States of Matter, Gas Laws, and Attractive Forces: Chapter 7 Study Notes

Gases and Gas Laws

Gases and Pressure

Gases are a fundamental state of matter characterized by widely spaced particles in constant motion. The concept of pressure is central to understanding gas behavior.

• Pressure is defined as the force exerted against a given area.

• Common units of pressure include:

  • Atmosphere (atm): Standard unit for atmospheric pressure.

  • Pounds per square inch (psi): Measures force in pounds per square inch; atmospheric pressure at sea level is about 14.7 psi.

  • Pascal (Pa): The SI unit of pressure.

  • Millimeters of mercury (mmHg): Based on the height of a mercury column in a barometer; 760 mmHg equals 1 atm.

• Example: A mercury barometer measures atmospheric pressure by the height of mercury it supports; at sea level, this is 760 mmHg.

Kinetic Molecular Theory of Gases

The kinetic molecular theory explains the unique behaviors of gases and forms the basis for the gas laws.

• Gas particles are far apart; most of the volume is empty space.

• Particles are in constant, random motion with a range of speeds.

• There are no attractive forces between gas particles.

• Gas particles have kinetic energy proportional to absolute temperature; higher temperature means faster movement.

• A gas that perfectly follows these assumptions is called an ideal gas.

Gas Laws

Gas laws describe the relationships between pressure, volume, temperature, and amount of gas.

• Boyle's Law: At constant temperature, the volume of a fixed amount of gas is inversely proportional to its pressure.

$P_1 V_1 = P_2 V_2$

• Example: Doubling the pressure on a gas halves its volume.

• Application: Breathing involves changes in lung volume and pressure, following Boyle's Law.

• Charles's Law: At constant pressure, the volume of a fixed amount of gas is directly proportional to its absolute temperature (in Kelvin).

$\frac{V_1}{T_1} = \frac{V_2}{T_2}$

• Example: Doubling the absolute temperature doubles the volume.

• Gay-Lussac’s Law: At constant volume, the pressure of a fixed amount of gas is directly proportional to its absolute temperature.

$\frac{P_1}{T_1} = \frac{P_2}{T_2}$

• Combined Gas Law: Combines Boyle’s, Charles’s, and Gay-Lussac’s laws for a fixed amount of gas.

$\frac{P_1 V_1}{T_1} = \frac{P_2 V_2}{T_2}$

• Ideal Gas Law: Relates pressure, volume, temperature, and moles of gas.

$PV = nRT$

• Where P = pressure, V = volume, n = moles, R = ideal gas constant, T = temperature in Kelvin.

Liquids and Solids: Predicting Properties Through Attractive Forces

Changes of State and Attractive Forces

Substances change state (solid, liquid, gas) as temperature changes, due to changes in particle motion and the strength of attractive forces.

• Key transitions: melting/freezing, evaporation/condensation, sublimation/deposition.

Vapor Pressure and Boiling Points

• Vapor pressure: The pressure exerted by a vapor in equilibrium with its liquid at a given temperature.

• Each substance has a characteristic vapor pressure that increases with temperature.

• Boiling point: The temperature at which vapor pressure equals atmospheric pressure.

• Stronger attractive forces result in higher boiling points and lower vapor pressures.

Types of Attractive Forces

Intermolecular forces determine the physical properties of substances.

• London (dispersion) forces: Weak, temporary attractions due to momentary uneven electron distribution; significant in nonpolar molecules.

• Dipole-dipole attractions: Occur between polar molecules with permanent dipoles.

• Hydrogen bonding: Strong dipole-dipole attraction involving hydrogen bonded to N, O, or F and a lone pair on another N, O, or F atom.

• Ion-dipole attractions: Between ionic charges and polar molecules; important in solutions.

• Ionic (salt bridge) attractions: Strongest; between oppositely charged ions.

Effect of Molecular Structure on Physical Properties

• Nonpolar molecules (e.g., alkanes) interact via London forces; larger surface area increases boiling/melting points.

• Branched alkanes have lower boiling points than straight-chain isomers due to less surface contact.

• Melting points follow similar trends as boiling points.

Summary Table: Types of Intermolecular Forces

Type of Force	Occurs Between	Relative Strength
London (dispersion)	All molecules (esp. nonpolar)	Weakest
Dipole-dipole	Polar molecules	Moderate
Hydrogen bonding	H with N, O, or F	Strong
Ion-dipole	Ions and polar molecules	Very strong
Ionic (salt bridge)	Oppositely charged ions	Strongest

Attractive Forces and Solubility

Golden Rule of Solubility: Like Dissolves Like

Solubility depends on the similarity of polarity and types of attractive forces between solute and solvent.

• Hydrophilic: Water-loving; polar or ionic substances soluble in water.

• Hydrophobic: Water-hating; nonpolar substances not soluble in water.

• Nonpolar compounds (e.g., oils) are insoluble in water due to lack of shared attractive forces.

• Polar compounds (e.g., sugar) dissolve in water via dipole-dipole and hydrogen bonding.

• Ionic compounds dissolve in water via ion-dipole interactions and hydration.

Amphipathic Compounds and Emulsifiers

• Amphipathic molecules: Contain both polar (hydrophilic) and nonpolar (hydrophobic) regions (e.g., fatty acids, soaps).

• Soaps form micelles in water: nonpolar tails cluster inside, polar heads face outward, allowing oils to be emulsified and washed away.

• Emulsifiers enable mixing of polar and nonpolar substances.

Dietary Lipids

Fats vs. Oils

• Fats: Solid at room temperature; derived from animals; composed mainly of saturated hydrocarbon tails (no double bonds).

• Oils: Liquid at room temperature; derived from plants; contain unsaturated (cis double bonds) hydrocarbon tails, causing kinks and less packing.

• Straight, saturated tails allow for stronger London forces and higher melting points.

• Kinked, unsaturated tails reduce London forces, increasing fluidity and lowering melting points.

Attractive Forces and the Cell Membrane

Phospholipids and Membrane Structure

• Phospholipids: Main components of cell membranes; consist of a glycerol backbone, two nonpolar fatty acid tails, and a polar phosphate-containing head.

• Phospholipids are amphipathic and form a bilayer in water: polar heads face aqueous environments, nonpolar tails face inward.

• The bilayer creates a nonpolar interior, forming the structural foundation of cell membranes.

Membrane Proteins and Fluid Mosaic Model

• Proteins span the bilayer (integral) or associate with surfaces (peripheral), enabling selective transport and signaling.

• Carbohydrates on the membrane surface act as cell signals.

• The fluid mosaic model describes the dynamic, flexible nature of the membrane.

Cholesterol in Membranes

• Cholesterol: A steroid lipid with a polar OH group and nonpolar steroid nucleus.

• Cholesterol inserts into the bilayer, modulating membrane fluidity by interacting with phospholipid tails.