Lecture 26: Liquids 1: phase changes & heat capacity

Transcription

1 Lecture 26: Liquids 1: phase changes & heat capacity Read: BLB 5.5; 11.4 HW: BLB 5:48,49,51; 11:33,37,39 Know: viscosity, surface tension cohesive & adhesive forces phase changes heat capacity calorimetry Need help?? Get help!! TAs in CRC (211 Whitmore) and SI hours on Chem 110 website; my office hours (Mon 12:30-2 & Tues 10:30-12 in 324 Chem Bldg [or 326 Chem]) Bonus deadline for BST #8: Intermolecular forces, March 26 Bonus deadline for BST #9: Solutions & dilutions, April 2 Exam 3: Monday, April 6:30 Check out the Sheets Page 1 Lecture 26

2 Manifestations of IM forces cohesive forces: forces between molecules for example: viscosity: resistance to flow surface tension: E needed to increase vapor pressure (more next time) BP (MP) ΔH vap (ΔH sub ) adhesive forces: forces between a & the surface for example: water glass interactions water oil interactions meniscus formation capillary action glue non-stick surfaces Sheets Page 2 Lecture 26

3 Properties of liquids viscosity: resistance to flow; IM forces, viscosity surface tension: energy needed to increase surface area; IM forces, surface tension IM interactions are energetically favorable; that is, heat is required to break them the more interactions, the better BUT surface molecules have fewer interactions; minimize energy by minimizing surface area Sheets Page 3 Lecture 26

4 Sheets Page 4 Lecture 26

5 Kinetic energy & intermolecular forces kinetic energy (KE) prevents molecules from interacting with each other, i.e., KE attractive intermolecular forces (IMF) gas: liquid: solid: KE >> IMF KE IMF KE << IMF KE T energy IN (heating): solid liquid gas Sheets Page 5 Lecture 26

6 Phase changes & temperature energy change: ΔH sub, ΔH fus, ΔH vap Sheets Page 6 Lecture 26

7 Phase diagrams Go to: phase diagram: graphically summarizes conditions where equilibria exist between phases; shows the relationship between the phases as functions of T & P (for example) normal melting point (P = 1 atm) pressure dependence of melting point normal boiling point (P = 1 atm) pressure dependence of boiling point critical point triple point supercritical fluid coexistence curves Sheets Page 7 Lecture 26

8 Calorimetry See: & check out heating curve & phase diagram experimental measure of heat flow; used to determine ΔH rxn molar heat capacity: amount of heat required to raise 1 mole of substance 1 C (or 1 K); an intensive variable specific heat (capacity): amount of heat required to raise 1 g of substance 1 C (or 1 K); an intensive variable; for H 2 O C = J/(g C) q = C m ΔT q = quantity of heat (measure of energy) C = specific heat (heat capacity per gram) [energy/(mass C)] [see BLB Table 5.2] m = mass ΔT = T final T initial m C = heat capacity (an extensive property that is, it depends on how much stuff you have) Sheets Page 8 Lecture 26

9 Heating curve when heat is added to a system: 2 types of changes occur 1. within single phase (in blue), changes are ; q = n C p ΔT since T, kinetic energy, enthalpy (ΔH), molecular motion ; separation between molecules, molecular attractions, & molecular order 2. between phases, ( in red) abrupt changes from one physical state to another; q = n ΔH x (x = melting ( fusion), vaporization) since T is constant, kinetic energy is constant, BUT enthalpy (ΔH), molecular separation, molecular attractions & molecular order Sheets Page 9 Lecture 26

10 Example: use your units!!, very important!!! 2 moles of ice at 25 C are heated to 125 C. How much energy is needed? C p (ice) = 37.6 J/mol K C p (water) = 75.3 J/mol K C p (steam) = 33.1 J/mol K ΔH fusion = ice 25 C! 0 C: "T = q 1 = 2. ice 0 C! water 0 C: "T = q 2 = kj/mol ΔH vapor = water 0 C! 100 C: "T = q 3 = kj/mol 4. water 100 C! steam 100 C: "T = q 4 = 5. steam 100 C! 125 C: "T = q 5 = #q = Sheets Page 10 Lecture 26

11 Example: A swimmer emerging from a pool is covered with a film containing 75 g of water. (a) How much heat must be supplied to evaporate this water? (b) Why does the swimmer feel a chill? ΔH vapor = kj/mol water film = system; swimmer = surroundings Sheets Page 11 Lecture 26

12 Coffee cup calorimeter (constant pressure calorimetry) [ heat lost by the reaction (q rxn ) is equal in magnitude to but opposite in sign to heat gained by the solution (q soln ) (and vice versa) (more also later) weʼll hit this again when we cover thermochemistry!!] we can measure temperature and calculate enthalpy (heat lost or gained) Sheets Page 12 Lecture 26

13 Before next class: Read: BLB 11.3; HW: BLB 11:4,6,31,43,45,47,50,53,56 Sup 11:5 13 Know: phase changes vapor pressure & boiling points concentrations & dilutions Answers: p. 10: q tot = 112 kj p. 11: q tot = kj Sheets Page 13 Lecture 26