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Chapter 6: Solutions and Dilutions

Molarity Calculations:

Definition: Molarity (M) is defined as moles of solute per liter of solution.
Formula: M = \frac{\text{moles of solute}}{\text{liters of solution}}
Example: If 5 g of NaCl is dissolved in enough water to make 0.5 L of solution:
- Calculate moles of NaCl: Moles,of,NaCl = \left( \frac{5,\text{g}}{58.44,\text{g/mol}} = 0.0857,\text{mol} \right)
- Calculate molarity: Molarity = \left( \frac{0.0857,\text{mol}}{0.5,\text{L}} = 0.1714,\text{M} \right)

Volume to Mole Conversions:

Use molarity to convert between volume and the number of moles using the formula: Moles = Molarity \times Volume ,(L)
Example: To find moles in 2 L of a solution with a molarity of 3 M, calculate: $ Moles = 3 , M \times 2 , L = 6 , mol. $

Dilution Calculations:

Understand the dilution formula: M_1V_1 = M_2V_2
Example: To dilute a solution, mix 100 mL of 2 M HCl with water to reach a final volume of 500 mL.
- Calculate new molarity: M_2 = \frac{M_1V_1}{V_2} = \frac{2,\text{M} \times 100 \text{mL}}{500 \text{mL}} = 0.4,\text{M}
This shows that by adding water, the concentration of the solute decreases as the total volume increases.

Stoichiometric Calculations:

Perform calculations using molarity and volume to find the amounts of reactants/products in chemical reactions. This is crucial in determining the proportions required in reactions for accurate and efficient chemical processes.

Interactions Between Particles:

Solute-Particle Interactions: Weak attractions among individual solute particles. These interactions can affect how easily the solute dissolves in the solvent.
Solvent-Particle Interactions: Attractions among solvent molecules, which dictate the solvent's physical properties such as boiling point and viscosity.
Solvent-Solute Interactions: New attractions formed during the solution process, which are critical for the dissolving process.
Example: In NaCl dissolved in water, ion-dipole interactions occur between Na+ ions and water molecules, facilitating solubility.

Solubility in Water:

Determine solubility based on interactions. Ionics and polar molecules are generally soluble in water due to dipole interactions, while non-polar molecules often remain insoluble.
Role of Solute-Solvent Interactions: Strong interactions favor solubility by lowering the energy barrier for dissolution.

Predicting ΔH:

Endothermic (+ΔH): Absorbs heat – typical in breaking established interactions between solute or solvent particles.
Exothermic (-ΔH): Releases heat – occurs when new, stronger interactions are formed (solute-solvent).
Consider ΔH between different interactions to understand energy changes during dissolution.

Relation to Temperature:

Increased temperature often increases solubility of solids in liquids, as increased kinetic energy allows more solute particles to overcome solute-solvent interactions.

Entropy (S):

Explains the natural tendency for systems to move towards disorder. Example: The process of a solid unfolding upon dissolution increases entropy, which is favorable for dissolution processes.

ΔH and Interaction Strengths:

If stronger interactions are formed than broken, the dissolution is exothermic; if weaker interactions are formed than broken, it is endothermic.
This concept is vital for predicting solubility behavior in varying conditions.

Free Energy Change (ΔG):

Predicts spontaneous reactions and solubility based on the balance of enthalpy (ΔH) and entropy (S) changes. A negative ΔG indicates a spontaneous process, indicating that dissolution is favorable under given conditions.

Identifying Models:

Arrhenius Model: Acids produce H+ ions in water, while bases produce OH- ions; this model is limited to aqueous solutions.
Bronsted-Lowry Model: Accurately describes acids as proton donors and bases as proton acceptors, allowing for a broader range of acid-base reactions outside aqueous solutions.
Lewis Model: Defines acids as electron pair acceptors and bases as electron pair donors, extending the concept of acid-base reactions to a wider array of chemical species.

Classification of Compounds:

Match compounds to the appropriate model based on functional groups or ionizable protons.
Example: HCl acts as an acid in all three models, and NaOH acts as a base under Arrhenius and Bronsted models due to its ability to produce hydroxide ions.

Acid-Base Reactions:

Analyze using Bronsted or Lewis model, allowing identification of acids and bases and predicting their respective products.
Examples: HCl + NH3 (Bronsted model) illustrates a proton transfer reaction.

Molecular Structures:

Recognizing features in molecular structures that indicate acid/base properties is crucial (e.g., presence of electronegative atoms often signals acidity).

Mechanisms:

Illustrate acid-base reactions through proton transfer (Bronsted-Lowry) and curved arrow notation (Lewis model).

Identify Products:

Recognizing reactions based on molecular structure and charge enables predictions of products and helps facilitate a deeper understanding of acid-base chemistry.

For each checklist item, summarize what you know utilizing: Notes, Poll Everywhere, Worksheets, Homework, Quizzes, and Exam Review Questions.