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Energy The capacity to do work or to produce heat. Law of Conservation of Energy Energy can be converted from one form to another but can neither be created nor destroyed. (Euniverse is constant) The Two Types of Energy Potential: due to position or composition can be converted to work Kinetic: due to motion of the object KE = 1/2 mv2 (m = mass, v = velocity) Temperature v. Heat Temperature reflects random motions of particles, therefore related to kinetic energy of the system. Heat involves a transfer of energy between 2 objects due to a temperature difference State Function Depends only on the present state of the system - not how it arrived there. It is independent of pathway. System and Surroundings System: That on which we focus attention Surroundings: Everything else in the universe Universe = System + Surroundings Exo and Endothermic Heat exchange accompanies chemical reactions. Exothermic: Heat flows out of the system (to the surroundings). Endothermic: Heat flows into the system (from the surroundings). First Law First Law of Thermodynamics: The energy of the universe is constant. First Law E = q + w E = change in system’s internal energy q = heat w = work Work work = force distance since pressure = force / area, work = pressure volume wsystem = PV 06_73 P= P = F A F A Area = A A h (a) Initial state h (b) Final state V Enthalpy Enthalpy = H = E + PV E = H PV H = E + PV At constant pressure, qP = E + PV, where qP = H at constant pressure H = energy flow as heat (at constant pressure) Heat Capacity heat absorbed J J C = = or increase in temperature C K Some Heat Exchange Terms specific heat capacity heat capacity per gram = J/°C g or J/K g molar heat capacity heat capacity per mole = J/°C mol or J/K mol 06_74 Thermometer Styrofoam cover Styrofoam cups Stirrer 06_75 Stirrer Ignition wires Thermometer Insulating container Steel bomb Water Reactants in sample cup Hess’s Law Reactants Products The change in enthalpy is the same whether the reaction takes place in one step or a series of steps. Calculations via Hess’s Law 1. If a reaction is reversed, H is also reversed. N2(g) + O2(g) 2NO(g) H = 180 kJ 2NO(g) N2(g) + O2(g) H = 180 kJ 2. If the coefficients of a reaction are multiplied by an integer, H is multiplied by that same integer. 6NO(g) 3N2(g) + 3O2(g) H = 540 kJ Standard Enthalpies of Formation Change in enthalpy that accompanies the formation of one mole of substance from its elements, with all substances in their standard states. Hfo Standard States Compound - For a gas, pressure is exactly 1 atmosphere. - For a solution, concentration is exactly 1 molar. - Pure substance (liquid or solid), it is the pure liquid or solid. Element - The form [N2(g), K(s)] in which it exists at 1 atm and 25°C. Change in Enthalpy Can be calculated from enthalpies of formation of reactants and products. Hrxn° = npHf(products) nrHf(reactants) 06_77 C(s) CH4(g) (a) (c) CO2(g) 2H2(g) (d) 2O2(g) (b) Reactants 2O2(g) Elements Products 2H2O(l) 06_1551 Reactants Elements 0 kJ C(s) 2O2(g) 2O2(g) 75 kJ 2H2(g) - 394 kJ - 572 kJ Energy CH4(g) = Reactants = Elements = Products CO2(g) 2H2O(l) Products 06_78 2N2(g) (a) (c) 4NO2(g) 4NH3(g) 6H2(g) (d) (b) 7O2(g) Reactants 7O2(g) Elements Products 6H2O(l) 06_1552 Reactants Elements 0kJ 2N2(g) 7O2(g) 6H2(g) 7O2(g) 4 x 34 kJ 6 x (-286 kJ) 4 x (-64 kJ) Energy 4NH3(g) = Reactants = Elements = Products 4NO2(g) 6H2O(l) Products Infrared radiated by the earth 06_80 Visible light from the sun CO2 and H2O molecules Earth Earth’s atmosphere