CHEM 118 Chapter Notes - Chapter 13: Colloid, Strong Electrolyte, Boiling-Point Elevation

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1 Feb 2017

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Solution: a homogeneous mixture that forms from intermolecular forces. Water molecules will form a shell around the ions that are dissolving in them. Solubility is affected by strength of intermolecular forces. A positive end attracts to a negative end of another molecule. Ion-dipole: interaction of an ion and a dipole of a molecule. A temporary attractive force that results from electrons in two adjacent atoms moving around. Hydrogen: strong bond between hydrogen and a highly electronegative atom. Entropy is a measure of energy randomization (disorder) Enthalpy is the sum of the internal energy. Enthalpy of solution = enthalpy of solute + enthalpy of solvent + enthalpy of mix. To ensure energy change: overcome all attractions among solute particles (endothermic, overcome solvent molecule attractions (endothermic, form new solute-solvent molecules (exothermic) The overall change in energy depends on the change of energy during each of the three steps.

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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.

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* 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:

(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:

CHEM 118 Chapter Notes - Chapter 13: Colloid, Strong Electrolyte, Boiling-Point Elevation

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