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Thermodynamics: the First Law Atkins, Chapter 2

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Thermodynamics: the First Law Atkins, Chapter 2 Thermodynamics: The First Law Atkins, Chapter 2 • Open System: Mass, heat, energy flow freely • Closed System: Heat, energy flow freely • Isolated System: No mass, heat, or energy flow System System System HEAT HEAT q < 0 q > 0 Exothermic EndothermicNils Walter: Chem 260 Internal Energy U Internal Energy U: The sum of all of the kinetic and potential energy contributions to the energy of all the atoms, ions, molecules, etc. in the system He gas Methanol Gas Translational Energy Rotational Energy Electronic Energy Vibrational Energy Nuclear Energy Bond Energy Nils Walter: Chem 260 The First Law of Thermodynamics: Internal Energy is Conserved •• TheThe changechange inin internalinternal energyenergy ((∆∆UU)) ofof aa closedclosed systemsystem isis equalequal to to the the sum sum of of the the heat heat ( (q)q) added added to to it it and and the the work work ( (w)w) donedone uponupon itit •• TheThe internalinternal energyenergy ofof anan isolatedisolated systemsystem isis constantconstant ∆U = q + w For a Closed System ∆U = 0 For an Isolated System Internal energy U is a state function Þ Quantity is independent of path Volume, Temperature, Pressure, and Quantity are other examples of state functionsNils Walter: Chem 260 Internal Energy can be exchanged with the surroundings as heat or work Closed system, ∆U = q + w expansion against external pressure Closed system, constant volume Heat is stored as internal energy and released as q↑ heat introduced volume-pressure work [J] T↑ ∆U = q + w w = -Fdx = -p∆V ∆ = q - pex V w = 0; no work done Þ ∆ U = qv Nils Walter: Chem 260 Internal Energy and Enthalpy Enthalpy definition: Η = U + pV Most convenient for processes at constant pressure: •Cooking dinner • Digesting dinner •Drying the laundry •Synthesizing a compound in lab At constant pressure, if only pV work is done: V ∆ ò 2 U = q + w = qp - p dV V1 V ò 2 ∆ = qp - p dV = qp - p(V2-V1) = qp - p V V p independent of V 1 Enthalpy is the heat transferred ∆Η = ∆U + p∆V = q in a process at constant pressure p (assuming only pV work) Nils Walter: Chem 260 Enthalpy • Open System: Mass, heat, energy flow freely • Closed System: Heat, energy flow freely • Isolated System: No mass, heat, or energy flow System System System HEAT HEAT q < 0 q > 0 ∆ Nils ∆Walter: Chem 260 Exothermic H < 0 Endothermic H > 0 Enthalpy and Internal Energy are State Functions We only need be concerned with the change in enthalpy (∆H) or change in internal energy (∆U), not the path of how we got there Þ We can arbitrarily assign H = 0 for each element in its standard state = state of aggregation at p = 1 bar, T = 298.15 K s e Standard Formation Reaction: Formation of i p l one mole of a substance from the elements in a h t their standard states. n e CS +87.86 kJ/mol → n 2 (l) C + 2 S CS r (graphite) (s, rhombic) 2(l) o a i l t o a C C m (graphite) (graphite) m 0 r d o O , S r 2 (g) (s, rhombic) f a f d o n CO -110.52 kJ/mol → a (g) C(graphite) + 1/2 O2 (g) CO(g) t s = ∅ H f CO -393.51 kJ/mol → 2 (g) NilsC(graphite) Walter: + O Chem2 (g) 260 CO2(g) ∆ ∆ ∆ ∆.
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