Phase Diagrams CHEM HONORS

Kinetic Theory of Matter Kinetic Theory of Matter: molecules are always moving Measure kinetic energy with a thermometer known as temperature The greater the material’s internal energy, the higher the temperature of that material Heat is the energy flow btwn objects of different temperature (heat ≠ temperature!) Brownian motion describes how visible particles are seen moving due to invisible molecules bumping into them

Phases of Matter Solid - definite volume and definite shape - molecules are packed together tightly and move slowly Liquid - definite volume, indefinite shape - molecules loosely packed and move with greater speed, hence a liquid can flow and spread Gas - indefinite volume, indefinite shape - molecules loosely arranged and move so rapidly they fill container

Phase Changes Melting: solid  liquid Freezing: liquid  solid Evaporation: liquid  gas Condensation: gas  liquid Sublimation: solid  gas Deposition: gas  solid

Intermolecular Forces © 2015 Pearson Education, Inc. Heating Curves A plot of temperature vs. heat added is called a heating curve. Within a phase, heat is the product of specific heat, sample mass, and temperature change. The temperature of the substance does not rise during a phase change. For the phase changes, the product of mass and the heat of fusion of vaporization is heat.

Checkpoint Calculate the enthalpy change upon converting 1.00 mol of ice at -25 o C to steam at 125 o C under a constant pressure of 1 atm. The specific heats of ice, liquid water, and steam are 2.03, 4.18, and 1.84 J/ g-K, respectively. For H 2 O, deltaH fus = 6.01 kJ/mol and deltaH vap =40.67 kJ/mol.

Phase Diagram A phase diagram is a graph of pressure vs. temperature for a substance. It shows ◦melting, boiling, and sublimation points at different pressures. ◦the triple point and critical point.

Phase Diagram Triple Point – represents the only point in the phase diagram in which all three states are in equilibrium Critical Point – is the highest temperature and pressure at which a gas and a liquid can coexist at equilibrium A supercritical fluid (SCF) is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid.

Intermolecular Forces © 2015 Pearson Education, Inc. Phase Diagram of Water Note the high critical temperature and critical pressure. ◦These are due to the strong van der Waals forces between water molecules.

Intermolecular Forces © 2015 Pearson Education, Inc. Phase Diagram of Water Unusual feature for water: The slope of the solid– liquid line is negative. This means that as the pressure is increased, the melting point decreases.

Intermolecular Forces © 2015 Pearson Education, Inc. Phase Diagram of Carbon Dioxide Unusual features for carbon dioxide: cannot exist in the liquid state at pressures below 5.11 atm (triple point) CO 2 sublimes at normal pressures.

Checkpoint Use the phase diagram for methane to answer the following questions. (a) what are the approximate temperature and pressure of the critical point? (b) what are the approximate temperature and pressure of the triple point?

Checkpoint (c) Is methane a solid, liquid, or gas at 1 atm and 0 o C? (d) If solid methane at 1 atm is heated while the pressure is held constant, will it melt or sublime? (e) If methane at 1 atm and 0 o C is compressed until a phase change occurs, in which state is the methane when the compression is complete?