Chapter 11: Solutions and Colloids – General Chemistry Study Notes

Solutions and Colloids

Introduction to Solutions

A solution is a homogeneous mixture composed of a solvent (the substance present in greater amount) and a solute (the substance present in lesser amount). Solutions are fundamental in chemistry due to their widespread occurrence and importance in chemical processes.

• Colloids are mixtures where the dispersed particles are larger than molecules but do not settle out upon standing, exhibiting properties between those of solutions and suspensions.

Solubility Principles: "Like Dissolves Like"

The solubility of a solute in a solvent is largely determined by the nature of their intermolecular forces (IMFs). The general rule is "like dissolves like":

• Polar solvents dissolve polar solutes.

• Non-polar solvents dissolve non-polar solutes.

• If solute and solvent IMFs are similar, the pair will be soluble; if not, they are likely insoluble.

• Example: Water (polar) dissolves salt (ionic/polar), but not oil (non-polar).

Types of Intermolecular Forces (IMFs) in Solutions

IMFs are the forces that hold molecules together and influence solubility:

• Dispersion (London) forces: Present in all molecules, often weak, especially in non-polar substances.

• Dipole-dipole interactions: Stronger, occur between polar molecules.

• Hydrogen bonding: The strongest IMF, occurs when H is bonded to N, O, or F.

• Ionic interactions: Ions are attracted to oppositely charged dipoles in polar solvents.

• Dipole-induced dipole: A dipole can induce a dipole in a nearby atom or molecule (e.g., H2O and Xenon).

Solute-Solvent Interactions: Alcohols in Water

The solubility of alcohols in water decreases as the hydrocarbon chain length increases due to the decreasing influence of hydrogen bonding relative to dispersion forces.

Name	Formula	Structure	Solubility (g/100g H2O)	IMFs
Methanol	CH3OH	linear	miscible	H-bond; Dispersion
Ethanol	C2H5OH	linear	miscible	H-bond; Dispersion
Propanol	C3H7OH	linear	miscible	H-bond; Dispersion
Butanol	C4H9OH	linear	7.9	H-bond; Dispersion
Pentanol	C5H11OH	linear	2.7	H-bond; Dispersion
Hexanol	C6H13OH	linear	0.6	H-bond; Dispersion

Formation of Solutions: Thermodynamic Criteria

Two main criteria favor the spontaneous formation of a solution:

• Decrease in internal energy (U) of the system ($\Delta U$).

• Increase in entropy (S) of the system ($\Delta S$), which is a measure of disorder.

The process of solution formation involves three steps:

1. Separating solute particles (endothermic, $\Delta H_{solute}$).

2. Separating solvent particles (endothermic, $\Delta H_{solvent}$).

3. Mixing solute and solvent particles (exothermic, $\Delta H_{mix}$ or $\Delta H_{hydration}$).

Solvation refers to stabilization of molecules/ions by interaction with solvent. If the solvent is water, this is called hydration.

Energetics of Solution Formation

Exothermic Process (Releases Heat)

• Occurs when the energy released by mixing ($\Delta H_{mix}$) is greater than the energy required to separate solute and solvent particles.

• Example: Dissolving salt in water.

• $\Delta H_{solution} = \Delta H_{solvent} + \Delta H_{solute} + \Delta H_{mix}$

Endothermic Process (Requires Heat)

• Occurs when more energy is required to separate solute and solvent particles than is released by mixing.

• Example: Mixing oil and water.

• Net endothermic process: $\Delta H_{solution} > 0$

Enthalpy, Entropy, and Dissolving Solutes

When solute and solvent molecules spread out over a larger volume, entropy increases. Processes with $\Delta S > 0$ tend to be product-favored.

• KCl in water: Soluble; overall process is exothermic ($\Delta H_{solution} > 0$).

• CaCO3 in water: Insoluble; overall process is endothermic ($\Delta H_{solution} < 0$).

• Endothermic dissolution can occur if $\Delta S > 0$.

Solubility Example: Water vs. Benzene

Predicting solubility based on polarity:

• Non-polar compounds (e.g., hydrocarbons) are soluble in benzene (non-polar), insoluble in water (polar).

• Polar compounds (e.g., aldehyde CH2O) are soluble in water, insoluble in benzene.

Example: Aldehyde (CH2O) is more soluble in water than in benzene.

*Additional info: These notes cover the foundational concepts of solution chemistry, including types of mixtures, intermolecular forces, thermodynamics of solution formation, and practical examples of solubility prediction. Further topics such as concentration units, colligative properties, and colloid behavior are typically included in a full chapter but are not present in the provided slides.*