AP Chemistry Unit 3.3 Solids, Liquids, and Gas

The Three States of Matter

• Gas: molecules/atoms have enough energy to move freely
  • Particles so far apart from each other that intermolecular forces not considered
  • Indefinite shape and volume
• Liquid: strong intermolecular forces and molecular motions
  • Particles are always in contact but have enough energy to slide past each other
  • Indefinite shape & definite volume
• Solid: strongest intermolecular forces, but the molecular motions are minimal
  • Particles don’t have enough energy to move → always in contact and in fixed position
  • Definite shape and definite volume

Characteristics

• Particles retain their chemical identity in all 3 states, but the volume, density, and interparticle distances are different
• Liquids & solids are incompressible (condensed state) & their density does not change with temperature
  • These similarities are due to the molecules being close together in solids and liquids; far apart in gases

The Liquid State

• Liquids have low compressibility, lack of rigidity, and high density compared to gasses
• Surface tension: tendency of molecules to be pulled from the surface to the interior of a liquid; resistance of a liquid to an increase in its surface area
  • Stronger InterMF = stronger surface tension b/c molecules resist being stretched/broken
• Viscosity: a measure of a liquid’s resistance to flow
• Capillary action: spontaneous rising of a liquid in a narrow tube
  • Capillary action depends on the cohesive and adhesive forces present → during capillary action, the liquid molecules simultaneously adhere to the tubing while pulling each other up
    • Cohesive forces: intermolecular forces among the molecules of the liquid
      • Molecules attracted to same molecules
    • Adhesive forces: forces between the liquid molecules and their container
      • Molecules attracted to another type of molecule

Concave Meniscus Formed by Polar Water

• Water has both strong adhesive and cohesive forces, but bcuz the adhesive forces are stronger → water will have a concave meniscus (water is attracted to the glass)

• Nonpolar liquids have stronger cohesive than adhesive forces (not attracted to glass) → convex meniscus

 Changes of State

• Phase Changes: when a substance changes from solid to liquid to gas

• Melting point: temp at which the substance goes from a solid to a liquid (or from a liquid to a solid)
  • The strength of the InterMF determines the temp at which these phase changes will occur
• Boiling point: temp at which a substance goes from a liquid to a gas (or from a gas to a liquid)
• 2 Key Points

1. 1. At a substance’s MP or BP, two phases can exist simultaneously
   2. The temp of a substance does not change as the substance goes from one phase to another

• • • Only after all of the substance has changed phases does adding heat change the temp of the substance
• Heat of fusion:ΔH_fus·: The enthalpy change that occurs at the melting point when a solid melts
• 
  • • When curve quickly changes slope to 0, all energy is used to overcome intermolecular forces holding the substance’s molecules together

Evaporation

• Evaporation: liquid becomes gas below a substances BP (occurs only for particles at the surface of a liquid)
• The particles with the highest KE can overcome the InterMF forces within the liquid and evaporate as gas
• Is a cooling process because the particles with the highest KE diffuse away from the liquid so the average KE of remaining particles decrease

 Boiling

• Boiling: process by which a liquid becomes a vapor when it is heated to its boiling point
• As temp of liquid increases, vapor pressure increases until the vapor pressure of the liquid become equal to the surrounding atmospheric pressure → At this temp the liquid will boil (BP)
  • As atmospheric pressure increase, BP of liquid increases
  • The normal BP is the temperature at which the liquid boils at standard pressure

Evaporation vs Boiling

• Vaporization occurs in two ways: boiling and evaporation
• Evaporation is slower, occurs only from the surface of the liquid, does not produce bubbles, and leads to cooling.
• Boiling is faster, can occur throughout the liquid, produces lots of bubbles, and does not result in cooling.

Vapor Pressure and Changes of State

• Vapor Pressure: Liquid molecules at the surface escape into the gas phase → gas particles create pressure above the liquid in a closed container
• Gases are often collected over water so the vapor pressure of water must be subtracted from the total pressure in calculations
• Weaker IMF → Lower BP → will have higher vapor pressure before reaching boiling point
  • Liquids are said to be volatile—they evaporate rapidly from an open dish
• Stronger IMF → higher BP/fewer molecules break away → will have lower vapor pressure before reaching boiling point.
• Heat of vaporization ΔH_vap•:The energy required to vaporize 1 mole of a liquid at a pressure of 1 atm
  • Water has high HoV so can absorb lots of heat and resist chemical change; needs lots of energy to freeze → cools air when it is warm and releases heat in the winter, stabilizes ocean temperature, climate
• Generally, the vapor pressure of a liquid is related to temperature and intermolecular forces
  • Vapor pressure increases significantly with temperature.
    • Temperature of the liquid increases = more molecules will have the minimum energy needed to overcome InterMF and escape into the vapor phase

Phase Diagrams

• Phase diagram: way of representing the phases of a substance as a function of temperature and pressure. (in a closed system)
  • Lines represent phase changes
• Triple point: condition of temp and pressure where all three phases are present
• Critical temperature: temperature above which the vapor cannot be liquefied no matter what pressure is applied
• Critical pressure: pressure required to produce liquefaction at the critical temperature
• Critical point: critical temperature + critical pressure

Phase Diagram for Water

• Density and Phase Diagrams: the slope of the line between the solid and liquid region indicates which of these 2 phases is denser 1
• MP Curve has positive slope (/) → solid is denser
  • Increasing pressure = increases melting point
• MP curve has negative slope (\)→ liquid is denser
  • Increasing pressure = decreases melting point
    • Water has a negative slope, but most other substances have a positive slope