States of Matter Notes

States of Matter

Learning Objectives

• Investigate and describe the three states of matter: solid, liquid, and gas.
• Develop models to explain the behavior of gases with changes in temperature, pressure, and volume.
• Develop a model of intermolecular forces and use it to explain states of matter.

Particle Motion

• Draw and describe the motion of particles in the three states of matter.

Kinetic Energy

• Kinetic energy is the energy that particles possess due to their motion.
• The state of a substance is determined by conditions of pressure and temperature.

Kinetic Theory of Gases

• The kinetic theory as it applies to gases includes the following assumptions:
  • Gas particles are small, hard spheres with insignificant volume.
  • Gas particles are relatively far apart compared to liquids and solids.
  • There is empty space between gas particles.
  • No attractive or repulsive forces exist between gas particles.
• Collisions between particles are perfectly elastic, meaning there is no loss of kinetic energy.
• The motion of particles is rapid, constant, and random, moving in straight lines until collision.

Modeling Gas Particles

• Draw extra particles in a cube of gas, doubling the quantity, and show how they interact, considering the kinetic theory.

Elastic Collisions

• Describe an elastic collision between gas molecules.

Standard Temperature and Pressure (STP)

• STP is a reference condition used to compare gas properties.
• Temperature: 0 degrees Celsius (273.15 Kelvin).
• Pressure: 1 atmosphere (100 kilopascals).

Common Gases

• Examples of elemental and complex gases, including diatomic gases.

Bonding in Gases

• Identify the type of bonding (ionic, covalent, metallic) for compounds that are gases at STP.
• Examples: hydrogen sulfide (H2S), ammonia (NH3), methane (CH4), nitrous oxide (N2O).
  *Note any patterns of bonding that can predict whether a compound will be a gas at STP.

Gas Pressure

• Gas pressure results from the force exerted by a gas per unit surface area of an object.
• Moving bodies exert force on collision.
• Gas pressure is the result of billions of rapidly moving particles colliding with an object.
• Factors affecting gas pressure:
  • Low temperature: Lower pressure
  • High temperature: Higher pressure
  • Decrease volume: Higher pressure
  • Add particles: Higher pressure

Car Tire Pressure

• Explain why car tire pressure decreases when it gets cold outside, relating it to the behavior of gas molecules.

Units of Pressure

• Common units of pressure: kPa, psi, atm, mm Hg
• Conversion factors allow comparison of units.

Atmospheric Pressure

• Air exerts pressure because gravity holds air particles within Earth's atmosphere.
• Collisions of atoms and molecules in air with objects result in atmospheric pressure.
• Atmospheric pressure decreases as altitude increases due to decreasing atmospheric density.

Barometer

• A barometer measures atmospheric pressure.
• At sea level, air supports a 760-mm column of mercury.
• On Mount Everest (9000 m), air supports a 253-mm column of mercury.

Pressure Conversion Problems

• Sample problem: Convert 450 kPa to atmospheres and millimeters of mercury.
• Practice problem: What pressure, in kilopascals and atmospheres, does a gas exert at 385 mm Hg?
• Practice problem: The pressure at the top of Mount Everest is 33.7 kPa. Is that pressure greater or less than 0.25 atm?

Kinetic Energy and Particle Motion

• Solids: Vibration
• Liquids: Vibration, Rotation, Translation
• Gases: Vibration, Rotation, Translation

Vibrational Energy

• Will the average vibrational energy of nitrogen gas (N₂) at 22°C be greater than, less than, or equal to the average vibrational energy of NaCl at 22°C? Make a claim and support it with evidence.

Fluids

• Substances that can flow are called fluids (liquids and gases).
• The ability to flow allows fluids to conform to the shape of their containers.

Liquids and Intermolecular Forces

• Gases and liquids are fluids.
• Fluids flow.

Intermolecular Forces

• Types of intermolecular forces:
  • Hydrogen bond
  • Dipole interaction
  • Dispersion forces
• Relative strength of intermolecular forces.

Argon and Dispersion Forces

• The element argon (Ar) has eight valence electrons and is a liquid at very low temperatures. Sketch a model of two argon atoms and where each atom's electrons need to be in order for an attractive dispersion force to occur. Make a similar sketch that explains why larger molecules typically experience larger dispersion forces.

Solids and Attractive Forces

• Molecular solid: Dipole interactions
• Metallic solid: Metallic bonds
• Ionic solid: Ionic bonds
• Covalent network solid: Covalent bonds

Types of Crystalline Solids

State of Matter at Room Temperature

• Explain why carbon dioxide is a gas, water is a liquid, and salt is a solid at room temperature based on how the particles are held together.

Crystalline vs. Amorphous Solids

• In a crystal, particles are arranged in an orderly, repeating, three-dimensional pattern called a crystal lattice.
• Amorphous solids lack an ordered internal structure; atoms are randomly arranged (e.g., rubber, plastic, asphalt).

Crystal Structure

• Crystalline: Cleavage
• Amorphous: Fracture

Cleavage in Ionic Crystals

• Ions with opposite charges align to form a crystal with strong attraction between planes.
• When shear forces are applied, ions shift, causing similarly charged ions to repel and break the crystal along the plane.

Crystal Systems

• The shape of a crystal depends on the arrangement of particles within it.
• The smallest group of particles within a crystal that retains the geometric shape of the crystal is a unit cell.

Seven Crystal Systems

• Cubic: $a = b = c$, $\alpha = \beta = \gamma = 90^\circ$
• Tetragonal: $a = b \neq c$, $\alpha = \beta = \gamma = 90^\circ$
• Orthorhombic: $a \neq b \neq c$, $\alpha = \beta = \gamma = 90^\circ$
• Monoclinic: $a \neq b \neq c$, $\beta = \gamma = 90^\circ \neq \alpha$
• Triclinic: $a \neq b \neq c$, $\alpha \neq \beta \neq \gamma \neq 90^\circ$
• Hexagonal: $a = b \neq c$, $\alpha = \beta = 90^\circ$, $\gamma = 120^\circ$
• Rhombohedral: $a = b = c$, $\alpha = \beta = \gamma \neq 90^\circ$

Cubic Systems

• Three types of unit cells for cubic systems.

Importance of Atomic Arrangement

• Why is the arrangement of constituent atoms/molecules more important in determining properties of a solid than a liquid or gas?
• Why are structures of solids usually described in terms of positions of atoms rather than their motion?