Solubility, Polarity, and Intermolecular Forces Study Guide

The Mechanism of Dispersion Forces:
- Dispersion forces (a type of intermolecular force) arise because electrons are in a state of constant motion within a molecule.
- If you could stop time for a moment, you would see an uneven charge distribution; electrons do not remain symmetrically distributed while they are moving.
Strength of the Force:
- Dispersion forces are considered weak because the uneven charge distribution is not constant.
- Because the distribution is constantly changing, the temporary dipoles do not remain fixed, resulting in a weaker attraction compared to permanent dipoles.

Setup and Density Observations:
- The experiment utilizes a fume hood because Breathing in hexane vapors is unwise.
- Volume: Approximately $15\,mL$ of water were poured into the first three test tubes and approximately $15\,mL$ of hexane ( $C_6H_{14}$ ) were poured into the last three test tubes.
- Immiscibility: Polar water and nonpolar hexane form distinct layers. They are immiscible, which is defined as liquids that do not dissolve in each other.
- Density Comparison:
  - Density of Water: $1.0\,g/mL$
  - Density of Hexane: $0.66\,g/mL$
  - The result is that hexane floats on top of the water layer.
Solubility Tests:
- Iodine ( $I_2$ ): Added to hexane, it begins to dissolve immediately. When added to water, nothing happens even with stirring.
- Copper (II) Chloride ( $CuCl_2$ ): This is an ionic compound. When added to hexane, it does not dissolve (it sits at the bottom, creating a reflection at the liquid's surface). When added to water, it dissolves.
- Combined Test: In a test tube containing both solvents, Iodine (nonpolar) dissolves in the hexane layer, while Copper (II) Chloride (ionic) dissolves in the water layer. (Note: Iodine is more dense than both hexane and water, often falling to the bottom initially, making it slightly difficult to mix).

Core Rule: Substances with similar polarities will generally dissolve in one another.
- Polar/Ionic Solutes: Will dissolve in polar solvents (e.g., water).
- Ionic Dissociation: An ionic compound like Copper (II) Chloride dissociates in water, resulting in copper (II) ions ( $Cu^{2+}$ ) and chloride ions ( $Cl^-$ ).
- Nonpolar Solutes: Will dissolve in nonpolar solvents (e.g., hexane).
The "Dissimilar" Rule: If a solute and solvent are not alike (e.g., polar solute in nonpolar solvent, or vice versa), they will not mix/dissolve.

Updates to Textbook Values: Older textbooks used different thresholds (e.g., $0$ to $0.3$ for nonpolar, over $1.7$ for ionic). Modern standards help eliminate ambiguity.
New Electronegativity ( $\Delta EN$ ) Ranges:
- Nonpolar Covalent: $0$ to $0.4$ .
- Polar Covalent: $0.4$ to $2.0$ .
- Ionic: $\ge 2.0$ .
Case Study: Carbon-Hydrogen ( $C-H$ ) Bonds:
- Carbon ( $EN = 2.5$ ), Hydrogen ( $EN = 2.1$ ).
- Difference ( $\Delta EN$ ) = $0.4$ .
- Under these values, the $C-H$ bond is definitively classified as nonpolar.
Case Study: Hydrogen Fluoride ( $HF$ ):
- Fluorine ( $EN = 4.0$ ), Hydrogen ( $EN = 2.1$ ).
- Difference ( $\Delta EN$ ) = $1.9$ .
- While old books might label this as ionic ( $> 1.7$ ), it is a covalent molecule. The new threshold ( $< 2.0$ ) correctly classifies it as polar covalent.

Iodine ( $I_2$ ):
- Electronegativity difference is $0$ (identity subtraction). It is the most nonpolar substance possible and mixes with nonpolar hexane.
Methanol ( $CH_3OH$ ):
- Classified as an Alcohol (identifiable by the "-ol" suffix).
- Contains a Hydroxyl group ( $OH$ ), which is polar but does not dissociate into ions. This is distinct from a Hydroxide (like $KOH$ ), which is an electrolyte.
Carbon Diselenide ( $CSe_2$ ):
- A covalent compound.
- Structure: Linear (similar to $CO_2$ ). Selenium is in the same chemical group as Oxygen and Sulfur, leading to similar bonding properties and valence electrons.
- Polarity: Nonpolar. Even if bonds are polar, the dipoles point in opposite directions and cancel out due to the linear symmetry.
Oxygen Dichloride ( $OCl_2$ ):
- Structure: Bent (similar to water) due to lone pairs on the central atom.
- Polarity: Polar. It will mix with polar substances like methanol.
Carbon Disulfide ( $CS_2$ ):
- Linear and nonpolar. It will not mix with polar methanol.
Carbon Tetrachloride ( $CCl_4$ ):
- Geometry: Tetrahedral.
- Polarity: Nonpolar. While individual bonds are polar, the symmetrical nature of the tetrahedral shape negates the dipole moments. It will not dissolve in water.
Ammonia ( $NH_3$ ):
- Geometry: Pyramidal.
- Polarity: Polar. It has a lone pair creating a distinct negative side and positive side. It dissolves in water (e.g., Windex).

Definition: An electrolyte is a substance that creates an aqueous solution capable of conducting electricity.
Requirement: To conduct electricity, the solution must contain ions.
Mechanism: The substance must dissociate into ions when placed in a solvent.
Strong Electrolytes: Compounds like Potassium Hydroxide ( $KOH$ ) dissociate completely in water to produce ions.
Non-electrolytes: Alcohols (containing covalent hydroxyl groups) do not dissociate and therefore do not conduct electricity.

Question: Why was hexane on top of the water?
Response: Because it is less dense. Hexane is approximately $0.6\,g/mL$ while water is $1.0\,g/mL$ .
Question: Will Iodine mix with water?
Response: No, the nonpolar iodine will not mix with polar water because they are not alike.
Discussion on Carbon Diselenide and Methanol: When asked if $CSe_2$ (nonpolar) and Ethanol/Methanol (polar) would mix, the answer is no. To mix, they must both be polar/ionic or both be nonpolar.
Lab Observations: During the student activity, sucrose was noted as being soluble/dissolved, while some salts were noted as insoluble. Ethanol was generally noted as soluble in the context provided.