CHEMISTRY. CHM201 Class #10 CHEMISTRY. Chapter 5. Particulate Review. Thermochemistry: Energy Changes in Reactions

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1 CHEMISTRY Fifth Edition Gilbert Kirss Foster Bretz Davies CHM201 Class #10 Chemistry, 5 th Edition Copyright 2017, W. W. Norton & Company CHEMISTRY Fifth Edition Gilbert Kirss Foster Bretz Davies Chapter 5 Thermochemistry: Energy Changes in Reactions Chemistry, 5 th Edition Copyright 2017, W. W. Norton & Company Particulate Review What is the equation for the titration between hydrochloric acid and sodium hydroxide? A. HCl(aq) + KOH(aq) H 2 O(l) + KCl(aq) B. HCl(aq) + NaOH(aq) H 2 O(l) + NaCl(aq) C. HClO(aq) + NaOH(aq) H 2 O(l) + NaClO(aq) D. 2 HCl(aq) + NaOH(aq) H 2 (g) + 2 NaOCl(aq) 3 1

2 Particulate Review When performing the titration between hydrochloric acid and sodium hydroxide which solution is in the buret? A. Hydrochloric acid B. Sodium hydroxide C. Water D. Sodium chloride 4 Particulate Preview When ozone molecules absorb UV light from the sun, the ozone breaks down into oxygen molecules and oxygen atoms. Does bond breaking occur when energy is absorbed or released? A. Absorbed B. Released C. Neither 5 Chapter 5 Outline for Class # Sunlight Unwinding 5.2 Forms of Energy 5.3 Systems, Surroundings, and Energy Transfer 5.4 Enthalpy and Enthalpy Change 6 2

3 Definitions Thermodynamics the study of energy and its transformations Thermochemistry the study of the relation between chemical reactions and changes in energy Thermochemical equation: 2 H 2 (g) + O 2 (g) 2 H 2 O(l) + energy Thermal equilibrium a condition in which temperature is uniform throughout a material and no energy flows from one point to another 7 Definitions Heat the energy transferred between objects because of a difference in their temperatures Work a form of energy: the energy required to move an object through a given distance w = F d w is work. F is force. d is distance. 8 Two Types of Energy Potential energy (PE): the energy stored in an object because of its position PE = m g h m = mass g = acceleration due to gravity h = vertical distance Kinetic energy (KE): the energy due to motion of the object KE = 1 / 2 mu 2 m = mass u = velocity 9 3

4 Work and Energy 10 Potential Energy: A State Function Depends only on the difference between initial and final state of the system and not how it achieved that state 11 Total Energy Total energy = PE + KE = m g h + ½mu

5 The Nature of Energy Law of conservation of energy Energy can be neither created nor destroyed. Energy can be converted from one form to another. Potential energy kinetic energy Chemical energy heat Thermal energy kinetic energy of atoms, ions, and molecules 13 Energy at the Molecular Level Kinetic energy at the molecular level: Mass, velocity of the particle (KE = ½mu 2 ) Temperature As T increases, molecular motion and KE increase. Potential energy at the molecular level: Electrostatic interactions: E ( Q Q ) 1 2 el d 14 Electrostatic Potential Energy 15 5

6 Energy of Chemical Reactions 16 Terminology of Energy Transfer System the part of the universe that is the focus of a thermochemical study Isolated exchanges no energy or matter with surroundings Closed exchanges energy but no matter with surroundings Open exchanges both energy and matter with surroundings Surroundings everything in the universe that is not part of the system Universe = system + surroundings 17 Examples of Systems 18 6

7 Endothermic Heat Flow Exothermic process: energy flows out of system to surroundings (q < 0) Endothermic process: energy flows into system from surroundings (q > 0) q is quantity of energy transferring. 19 Phase Changes and Heat Flow Exothermic 20 Energy and Phase Changes Absorbed heat increases kinetic energy of molecules. Loss of kinetic energy is caused by release of heat by molecules. 21 7

8 Internal Energy Internal energy (E): State function E = E final E initial Sum of KE and PE of all components of the system Types of molecular motion: (a) translational (b) rotational (c) vibrational 22 Change in Internal Energy E = q + w E = change in system s internal energy q = heat, w = work Work w = P V Where P = pressure, V = change in volume Work done by the system = energy lost by the system, hence the negative sign. E = q + w = q + ( P V) = q P V 23 Units of Energy Calorie (cal) The amount of heat necessary to raise the temperature of 1 g of water by 1 o C Joule (J) The SI unit of energy J = 1 cal Energy = heat and/or work (same units!) 24 8

9 First Law of Thermodynamics First law of thermodynamics = law of conservation of energy Energy of the universe is constant! Universe = system + surroundings Energy gained or lost by a system must equal the energy lost or gained by the surroundings. E system = E surroundings 25 Energy Flow Diagram 26 Practice: Calculation of Work Calculate the work in L atm and joules associated with the expansion of a gas in a cylinder from 54 L to 72 L at a constant external pressure of 18 atm. (Note: 1 L atm = J) Collect and Organize: Expansion of gas in a cylinder: V initial = 54 L V final = 72 L P = 18 atm We need to calculate the amount of work performed. 27 9

10 Practice: Calculation of Work Calculate the work in L atm and joules associated with the expansion of a gas in a cylinder from 54 L to 72 L at a constant external pressure of 18 atm. (Note: 1 L atm = J) Analyze: w = P V 1 L atm = J The sign associated with w will be negative, since work is done by the system. 28 Practice: Calculation of Work Calculate the work in L atm and joules associated with the expansion of a gas in a cylinder from 54 L to 72 L at a constant external pressure of 18 atm. (Note: 1 L atm = J) Solve: w = -(18 atm) (72-54 L) = L atm w = - (324 L atm) (101.32J L atm) = J = -33 kj 29 Practice: Calculation of Work Calculate the work in L atm and joules associated with the expansion of a gas in a cylinder from 54 L to 72 L at a constant external pressure of 18 atm. (Note: 1 L atm = J) Think About It: The negative sign indicates work done by the system energy lost from the system to the surroundings

11 Endothermic Enthalpy, Change in Enthalpy Enthalpy (H): The sum of the internal energy and the pressure volume product of a system H = E + PV Enthalpy change ( H): The energy absorbed by reactants (endothermic) or energy given off by products (exothermic) for a reaction carried out at constant pressure: H = E + P V 31 Enthalpy, Change in Enthalpy Enthalpy change ( H) H = E + P V H = q P = E + P V H > 0, endothermic; H < 0, exothermic Subscripts indicate H for specific processes. 32 Enthalpy Change Exothermic 33 11