Phases of Matter

1. Phases of Matter Chp 13 and 14

2. Phases of Matter • Solid – molecules are held tightly together by intermolecular forces, molecules move slowly • Liquid – some intermolecular forces still exist, but they are becoming weaker as molecules speed up • Gas – all intermolecular forces have been broken, molecules move very quickly and rarely interact • It take ENERGY to break those intermolecular forces and cause a phase change!!!

3. Phase Changes • Evaporation – liquid to gas, is a cooling process since hottest molecules escape (why we sweat) • Boiling – evaporation throughout a substance, it can be decreased as air pressure decreases • Condensation – gas to liquid, is a warming process since hot gas molecules bring their energy with them • Melting – solid to liquid • Freezing – liquid to solid • Sublimation – solid to gas, skipping liquid phase • This occurs especially at low pressures since the boiling point becomes lowered so much

4. Intermolecular forces • Exist between molecules • 3 types: • Dipole-dipole: occurs between polar molecules • Hydrogen bond: special dipole that occurs when hydrogen is involved (stronger than regular dipole-dipole interactions) • London dispersion force: occurs for noble gases and nonpolar molecules, very weak and doesn’t last long • The stronger the intermolecular forces involved, the higher the boiling point since more energy is needed to break those interactions.

5. Heating/Cooling Curves • Graph that represents at what temperature phase changes occur and how heat and temperature are related • Note, the temperature doesn’t change during a phase change (boiling water stays at 100oC until is becomes a gas) • Heat of fusion – energy needed to turn a solid into a liquid (or released when a liquid turns into a solid) • 6.02 kJ/mol for water • Heat of vaporization – energy needed to turn a liquid into a gas (or released when a gas turns into a liquid) • 40.6 kJ/mol for water • HOV is always higher than HOF because it takes more energy to completely break the intermolecular forces

6. Using HOV and HOF • Ex. How much energy is required to melt 8.5 g of ice at 0oC? 8.5 g 1 mol 6.02 kJ = 2.8 kJ 18 g 1 mol

7. Combining temperature and phase changes • Ex. How much energy is required to heat 25 g of water from 25oC to 100oC as steam? • 2 parts: heat 75oC, then hov to get steam • Part 1 Q = smT Q = 4.184 (25)(75) = 7800 J or 7.8 kJ • Part 2 25 g 1 mol 40.6 kJ = 57 kJ 18 g 1 mol • Now, we add since both must occur 7.8 kJ + 57 kJ = 64.8 kJ

8. Pressure • All gases, including the atmosphere, exert a pressure • Result of gravity pulling on the gas’s mass • 760 mm Hg at sea level on Earth • Has a variety of units • 1 standard atmosphere = 1 atm = 760 mm Hg = 760 torr = 101,325 Pa = 14.69 psi • We can convert between units using a T-chart • We typically use pascals in science

9. Unit conversion • Ex. What is 7.3 atm in mm Hg? 7.3 atm 760 mm Hg = 5548 mm Hg 1 atm • Ex. What is 7.3 mm Hg in Pascals? 7.3 mm Hg 101325 Pa = 973.2 Pa 760 mm Hg

10. Boyle’s Law • As the volume of a gas is decreased, it’s pressure increases • P1V1 = P2V2 • Ex. If a 1.5 L sample of freon gas at 56 torr is increased to a pressure of 150 torr, what is the new volume of the gas? 56 (1.5) = 150 (V) V = 0.56 L *Any units are fine, as long as they are the same on both sides of the equation.

11. Charles’ Law • As the temperature of a gas increases, it’s volume increases (temp must be in K) • At -273oC, all gases occupy a volume of 0, which is impossible, so that is coldest temp possible (absolute zero) • V1 = V2 T1 T2 • Ex. If a 2 L gas sample at 298 K is cooled to 278 K, what is it’s new volume? 2 = V2 298 278 V2 = 1.9 L

12. Combined Gas Law • Stick charles’ law and boyle’s law together • Pressure can be in any unit, as long as it’s the same on both sides, but temperature must be in kelvin • P1V1 = P2V2 T1 T2

13. Avogadro’s law • As the number of moles of gas increases, so does the volume • V1 = V2 n1 n2 • If 0.5 mol of oxygen occupy 12.2 L, what volume would 0.33 mol of oxygen occupy? 12.2 = V2 0.5 0.33 V2 = 8.1 L

14. Ideal Gas Law • Combines all gas laws into one • PV = nRT R = 0.08206 Latm/molK *so, temp must be in kelvin, volume must be in liters and pressure must be in atm • Ex. A 8.56 L sample of hydrogen gas at 0oC has a pressure of 1.5 atm. How many moles are present in the sample? 1.5 (8.56) = n (0.08206) (273) n = 0.57 mols

15. Some Terms • STP – standard temperature at pressure • 1 atm and 0oC • Molar volume – the volume that one mole of any gas takes up at STP • 22.4 L at STP • Kinetic molecular theory – predicts why gases have the properties that they do • As temp increases, particles move faster and collide with the container more often and interact with each other less

16. Dalton’s Law of Partial Pressure • When gases are mixed, their pressures add together to create a new pressure in the container • Occurs because gases move so fast, what the particle is doesn’t matter as much as how many there are • Ptotal = P1 + P2 … • Ptotal = ntotal (RT) V

17. An example • If 12 g of oxygen and 46 g of He are pumped into a 5.0 L container at 25oC, what will the total pressure in the container be? • First g convert to moles using molar mass 12 g 1 mol = 0.375 mol 46 g 1 mol = 11.5 mol 32 g 4 g • Next oC becomes K by adding 273 25 + 273 = 298 K • Then plug into formula Ptotal = (.375 + 11.5) (0.08206)(298) 5 Ptotal = 58.1 atm