MIT OpenCourseWare http://ocw.mit.edu 8.21 The Physics of Energy Fall 2009 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms. Introduction & apologies Case study I: Ethanol A tour of internal energy II: Cement Asides: Enthalpies of formation &reaction III: The hydrogen economy 8.21 Lecture 7 Chemical (and Biological) Energy September 23, 2009 8.21 Lecture 7: Chemical (and Biological) Energy 1 Introduction & apologies Case study I: Ethanol A tour of internal energy II: Cement Asides: Enthalpies of formation &reaction III: The hydrogen economy Chemical and Biological Energy How can we analyze statements like “Burning ethanol produces less CO than burning gasoline.” • 2 “Why does cement manufacturing produce so much more CO • 2 per Joule of energy consumption than other manufacturing?” “Hydrogen is the fuel of the future!” • 8.21 Lecture 7: Chemical (and Biological) Energy 2 Introduction & apologies Case study I: Ethanol A tour of internal energy II: Cement Asides: Enthalpies of formation &reaction III: The hydrogen economy Chemical (and Biological) Energy A course unto itself SOURCES TRANS FORMATION AND STORAGE USES Fossil fuels Batteries Chemical Electrical Heating & Cooling • • ↔ • Biofuels Fuel Cells Chemical Electrical Transportation • • ↔ • Bio Engines Mechanical Manufacturing Wood, waste, etc. • Chemical ↔ • ! • Working fluids Mechanical Heat ... • ↔ • Photosynthesis Radiation Biochemical • ↔ Emphasize physics issues, especially as they • impact the rest of the course Almost all chemistry (apologies!) CO2! • Review basic thermochemistry! • 8.21 Lecture 7: Chemical (and Biological) Energy 3 Introduction & apologies Case study I: Ethanol A tour of internal energy II: Cement Asides: Enthalpies of formation &reaction III: The hydrogen economy Outline A tour of the internal energy of matter Thermodynamic asides • • Change of internal energy with temperature — Internal energy — a state variable • • heat capacities Enthalpy — separating work from internal • Solid liquid gas — changes of phase or energy • latent⇔ heat ⇔ Molecules atoms — heat of formation • ⇔ Atoms electrons and nuclei — atomic • binding⇔ Nuclei protons and neutrons — nuclear • binding⇔ Matter Rest mass energy • ⇔ Case Studies Thermodynamic aside • • Enthalpies of reaction and combustion. Energy content of ethanol versus gasoline or • • natural gas Making cement • Hydrogen as a fuel or an energy storage system • 8.21 Lecture 7: Chemical (and Biological) Energy 4 Introduction & apologies Thermo Case study I: Ethanol A tour of internal energy II: Cement Asides: Enthalpies of formation &reaction III: The hydrogen economy Work and heat not state variables A little thermodynamics • Add work and extract heat and leave system in exactly the same state it began Looking back to Unit 4 Internal energy — U — is a state variable And ahead to future work • dU = dQ dW = dQ pdV − − System — very large numbers of molecules • ( N 1024) HEAT ADDED MINUS WORK DONE BY ∼ 0 ∼ Settle down. Achieve equilibrium state. U = U(T,ρ) or U = U(T, p). • STATE VARIABLE: intrinsic characteristic Internal energy can be viewed the sum of • • of system in equilibrium thermal energy plus all the energy stored in the binding energy of molecules, atoms, State variables: Pressure p • and nuclei. Density ρ = N/V Temperature T Internal energy = For single phase systems (like a gas or a (1) Energy necessary to assemble • liquid) two of p, ρ,T specify the state of the material at T =0: energy in nuclear, system. atomic, and chemical bonds, N + Equation of state like P = V kBT = ρkBT determines the other. (2) Thermal energy necessary to raise material to temperature T . 8.21 Lecture 7: Chemical (and Biological) Energy 5 Introduction & apologies Case study I: Ethanol A tour of internal energy II: Cement Asides: Enthalpies of formation &reaction III: The hydrogen economy I. Internal energy Energy is conserved (First Law) Ice block images removed • due to copyright restrictions. What is the energy content of material? • It depends on the context • Consider a block of ICE and add energy THERMAL ENERGY I Motions of molecules WARM THERMAL ENERGY II Inter molecular binding energy MELT/VAPORIZE INTERATOMIC BINDING ENERGY Binds atoms in H2O ELECTROLYZE ATOMIC BINDING ENERGY Binds electrons to H and O nuclei IONIZE NUCLEAR BINDING ENERGY Binds protons and neutrons DISASSEMBLE into nuclei INTO p AND n RESTMASS ENERGY The mass itself! ?? WARM 38.1 J/mol-K MELT/VAPORIZE 6.01/40.7 kJ/mol ELECTROLYZE 241 kJ/mol IONIZE 199.8 MJ/mol DISSASSEMBLE 3 TJ/mol NUCLEI REST MASS 1.6 PJ/mol 8.21 Lecture 7: Chemical (and Biological) Energy 6 Introduction & apologies Case study I: Ethanol A tour of internal energy II: Cement Asides: Enthalpies of formation &reaction III: The hydrogen economy How to relate these energies to the atomic world and the world of electromagnetic radiation? Energy per mole Energy per molecule (or atom, or nucleus) ⇒ Conversion factor: 1 MJ/mole = 10.4 eV/molecule Shorter wavelength ! Or: 1 eV/molecule = 96.5 KJ/mole Higher energy Relate to type of electromagnetic radiation... λ = c/ν = 2π!c/E ν = E/(2π!) 1 MJ/mole 10.4 eV/atom λ = 119 nm ⇔ ⇔ 1 MJ/mole 2.5 1015 Hz ⇔ × ANNIHILATE 1.6 PJ/mol 4 YHz FUSE 3 TJ/mol 387.5 ZHz IONIZE 199.8 MJ/mol 500 PHz ELECTROLYZE 241 kJ/mol .6 PHz 470 nm ⇔ MELT/VAPORIZE 6.01/40.7 kJ/mol 15 – 90 THz WARM 38.1 J/mol-K 95 GHz 8.21 Lecture 7: Chemical (and Biological) Energy 7 Introduction & apologies Case study I: Ethanol A tour of internal energy II: Cement Asides: Enthalpies of formation &reaction III: The hydrogen economy THERMAL ENERGY I Motions of molecules WARM THERMAL ENERGY II Inter molecular binding energy MELT/VAPORIZE INTERATOMIC BINDING ENERGY Binds atoms in H2O ELECTROLYZE ATOMIC BINDING ENERGY Binds electrons to H and O nuclei IONIZE NUCLEAR BINDING ENERGY Binds protons and neutrons DISASSEMBLE into nuclei INTO p AND n RESTMASS ENERGY The mass itself! ?? 8.21 Lecture 7: Chemical (and Biological) Energy 8 Introduction & apologies Case study I: Ethanol A tour of internal energy II: Cement Asides: Enthalpies of formation &reaction III: The hydrogen economy THERMAL ENERGY — MOTION OF MOLECULES Add energy to the system. • Atoms vibration graph removed Kinetic: Gas molecules graph removed due to copyright restrictions. • ⇒ due to copyright restrictions. – Liquid/gas: translational and rotational – Solid: vibration Potential • ⇒ Vibration of atoms in Kinetic energy – Solid: stretching of bonds a crystal lattice of a gas – Liquid/gas: increasing average separation Proportionality between change in internal energy and change in temperature defines Heat capacity (as discussed in Lecture 4) 8.21 Lecture 7: Chemical (and Biological) Energy 9 Introduction & apologies Thermo Case study I: Ethanol A tour of internal energy II: Cement Asides: Enthalpies of formation &reaction III: The hydrogen economy A Little More Thermodynamics As described in Unit 4 (on Heat) Remember difference between constant volume and constant pressure. Add heat at constant volume dQ = dU Defines heat capacity at constant volume ∂Q ∂U = C ∂T ∂T ≡ V !V !V ! ! ! ! ! ! The distinction matters Add heat at constant pressure dQ = dU + pdV most for gases where So heat added at constant pressure does not all go into increasing volume changes can be the internal energy of a system. Some goes into work against the environment. significant! Remember enthalpy: H U + pV ≡ Add heat at constant pressure dQ = dH = dU + pdV Defines heat capacity at constant pressure ∂Q ∂H = C ∂T ∂T ≡ P !P !P ! ! ! ! ! 8.21 Lecture! 7: Chemical (and Biological) Energy 10 Introduction & apologies Thermo Case study I: Ethanol A tour of internal energy II: Cement Asides: Enthalpies of formation &reaction III: The hydrogen economy Some useful heat capacities Substance Specific Heat Molar Heat Capacity KJ/kg K Capacity J/mol K Substance Specific Heat Ice 2.09 37 Capacity KJ/kg K Water 4.19 75 Steel 0.51 Steam 2.01 34 Glass 0.78 Ethanol(l) 2.42 113 Granite 0.80 Copper 0.38 33 Wood 1.67 Liquid sodium 1.39 32 Soil 1.05 Air 0.73 21 Helium 3.125 12.5 Lots of stories here! 8.21 Lecture 7: Chemical (and Biological) Energy 11 Introduction & apologies Case study I: Ethanol A tour of internal energy II: Cement Asides: Enthalpies of formation &reaction III: The hydrogen economy THERMAL ENERGY I Motions of molecules WARM THERMAL ENERGY II Inter molecular binding energy MELT/VAPORIZE INTERATOMIC BINDING ENERGY Binds atoms in H2O ELECTROLYZE ATOMIC BINDING ENERGY Binds electrons to H and O nuclei IONIZE NUCLEAR BINDING ENERGY Binds protons and neutrons DISASSEMBLE into nuclei INTO p AND n RESTMASS ENERGY The mass itself! ?? 8.21 Lecture 7: Chemical (and Biological) Energy 12 Introduction & apologies Case study I: Ethanol A tour of internal energy II: Cement Asides: Enthalpies of formation &reaction III: The hydrogen economy MELT OR VAPORIZE— INTERMOLECULAR BINDING ENERGY Energy derived from change of phase — breaking the “bonds” of intermolecular forces. Latent heat of melting or vaporization Solid Liquid Gas MELT H O(solid) H O(liquid) 2 → 2 Even though measurement was made at constant 6.01 kJ/mol at 0◦ C and 1 atm pressure, the volume change is so small that the result is very nearly ∆U. Not so for liquid vapor ∆H ∆U = + 6.01 kJ/mol ↔ ≈ VAPORIZE Changing liquid to gas creates ∆H = ∆U + p∆Vgas ∆V 22 L =0.022 m3 H O(liquid) H O(gas) gas = ∆U + RT ∆ngas 2 → 2 ∼ 1 1 40.7 kJ/mol at 100 C and 1 atm ∆n RT =1mole 8.31 Jmol− K− 373 K =3.10 kJ ◦ gas × × ∆H = + 40.7 kJ/mol ∆U = 40.7 kJ/mol - 3.1 kJ/mol = 37.6 kJ/mol Latent heat