AP Chemistry 3.8 Representation of Solution Study Notes - New Syllabus 2024-2025
AP Chemistry 3.8 Representation of Solution Study Notes- New syllabus
AP Chemistry 3.8 Representation of Solution Study Notes – AP Chemistry – per latest AP Chemistry Syllabus.
LEARNING OBJECTIVE
Using particulate models for mixtures:
i. Represent interactions between components.
ii. Represent concentrations of components.
Key Concepts:
- Particulate Representations of Solutions
3.8.A.1 Particulate Representations of Solutions:
1. Particulate Nature of Solutions:
i. Solute and Solvent:
- The solute is the substance that is dissolved in the solvent. It can be in the form of molecules (like sugar or alcohol) or ions (like salt in water).
- The solvent is the substance that does the dissolving. In most common solutions, water is the solvent, but it can be any liquid or gas.
ii. Molecular or Ionic Level:
- At the molecular level, the solute particles interact with the solvent particles. For example, when sugar dissolves in water, the sugar molecules are separated and dispersed throughout the water. Water molecules surround and interact with the sugar molecules through hydrogen bonding or other types of intermolecular forces.
- In ionic solutions (e.g., salt in water), the solute dissociates into ions. Sodium chloride (NaCl) dissociates into Na⁺ and Cl⁻ ions. The water molecules surround these ions with the negative oxygen part of the water molecules surrounding the positively charged Na⁺ and the positive hydrogen part surrounding the negatively charged Cl⁻.
iii. Homogeneous Mixture:
- A solution is considered a homogeneous mixture, meaning the solute is evenly distributed throughout the solvent at the molecular or ionic level. This distribution is a result of the motion of molecules or ions, where the solute particles spread out evenly because of the random motion (Brownian motion) and collisions.
iv. Dissolving Process:
- Solvation: When the solute particles interact with the solvent, this process is known as solvation (or hydration if the solvent is water). The strength of these interactions depends on factors like the type of solute, solvent, and temperature.
2. Solution Composition and Concentration:
| Concentration Type | Formula | Units | Description | Example |
|---|---|---|---|---|
| Molarity (M) | Molarity = moles of solute / liters of solution | moles/L (mol/L) | The number of moles of solute dissolved in 1 liter of solution. | 1 M NaCl = 1 mole of NaCl in 1 liter of water |
| Molality (m) | Molality = moles of solute / kg of solvent | moles/kg | The number of moles of solute dissolved in 1 kg of solvent. | 1 m NaCl = 1 mole of NaCl in 1 kg of water |
| Mass Percent (w/w) | Mass percent = (mass of solute / mass of solution) × 100 | % (mass/mass) | The mass of solute as a percentage of the total mass of the solution. | 10% NaCl (w/w) = 10g NaCl in 90g solution |
| Volume Percent (v/v) | Volume percent = (volume of solute / volume of solution) × 100 | % (volume/volume) | The volume of solute as a percentage of the total volume of the solution. | 10% ethanol (v/v) = 10mL ethanol in 100mL solution |
| Parts Per Million (ppm) | ppm = (mass of solute / mass of solution) × 10⁶ | ppm (mg/L) | Concentration in parts per million (used for very dilute solutions). | 1 ppm lead = 1 mg lead in 1 liter of water |
| Parts Per Billion (ppb) | ppb = (mass of solute / mass of solution) × 10⁹ | ppb (µg/L) | Concentration in parts per billion (used for very trace amounts). | 1 ppb mercury = 1 µg mercury in 1 liter of water |
| Normality (N) | Normality = equivalents of solute / liters of solution | eq/L | Concentration based on the reactive capacity of solute. | 1 N HCl = 1 equivalent of H⁺ ions in 1 L of solution |
| Dilution Equation | C₁V₁ = C₂V₂ | — | Relation between initial and final concentrations and volumes in dilution. | Diluting 1 M NaCl to 0.1 M using 100 mL of solution |
3. Interactions in Solutions:
| Interaction Type | Description | Example in Solutions | Effect on Solubility |
|---|---|---|---|
| Hydrogen Bonding | Attraction between a hydrogen atom and an electronegative atom (O, N, F). | Water and ethanol, water and sugars (e.g., glucose). | Enhances solubility of polar molecules in polar solvents. |
| Ion-Dipole Interactions | Attraction between ions and the dipole of a polar molecule. | NaCl in water (Na⁺ and Cl⁻ surrounded by water molecules). | Strong solubility of ionic compounds in polar solvents. |
| Dipole-Dipole Interactions | Interaction between polar molecules due to opposite charges on their dipoles. | Acetone in water. | Helps polar molecules dissolve in polar solvents. |
| London Dispersion Forces | Weak interactions caused by temporary dipoles in nonpolar molecules. | Oxygen (O₂) in hexane, methane in nonpolar solvents. | Nonpolar molecules dissolve in nonpolar solvents. |
| Ion-Induced Dipole | An ion induces a temporary dipole in a nearby nonpolar molecule. | Na⁺ in iodine (I₂), ions in nonpolar liquids. | Weak contribution to solubility of nonpolar molecules in ionic solutions. |
| Dipole-Induced Dipole | A polar molecule induces a temporary dipole in a nonpolar molecule. | Water and iodine (I₂) | Slight solubility of nonpolar substances in polar solvents. |
Effect of Interactions on Solution Properties
Solubility: The stronger the interactions between solute and solvent particles, the more likely the solute will dissolve in the solvent. For example, ionic compounds dissolve well in water because of the strong ion-dipole interactions.
Boiling and Freezing Points: The strength of interactions in a solution can affect its physical properties. For example, hydrogen bonding in water leads to higher boiling and freezing points compared to other solvents of similar size.
Conductivity: Ionic solutions (like NaCl in water) will conduct electricity because the ions are free to move, which is a result of the ion-dipole interaction. Nonpolar solutions (like oil in water) won’t conduct electricity because there are no charged particles.
4. Macroscopic vs. Microscopic Views:
i. Macroscopic View:
The macroscopic view refers to what we can observe with our naked eye or with common laboratory instruments. This includes properties like color, texture, density, boiling point, solubility, and more.
ii. Microscopic View:
The microscopic view looks at the structure and behavior of individual particles (atoms, molecules, ions, etc.) that make up the substance. It focuses on how these particles interact, move, and organize at a tiny scale.
iii. Connecting the Macroscopic and Microscopic Views:
| Macroscopic Property | Microscopic Explanation | Example |
|---|---|---|
| Color | The color of a substance depends on how its molecules or ions absorb and reflect light at the molecular or atomic level. | Copper: At the microscopic level, copper absorbs specific wavelengths of light, giving it a reddish-brown color. |
| State of Matter | The state (solid, liquid, gas) is determined by the arrangement and movement of particles. Solids have closely packed particles with limited movement, liquids have particles that can move past one another, and gases have widely spaced particles that move freely. | Water (Solid – Ice): In ice, water molecules are rigidly arranged in a crystalline structure. When heated, these molecules gain enough energy to move past each other, turning the solid into a liquid. |
| Boiling Point | The boiling point occurs when the particles in a liquid gain enough energy to break the intermolecular forces and move into the gas phase. Stronger forces between molecules lead to higher boiling points. | Water vs. Ethanol: Water has hydrogen bonds between its molecules, requiring more energy (higher boiling point) to break these bonds compared to ethanol, which has weaker dipole-dipole interactions. |
| Solubility | The solubility of a substance depends on the interaction between the solute particles and the solvent particles. If the solute’s particles are attracted to the solvent’s particles, the substance will dissolve. | Sugar in Water: The polar water molecules form hydrogen bonds with the sugar molecules, allowing them to separate and dissolve in the solution. |
| Density | The density of a substance is influenced by the packing of its particles. Tighter packing leads to higher density. | Lead vs. Aluminum: Lead atoms are more tightly packed than aluminum atoms, making lead denser. |
| Electrical Conductivity | Conductivity depends on the presence of free-moving charged particles (like ions or electrons). In ionic compounds, these ions must be able to move freely (usually in a liquid or solution) to conduct electricity. | Saltwater: When NaCl dissolves in water, it dissociates into Na⁺ and Cl⁻ ions, which can move and carry an electric current, making the solution conductive. |
| Viscosity | Viscosity is determined by the friction between molecules as they move past each other. Larger molecules or stronger intermolecular forces result in higher viscosity. | Honey vs. Water: Honey has larger molecules and stronger intermolecular forces (hydrogen bonds) than water, so it has a higher viscosity. |
| Expansion/Contraction | As particles gain energy (typically from heat), they move more and spread out, causing the substance to expand. Conversely, they contract when they lose energy. | Thermal Expansion of Metals: When a metal is heated, its atoms vibrate more and move farther apart, causing the metal to expand. |
| Pressure (in gases) | The pressure exerted by a gas is the result of the collisions of gas molecules with the walls of their container. The frequency and force of these collisions determine the pressure. | Balloon Inflation: As more air molecules are pumped into a balloon, they collide more frequently with the inner walls, increasing the pressure inside the balloon. |
| Surface Tension | Surface tension arises from the cohesive forces between molecules at the surface of a liquid. Molecules at the surface experience an imbalance of forces, leading to a “skin” effect. | Water Droplets: Water molecules at the surface form hydrogen bonds with each other, creating surface tension that allows small insects to walk on water. |
iv. Key Concepts in Connecting Macroscopic and Microscopic Views:
Intermolecular Forces: These are the forces between molecules (e.g., hydrogen bonding, dipole-dipole interactions, London dispersion forces) that determine the physical properties of a substance. The strength and type of these forces are reflected in macroscopic properties like boiling point, melting point, and viscosity.
Particle Movement and Energy: The movement and energy of particles at the microscopic level directly affect macroscopic behaviors like thermal expansion, pressure, and state changes (e.g., solid to liquid to gas).
Collisions and Interactions: The frequency and nature of particle collisions affect properties like pressure (in gases) and electrical conductivity. The way particles interact with each other—whether they attract or repel—determines the solubility and density of substances.
v. Visualizing the Connection:
- Macroscopic View: You can see that a substance is a liquid because it flows and has a definite volume but no definite shape.
- Microscopic View: At the particle level, you see that molecules are moving past each other, but they are still close enough to maintain a fixed volume.
OLD Content
Representation of Solution
- Molarity (M): K.A Concentration
- Ex: A solution that is 1.0 molar (written as 1.0 M) contains 1.0 mole of solute per liter of solution.
- Note: brackets around something it means the “molarity of what is inside”
- Dilution: water is added to achieve the molarity desired for a particular solution
- Does not change the amount of moles present
- Mass percent (weight percent):
- Molality:
- Normality (N): Molarity x number of equivalents (definition of an equivalent depends on the reaction taking place in the solution)
- Acid–base reaction → number of protons = equivalents
- Oxidation–reduction reactions → number of e- in half-reaction = equivalents
Calculating Concentration of Ions
- Write out balanced formula for dissolution reaction
- Ex: 35g CaCl2 dissolved in 2.75L
Mole Fraction, Mass Percent, & Density Questions
- Find moles → mass percent / molar mass
- Molarity = (MP as a decimal )(1000)(density/molar mass)
- Remember that Density = g/cm3 (mL)
- With water (d =1g/cm3) can just convert mL to grams
Steps in Solution Formation
- Overcoming solute-solute interactions (endothermic) (ΔH₁)
- Breaking ionic compound= breaking ionic bonds; breaking covalent solute = breaking interMF
- Expanding the solvent (endothermic) (overcoming interMF) to make room for the solute (ΔH₂)
- Allowing the solute and solvent to interact to form bonds and a solution (exothermic) (ΔH₃)
- Enthalpy (heat) of solution (ΔHsoln): is the sum of the ΔH values for the steps of solution formation
- Might have a positive sign (energy absorbed) or a negative sign (energy released)
- Enthalpy (heat) of solution (ΔHsoln): is the sum of the ΔH values for the steps of solution formation