Slide 1 / 43 New Jersey Center for Teaching and Learning Progressive Science Initiative This material is made freely available at www.njctl.org and is intended for the non-commercial use of students and teachers. These materials may not be used for any commercial purpose without the written permission of the owners. NJCTL maintains its website for the convenience of teachers who wish to make their work available to other teachers, participate in a virtual professional learning community, and/or provide access to course materials to parents, students and others. Click to go to website: www.njctl.org Slide 2 / 43 AP Chemistry Unit 5 States of Matter Part A: Phase Changes www.njctl.org Slide 3 / 43 States of Matter Many scientists now believe that the temperature becomes warm enough in Mars equatorial regions to melt subsurface ice and cause small gullies to form in the soil.
Slide 4 / 43 Phase Changes Phase changes involve a change in the potential energy of a material brought about by either the strengthening or weakening of the interactions between representative particles. The table below lists the six phase changes of water and the potential energy change of each. States Change in Change in Endo or Phase Change Involved PE T surr Exothermic Melting s --> l +6.0 kJ/mol - Endo Evaporation l --> g +41 kJ/mol - Endo Condensation g --> l -41 kJ/mol + Exo Freezing l --> s -6.0 kJ/mol + Exo Sublimation s --> g +47 kJ/mol - Endo Deposition g --> s -47 kJ/mol + Exo Slide 5 / 43 Phase Changes Endothermic phase changes occur when the molecules have enough kinetic energy to overcome the coulombic attractions between the particles. Examining the melting of water (ice) liquid Potential Energy solid Since the kinetic energy of the molecules was converted to potential energy, the T surr will decrease. When a substance melts, the bonds are only stretched thus requiring less energy than boiling where the bonds are broken. Slide 6 / 43 Phase Changes Exothermic phase changes occur when the molecules have lost enough kinetic energy to permit coulombic attractions to form between the particles. Examining the condensing of water gas Potential Energy liquid Since the potential energy of the molecules was converted to kinetic energy, the T surr will increase.
Slide 7 / 43 Phase Changes At a certain temperature and pressure, two phase changes will occur at equal rates resulting in equilibrium. Standard Melting Point of H 2 O(0 C at 1 atm) H 2 O(s) <--> H 2 O(l) = equilibrium Temperature above 0 C at 1 atm H 2 O(s) --> H 2 O(l) = not at equilibrium Temperature below 0 C at 1 atm H 2 O(s) <-- H 2 O(l) = not at equilibrium Slide 8 / 43 Phase Changes The standard (1 atm) melting and boiling point temperatures are dependent on the types of interactions found within the substance. Particle Influenced Standard Substance Example Interactions by MP Ionic (intra) charge and Ionic MgO 2852 C ionic radii Covalent Covalent C(diamond) atomic radii 3550 C Network (intra) Covalent Metallic Au ionic radii 1064 C (intra) LDF's polarizability Molecular I 2 114 C (inter) (# of e-) Note the big difference in melting point between substances with intra vs. inter molecular interactions. Slide 9 / 43 1 Which of the following phase changes would be exothermic? A Na(s) --> Na(l) B Na(l) --> Na(g) C Na(s) --> Na(g) D Na(g) --> Na(l)
Slide 9 (Answer) / 43 1 Which of the following phase changes would be exothermic? A Na(s) --> Na(l) B Na(l) --> Na(g) Answer C Na(s) --> Na(g) D D Na(g) --> Na(l) [This object is a pull tab] Slide 10 / 43 2 Which of the following would be true when ethyl alcohol evaporates? I. The potential energy of the A I only substance increases. II. The kinetic energy of the B II only surroundings increases. C III only III. Coulombic attractions are broken. D I and III only E I, II, and III Slide 10 (Answer) / 43 2 Which of the following would be true when ethyl alcohol evaporates? I. The potential energy of the A I only substance increases. II. The kinetic energy of the B II only surroundings increases. C III only III. Coulombic attractions are Answer broken. D D I and III only E I, II, and III [This object is a pull tab]
Slide 11 / 43 3 Which of the following changes would be endothermic? A sublimation B deposition C condensation D freezing E All of these Slide 11 (Answer) / 43 3 Which of the following changes would be endothermic? A sublimation B deposition C condensation Answer A D freezing E All of these [This object is a pull tab] Slide 12 / 43 4 For which of the following are covalent bonds breaking? A CO 2 (s) --> CO 2 (l) B CO 2 (l) --> CO 2 (g) C C(s) --> C(g) D C(s) --> C(l) E NaCl(l) --> NaCl(g)
Slide 12 (Answer) / 43 4 For which of the following are covalent bonds breaking? A CO 2 (s) --> CO 2 (l) B CO 2 (l) --> CO 2 (g) C C(s) --> C(g) Answer C D C(s) --> C(l) E NaCl(l) --> NaCl(g) [This object is a pull tab] Slide 13 / 43 5 Which of the following BEST characterize the standard boiling point of a material? A Liquid and vapor phases are in equilibrium at any pressure B Liquid and vapor phases are in equilibrium at1 atm pressure C The rate of vapor formation exceeds that of liquid formation at any pressure D The rate of liquid formation exceeds that of vapor formation at 1 atm E The potential energy of the material is decreasing Slide 13 (Answer) / 43 5 Which of the following BEST characterize the standard boiling point of a material? A Liquid and vapor phases are in equilibrium at any pressure B Liquid and vapor phases are in equilibrium at1 atm pressure Answer B C The rate of vapor formation exceeds that of liquid formation at any pressure D The rate of liquid formation exceeds that of vapor [This object is a pull tab] formation at 1 atm E The potential energy of the material is decreasing
Slide 14 / 43 6 Which of the following correctly ranks the materials from lowest to highest melting point? A Na < Li < K B CaO < MgO < NaF C CH 4 < NH 3 < H 2 O D I 2 < Br 2 < F 2 E Na < H 2 O < I 2 Slide 14 (Answer) / 43 6 Which of the following correctly ranks the materials from lowest to highest melting point? A Na < Li < K B CaO < MgO < NaF Answer C CH 4 < NH 3 < H 2 O C D I 2 < Br 2 < F 2 E Na < H 2 O < I 2 [This object is a pull tab] Slide 15 / 43 Phase Changes The heat of fusion is the amount of energy that is required to melt a given quantity of a material. The same amount of energy is released when that substance freezes. The heat of fusion depends on the strength of the particle interactions within the solid. Substance Particle Substance Heat of Fusion Type Interactions Ionic Bonds NaCl Ionic 28 kJ/mol (Intra) Ionic Bonds NaF Ionic 32 kJ/mol (Intra) H-Bonds, H 2 O Molecular 6 kJ/mol LDF's (Inter) CH 4 Molecular LDF's (Inter) 1 kJ/mol Covalent Ag Metallic 11.3 kJ/mol Bonds (Intra)
Slide 16 / 43 Phase Changes The heat of vaporization is the amount of energy that is required to vaporize a given quantity of a material. The same amount of energy is released when that substance condenses. The heat of vaporization depends on the strength of the particle interactions within the solid. Substance Particle Substance Heat of Fusion Type Interactions LDF's, H- CH 3 OH Molecular 35 kJ/mol Bonds CH 3 CH 2 CH 3 Molecular LDF's 16 kJ/mol H-Bonds, H 2 O Molecular 41 kJ/mol LDF's CH 3 COCH 3 Molecular LDF's, DDF's 31 kJ/mol I 2 Molecular LDF's 42 kJ/mol Slide 17 / 43 Phase Changes Calculating the heats of fusion or vaporization requires measuring the energy absorbed or released as the material undergoes a phase change. Example: What is the heat of fusion for water (kJ/mole) assuming that the temperature of a 100 gram sample of water (Cp = 4.2 J/gC) decreased by 14.2 degrees when 18.0 grams of ice is melted in a container surrounded by the water. 1. Find the heat absorbed from water: Heat absorbed (q) = m* T*cp = 100 g x 14.2 C x 4.2 J = 5,960 J gC 2. Find kJ/mol: 18 g H 2 O x 1 mol = 1 mole H 2 O 18 g 5.96 kJ/ 1mole = 5.96 kJ/mol Slide 18 / 43 7 Which of the following would have the highest heat of fusion? A Na B Li C K D Mg E Be
Slide 18 (Answer) / 43 7 Which of the following would have the highest heat of fusion? A Na B Li Answer C K E D Mg E Be [This object is a pull tab] Slide 19 / 43 8 Which of the following would have the lowest heat of vaporization? A C 4 H 10 B CH 3 CH 2 CH 2 CHO C I 2 D H 2 O E C 2 H 4 Slide 19 (Answer) / 43 8 Which of the following would have the lowest heat of vaporization? A C 4 H 10 B CH 3 CH 2 CH 2 CHO Answer E C I 2 D H 2 O E C 2 H 4 [This object is a pull tab]
Slide 20 / 43 9 What is the heat of vaporization (J/mol) of ethanol assuming that 419 Joules of energy are needed to vaporize 0.5 grams of the material? Slide 20 (Answer) / 43 9 What is the heat of vaporization (J/mol) of ethanol assuming that 419 Joules of energy are needed to vaporize 0.5 grams of the material? Answer 38,090 J/mol [This object is a pull tab] Slide 21 / 43 10 How many grams of water must have been present if 3210 J of energy were needed to vaporize the sample? (The heat of vaporization of water is 41 kJ/mol)
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