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Watkins Chapter 5 Thermochemistry Chapter 5 (sections 1 thru 4 only) Force, Energy, Heat, Work Chemistry 1421 1 Brown & Lemay Watkins Chapter 5 The Nature of Force Mechanical Force (f ) is a push or pull applied to a material object • A material object has mass (m, kg) • Mechanical force changes the velocity (m/s) of the object: acceleration ( a, m/s2 or ms-2) • Newton's 2nd law of motion: f = ma (kgm/s2 or kgms-2 = Newton) • +f is a push, -f is a pull) Chemistry 1421 2 Brown & Lemay Watkins Chapter 5 The Nature of Force Gravitational Force ( f ) is a pull between two masses, m and M • If the masses are separated by distance d ... f = -GmM /d 2 • If M is earth mass and d is distance to earth center: f = -m(GM /d 2) = -mg • g = 9.80 m/s2 at mean sea-level (“gravitational acceleration”) • "weight" = -f = mg Chemistry 1421 3 Brown & Lemay Watkins Chapter 5 The Nature of Force Coulombic Force ( f ) is a push or pull between two charged particles (e.g., ions) • If the ions, separated by distance d, have charges ±z1 and ±z2 ... f = k z1 z2 / d 2 • Two kinds of Coulombic force: Repulsion pushes if (+z1, +z2) or (-z1, -z2) ( f > 0) Attraction pulls if (-z1, +z2) or (+z1, -z2) ( f < 0) Chemistry 1421 4 Brown & Lemay Watkins Chapter 5 The Nature of Force Some of the different kinds of force: • • • • • • • • • • • Chemistry 1421 Mechanical Gravitational Coulombic Torque Nuclear weak/strong Magnetic Electromagnetic Frictional/Drag Centrifugal/Centripetal Coriolis Static 5 Force f is a push or pull on a mass, and is usually associated with motion of that mass. Brown & Lemay Watkins Chapter 5 The Nature of Energy Force f moves an object a distance d • w = f d (Nm = kgm2/s2 = kgm2 s-2 = Joule) Work w is also called Mechanical Energy When the object moves at velocity v • The energy of motion is called Kinetic Energy Ek = ½ m v2 (kgm2/s2 = Joule) Chemistry 1421 6 Brown & Lemay Watkins Chapter 5 The Nature of Energy Stored Energy is called Potential Energy; for example, the energy of gravitational attraction between an object and the center of the earth. • Example: a ball on top of a wall. Gravitational PE = m g h • Potential energy is converted into kinetic energy when the ball falls. Chemistry 1421 7 Brown & Lemay Watkins Chapter 5 The Nature of Energy Potential energy can also be stored • in a battery or a capacitor (electrical potential energy); Each of these forms of energy • in a flywheel can be converted (kinetic potential energy); to other forms by • in the bonds between atoms various processes (chemical potential energy); and/or machines. • in an atomic nucleus (nuclear potential energy). Chemistry 1421 8 Brown & Lemay Watkins The Nature of Energy Chapter 5 Some of the different forms of energy: • • • • • • • • • • • Chemistry 1421 Mechanical Energy (E = fd, E = PDV ) Gravitational Energy (E = -GmM/d) Kinetic Energy (E = 1/2mv2) Coulombic Energy (E = kz1z2/d) Radiant (light) Energy (E = hn) (chapter 6) Nuclear Energy (E = mc2) Electrical Energy (E = VQ = VIt )(chapter 20) Heat flow (q) Chemical Heat (DHrxn) Chemical Potential Energy (DGrxn) (chapter 19) Waste Energy (TDS) (chapter 19) 9 Brown & Lemay Watkins Chapter 5 The Nature of Energy Different units of energy: • Joule, the SI Unit of energy: f × d 1 J = 1 Nm = 1 kg m2 s-2 • Calorie (used in the past for heat flow) 1 cal = 4.184 J (exactly) • Nutritional Calorie 1 Cal = 1000 cal = 1 kcal = 4.184 kJ • Electron Volt (used in atomic physics) 1 ev = 1.602×10-19 J Chemistry 1421 10 Brown & Lemay Watkins Chapter 5 The Nature of Energy We perceive the form of energy called "heat" as subjectively different from other forms of energy. All other forms of energy are distinquished from heat, and they are collectively called "work". Chemistry 1421 11 Brown & Lemay Watkins Chapter 5 Thermodynamics Literally: the study of heat movement Generally: the study of the movement and conversion of all forms of energy The First Law of Thermodynamics Energy cannot be created or destroyed; it can be moved from one place to another, and it can be converted from one form to another. Thermodynamics is the set of rules that govern • the movement of energy from one place to another; • the conversion of energy from one form to another. Chemistry 1421 12 Brown & Lemay Watkins Chapter 5 Thermodynamics DEFINITIONS System (sys) that part of the universe in which we are interested. Surroundings (surr) the rest of the universe. Chemistry 1421 13 Brown & Lemay Watkins Chapter 5 Thermodynamics DEFINITIONS Internal Energy Total energy of system Esys We cannot measure Esys We can measure the change in Esys during a process: DEsys Chemistry 1421 14 Brown & Lemay Watkins Chapter 5 Thermodynamics Initial State H2(g) & O2(g) DEFINITIONS Internal Energy Total energy of system Esys We cannot measure Esys We can measure the change in Esys during a process: DEsys For example, during a chemical reaction DEsys = Efinal - Einitial Esys H2(g) + ½ O2(g) → H2O(g) Chemistry 1421 DEsys < 0 H2O(g) Final State 15 Brown & Lemay Watkins Chapter 5 Thermodynamics Final State H2(g) & O2(g) DEFINITIONS Internal Energy Total energy of system Esys We cannot measure Esys We can measure the change in Esys during a process: DEsys For example, during a chemical reaction DEsys = Efinal - Einitial Esys H2O(g) → H2(g) + ½ O2(g) Chemistry 1421 DEsys > 0 H2O(g) Initial State 16 Brown & Lemay Watkins Chapter 5 First Law of Thermodynamics Relating DE to Heat and Work When a system undergoes a physical or chemical change, the change in its internal energy is equal to the sum of q, the heat flow into or out of the system w, the work done on or by the system: DEsys = q + w Work w is all forms of energy other than q. Both q and w are signed. Chemistry 1421 17 Brown & Lemay Watkins Chapter 5 First Law of Thermodynamics Example I "Endothermic" q>0 DEsys = q + w Heat Surroundings w>0 DEsys > 0 System Work work is done on the system by the surroundings Process I occurs in the system Chemistry 1421 18 Brown & Lemay Watkins Chapter 5 First Law of Thermodynamics Example II "Exothermic" q<0 DEsys = q + w Heat Surroundings w<0 DEsys < 0 System Work work is done by the system on the surroundings. Process II occurs in the system Chemistry 1421 19 Brown & Lemay Watkins Chapter 5 First Law of Thermodynamics Example III "Exothermic" q<0 DEsys = q + w Heat Surroundings w>0 DEsys ? System Work work is done on the system by the surroundings. Process III occurs in the system Chemistry 1421 20 Brown & Lemay Watkins Chapter 5 First Law of Thermodynamics Example IV “Endothermic" q>0 DEsys = q + w Heat Surroundings w<0 DEsys ? System Work work is done by the system on the surroundings. Process IV occurs in the system Chemistry 1421 21 Brown & Lemay Watkins DE = q + w Energy E Chapter 5 One form of work is expansion or “PV” work: w = -PDV, Pressure (atm) × DVolume (L) 1 L.atm = 101.325 J Often, PV work is the only form of work in a chemical process, and if DV = 0, then wV = -PDV = 0 and subscript means “this variable is held constant during the process”. DE = qv Heat flow qV in a constant volume process is a direct measure of DE. DV = 0 for a rigid container. Chemistry 1421 22 Brown & Lemay Watkins DE = qV Energy E Chapter 5 However, most chemical reactions are not carried out at constant volume. They are usually carried out at constant pressure (in the open atmosphere). In some reactions, both heat and work are involved (e.g., explosions). So a new energy function was invented for contant pressure processes. Chemistry 1421 23 Brown & Lemay Watkins DE = qV Energy E Chapter 5 For a contant pressure process (DP = 0), if the system expands (DV > 0) it does work on the surroundings: wp = -PDV DE = qp + wp = qP - PDV qP = DE + PDV = DH Energy H is called Enthalpy Chemistry 1421 24 Brown & Lemay Watkins DE = qV DH = qP Chapter 5 Enthalpy H At contant pressure, H is the “total heat energy in the system”, also called the “internal heat” or “heat content” When qp Joules of heat flow into or out of the system during the process (reaction) the heat content changes by an amount qp: DH = Hfinal - Hinitial = qP We cannot measure either H, but we can measure DH = qp, the flow of heat between sys. and surr. during a constant pressure process. Chemistry 1421 25 Brown & Lemay Watkins DE = qV DH = qP Chapter 5 Enthalpy of Reaction Consider an open beaker in which a reaction takes place at constant atmospheric pressure: Reactants → Products aA + bB → cC + dD HReact HProd qp = HProd - HReact Chemistry 1421 26 Brown & Lemay Watkins Enthalpy of Reaction There are two kinds of chemical reaction H ("heat content") DE = qV DH = qP Chapter 5 reactants H products H qP = DH < 0 heat out exo qP = DH > 0 heat in endo Consider an open beaker in which a reaction takes place at constant atmospheric pressure: Reactants → Products HReact or products H HProd qp = HProd - HReact reactants H positive DH = endothermic; negative DH = exothermic Chemistry 1421 27 Brown & Lemay Watkins DE = qV DH = qP Chapter 5 Enthalpy of Reaction For a chemical reaction, qp is called the reaction heat: qP = qrxn = DHrxn Reaction heat is an extensive property - the magnitude of DH is directly proportional to amounts of reactants and products: 1CH4(g) + 2O2(g) 1CO2(g) + 2H2O(g) DH1 = -803 kJ 2CH4(g) + 4O2(g) 2CO2(g) + 4H2O(g) DH2 = -1606 kJ Multiply a reaction, multiply the heat. Chemistry 1421 28 Brown & Lemay Watkins DE = qV DH = qP Chapter 5 Enthalpy of Reaction When a reaction is reversed, the sign of DH changes: CH4(g) + 2O2(g) CO2(g) + 2H2O(g) DH = -803 kJ CO2(g) + 2H2O(g) CH4(g) + 2O2(g) DH = +803 kJ Chemistry 1421 29 Brown & Lemay Watkins DE = qV DH = qP Chapter 5 Enthalpy of Reaction Enthalpy depends on state: at 100o C H2O(l) H2O(g) Hl Hg DH = Hg - Hl DHvap = +88 kJ DH = Hg – Hl > 0 H[steam] > H[water] You have to add heat to water to produce steam! H[vapor] > H[liquid] The process called “vaporization” is endothermic for all substances at all temperatures. Chemistry 1421 30 Brown & Lemay Watkins DE = qV DH = qP Chapter 5 Enthalpy of Reaction Four things to remember 1. DHrxn = qp = Hprod -Hreact 2. When you multiply a reaction, multiply DH by the same factor. 3. When you reverse a reaction, DH changes sign. 4. DH depends on state; in general, for a specific substance: H(s) < H(l) < H(g) Chemistry 1421 31 Brown & Lemay Watkins DE = qV DH = qP Chapter 5 Enthalpy of Reaction You can measure reaction heat (calorimetry) Chapter 5, Chem 1422 You can calculate reaction heat in two ways: • Chemical bond enthalpies (Chapter 8) • Hess's Law (Chapter 5, Chem 1422) Chemistry 1421 32 Brown & Lemay Watkins Chapter 5 Chapter 5, Part 1 Problems • Energy Calculations • ½mv2, mgh, PDV, fd • First Law; DE = q + w; DE = qV • DHrxn, DH = qP • Multiply reaction heats • Reverse reaction heats • States: H(s) < H(l) < H(g) Chemistry 1421 33 Brown & Lemay