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Chemistry 1301A/B Lecture Notes - Lecture 14: Intermolecular Force, Chemical Polarity, Bayerischer Rundfunk

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spectral
31 Aug 2017
School
Western University
Department
Chemistry
Course
Chemistry 1301A/B
Professor
Felix Lee

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Document Summary

It is important to realize that a molecule with polar bonds is not necessarily polar, but a molecule without polar bonds cannot possibly be polar. The resulting molecule has dipoles or partial positive ( + In general, the weakest of all of the intermolecular forces, this type of force exists between all types of compounds and is the only intermolecular force for pure covalent compounds. Dispersion forces can vary in molecular elements and molecular compounds. In larger molecules, there are greater dispersion forces as there are more atoms, therefore more electrons, therefore more polarization of these electrons, and therefore more dispersion forces (example: c2h6 has a greater boiling point than ch4). A chemical equation is used to show the interaction of matter. By convention, the reactants are always written on the left, and the products on the right, with reactants being separated from products with an arrow.

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Related Questions

The following table presents the solubilities of several gases in water at 25 °C under a total pressure of gas and water vapor of 1 atm. (a) What volume of CH4(g) under standard conditions of temperature and pressure is contained in 4.0 L of a saturated solution at 25 °C? (b) Explain the variation in solubility among the hydrocarbons listed (the first three compounds), based on their molecular structures and intermolecular forces. (c) Compare the solubilities of O2, N2, and NO, and account for the variations based on molecular structures and intermolecular forces. (d) Account for the much larger values observed for H2S and SO2 as compared with the other gases listed. (e) Find several pairs of substances with the same or nearly the same molecular masses (for example, C2H4 and N2), and use intermolecular interactions to explain the differences in their solubilities.

The corresponding volume is

b) All three molecules are nonpolar and form very weak interactions with the water molecules. The solubility of ethane is higher that the solubility of methane as ethane is more massive molecule and forms stronger dispersion forces with the water molecule. Ethene is the most soluble gas as the C atoms in ethene are planar, sp2 hybridized, and there are π electrons in the orbitals perpendicular to the carbon atoms. π electrons are less tightly held by the nucleus than σ electrons, allowing the dipole of water molecule to disturb the electron density more easily, leading to stronger dipole-induced dipole interactions.

c) The molecular weight is similar for three gases. O2 and N2 are nonpolar gases, but more water molecules surround O2 as it is larger molecule. The O–O bond (double bond) is longer than N–N bond (triple bond). The water attracts nonpolar gases through the weak dipole-induced dipole interactions. NO is the most soluble gas as it has polar bond and the overall molecule is polar as it is linear structure. Water dissolves it via stronger dipole-dipole interactions.

d) H2S is capable to form weak hydrogen bonds with the water molecules, and these interactions are much stronger than dipole-dipole and dispersion forces. SO2 is even more soluble in water as it reacts with the water forming H2SO3. The very high values for the solubility of gas are the sign that the gas chemically reacts with the solvent.

d) N2 and CH4. Solubility of N2 is lower as N2 is smaller molecule than CH4 and less hydrated.

NO and C2H6 have similar water solubility. NO is more soluble than ethane and form dipole-dipole interactions, but the dispersion forces of ethane and water are stronger than dispersion forces between NO and water as ethane is larger, more polarizable molecule. The sum of all intermolecular interactions is similar for these two gases, leading to the similar water solubility.

NO and O2. NO is more soluble as it is polar molecule and form stronger intermolecular forces with polar solvent molecules compared to nonpolar O2 molecule.

* Circle the intermolecular force(s) that is/are present in each of the following species. [Hint: there may be more than one correct answer.]

Ar (hydrogen bonding, dipole-dipole forces, dispersion forces)

CH4 (hydrogen bonding, dipole-dipole forces, dispersion forces)

HBr (hydrogen bonding, dipole-dipole forces, dispersion forces)

HF (hydrogen bonding, dipole-dipole forces, dispersion forces)

CHCl3 (hydrogen bonding, dipole-dipole forces, dispersion forces)

* For the following pairs of atoms/molecules, circle the species that has the higher boiling point.

Ne or Xe

Cl2 or F2

HBr or HF

HCl or HI

H2O or H2S

* Phase changes always associate with change of enthalpy. For the following situations, name the phase

transition and indicate whether the process is exothermic or endothermic.

(a) Dry ice (solid carbon dioxide) turns to gaseous carbon dioxide.

Answer:

(b) Water vapor turns to liquid water.

Answer:

(c) Solid iron turns to liquid iron.

Answer:

(d) Iodine vapor turns to solid iodine.

Answer:

(e) Liquid helium turns to helium gas.

Answer:

* True or false

1. Intermolecular forces are stronger than ionic and covalent bonds. Answer:

2. For molecules with similar size and molar mass, those with higher polarity would have higher boiling

point. Answer:

3. Ion-dipole forces exist between ionic potassium bromide and polar water.

Answer:

4. Dispersion forces exist between argon and helium. Answer:

5. Instantaneous dipoles exist in all molecules and atoms. Answer:

6. The strength of dispersion forces depends on the molar mass but the shape of the molecules have no effect on dispersion forces. Answer:

7. Ice has a higher density than water because ice has an ordered cagelike structure. Answer:

8. Liquids with weaker intermolecular interactions have lower viscosity. Answer:

9. Viscosity decreases with increasing temperature. Answer:

10. For any molecules, the heat of vaporization is always lower than the heat of fusion because it takes more energy to make molecules to move past one another in a solid lattice. Answer: