chem of solutions Lecture 1 : solutions

Introduction to Solubility

• Solubility is a key topic in chemistry concerning the formation of solutions.

• This lesson outlines the three-step process for forming a solution and the factors affecting solubility.

• Instructor: Chad from Chad's Prep, focusing on making science easy to learn.

Three-Step Process for Solution Formation

1. Breaking apart solvent molecules:

   • Solvent molecules must be separated to allow room for solute particles.

   • This is an endothermic process, so the change in enthalpy (ΔH) is positive.

2. Separating solute molecules:

   • Similar to the solvent, solute particles must also be broken apart.

   • This process is also endothermic and contributes positively to ΔH.

3. Mixing solute and solvent:

   • Once separated, solute particles mix with solvent particles, forming new intermolecular forces.

   • This step is exothermic, thereby reducing ΔH.

• The overall ΔH for solution formation is the sum of the changes from all three steps.

• Best case scenario: ΔH approaches 0, which indicates a favorable reaction for spontaneity.

Factors Affecting Spontaneity of Solution Formation

• ΔH (Enthalpy change):

  • Favorable if ΔH is negative; unfavorable if positive.

  • For spontaneous processes, ΔH should be as close to 0 as possible.

• Entropy of Mixing:

  • Entropy relates to the disorder of the system; higher entropy favors spontaneity.

  • Mixing solute and solvent increases disorder compared to the initial pure substances.

Key Principle: "Like Dissolves Like"

• Solubility is greatly influenced by the polarity of the solute and solvent.

• Polar solutes mix well with polar solvents (e.g., ethanol with water).

• Nonpolar solutes mix well with nonpolar solvents (e.g., butane with cyclohexane).

Solution Vocabulary

1. Miscible:

   • Liquids that mix in any proportion (e.g., ethanol and water).

2. Immiscible:

   • Liquids that do not mix significantly (e.g., oil and water).

3. Saturated:

   • A solution that has reached its maximum solute concentration.

4. Unsaturated:

   • A solution that can still dissolve more solute.

5. Supersaturated:

   • A solution that contains more dissolved solute than usually possible under certain conditions.

   • Example: Supersaturated sugar solution created during rock candy preparation.

6. Colloid:

   • A type of homogeneous mixture where larger particles stay suspended (e.g., milk).

   • Particles do not settle and scatter light differently than true solutions.

Factors Affecting Gas Solubility

• Solubility of gases is influenced mainly by temperature and pressure:

  1. Temperature:

     • Gases are less soluble in warmer solutions. As temperature rises, gases tend to escape from the liquid (e.g., boiling water releasing dissolved gases).

  2. Pressure:

     • Gases are more soluble under higher pressure. Henry's Law states that gas solubility is directly proportional to its partial pressure above the solution.

     • Example calculation for oxygen solubility in water using Henry's Law constant.

Factors Affecting Solid Solubility

• Higher temperatures generally increase the solubility of ionic solids (exceptions are rare).

• Examples:

  • Lead nitrate, potassium dichromate, potassium bromide all show increased solubility as temperature rises.

  • Sugar, although not ionic, also dissolves better at higher temperatures.

Conclusion

• A brief overview of concentration units will be the next topic of study.

• Note that understanding solubility concepts will aid in further chemistry topics.

Additional Resources

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• Reference other chemistry playlists and free content available on the Chad's Prep website.