子大小:**在某些情况下，具有相似偶极矩的分子可能由于大小和形状的不同而表现出不同的d-d力。与具有相同偶极矩的小分子相比，具有更多原子的大分子可能具有更强的d-d力。
值得注意的是，与氢键相比，偶极-偶极力相对较弱，氢键是一种特殊类型的偶极-偶极相互作用，涉及氢原子与高电负性原子(如氮，氧或氟)结合。当比较d-d力时，请记住，如果存在氢键，通常会导致比常规的偶极子-偶极子相互作用更强的分子间力。
总之，比较偶极-偶极力强度的关键是评估极性分子中偶极矩的大小，这取决于电负性差异和分子几何等因素。
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