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Transcript
C H E M I S T R Y Chapter 8 Thermochemistry: Chemical Energy Energy and Its Conservation Conservation of Energy Law: Energy cannot be created or destroyed; it can only be converted from one form to another. Energy: The capacity to supply heat or do work. Kinetic Energy (EK): The energy of motion. Potential Energy (EP): Stored energy. Units: 1 cal = 4.184 J (exactly) 1 Cal = 1000 cal = 1 kcal Energy and Its Conservation Energy and Its Conservation Thermal Energy: The kinetic energy of molecular motion and is measured by finding the temperature of an object Heat: The amount of thermal energy transferred from one object to another as the result of a temperature difference between the two Internal Energy and State Functions First Law of Thermodynamics: The total internal energy E of an isolated system is constant DE = Efinal - Einitial Internal Energy and State Functions CH4(g) + 2O2(g) CO2(g) + 2H2O(g) + 802 kJ energy DE = Efinal - Einitial = -802 kJ 802 kJ is released when 1 mole of methane, CH4, reacts with 2 moles of oxygen to produce 1 mole of carbon dioxide and two moles of water. Internal Energy and State Functions State Function: A function or property whose value depends only on the present state, or condition, of the system, not on the path used to arrive at that state Expansion Work C3H8(g) + 5O2(g) 6 mol of gas w=Fxd 3CO2(g) + 4H2O(g) 7 mol of gas Expansion Work Expansion Work: Work done as the result of a volume change in the system 8.5 Energy and Enthalpy DE = q + w q = heat transferred w = work = -PDV q = D E + PD V Constant Volume (DV = 0): Constant Pressure: qV = DE qP = DE + PDV Energy and Enthalpy qP = DE + PDV = DH Enthalpy change or Heat of reaction (at constant pressure) Enthalpy is a state function whose value depends only on the current state of the system, not on the path taken to arrive at that state. DH = Hfinal - Hinitial = Hproducts - Hreactants 8.6The Thermodynamic Standard State C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g) DH= -2043 kJ C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(l) DH = -2219 kJ Thermodynamic Standard State: Most stable form of a substance at 1 atm pressure and at a specified temperature, usually 25 °C; 1 M concentration for all substances in solution. Standard enthalpy of reaction is indicated by the symbol ΔHo C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g) Copyright © 2008 Pearson Prentice Hall, Inc. DH° = -2043 kJ Chapter 8/12 Enthalpies of Physical and Chemical Change Enthalpy of Fusion (DHfusion): The amount of heat necessary to melt a substance without changing its temperature Enthalpy of Vaporization (DHvap): The amount of heat required to vaporize a substance without changing its temperature Enthalpy of Sublimation (DHsubl): The amount of heat required to convert a substance from a solid to a gas without going through a liquid phase Enthalpies of Physical and Chemical Change Enthalpies of Physical and Chemical Change 2Al(s) + Fe2O3(s) 2Fe(s) + Al2O3(s) DHo = -852 kJ exothermic 2Fe(s) + Al2O3(s) 2Al(s) + Fe2O3(s) DHo = +852 kJ endothermic Examples Identify each of the following processes as endothermic or exothermic and indicate the sign of ΔHo Sweat evaporating from your skin Water freezing in a freezer Wood burning in fire Example An LP gas tank in a home barbeque contains 13.2 kg of propane, C3H8. Calculate the heat (in kJ) associated with the complete combustion of all of the propane in the tank C3H8(g) + CO2(g) 3 CO2(g) + 4 H2O(g) ΔHo = -2044kJ Example How much heat (in kJ) is evolved when 5.00g of aluminum reacts with a stoichiometric amount of Fe2O3? 2 Al(s) + Fe2O3(s) 2 Fe(s) + Al2O3(s) ΔHo = -852 kJ Calorimetry and Heat Capacity Measure the heat flow at constant pressure (DH). Calorimetry and Heat Capacity Measure the heat flow at constant volume (DE). Calorimetry and Heat Capacity Heat Capacity (C): The amount of heat required to raise the temperature of an object or substance a given amount. q = C x DT Molar Heat Capacity (Cm): The amount of heat required to raise the temperature of 1 mol of a substance by 1 °C. q = (Cm) x (moles of substance) x DT Specific Heat: The amount of heat required to raise the temperature of 1 g of a substance by 1 °C. q = (specific heat) x (mass of substance) x DT Calorimetry and Heat Capacity Molar Heat Capacity (Cm): The amount of heat necessary to raise the temperature of 1 mol of a substance by 1 oC q = Cm x Moles of substance x DT Calorimetry and Heat Capacity © 2012 Pearson Education, Inc. Chapter 8/23 Examples Assuming that Coca Cola has the same specific heat as water [4.183 J/goC], calculate the amount of heat (in kJ) transferred when one can (about 350.0 g) is cooled from 25.0oC – 3.0oC 8.9 Hess’s Law Hess’s Law: The overall enthalpy change for a reaction is equal to the sum of the enthalpy changes for the individual steps in the reaction. Haber Process: 3H2(g) + N2(g) 2NH3(g) DH°total = ??? Multiple-Step Process - Given 2H2(g) + N2(g) N2H4(g) N2H4(g) + H2(g) 2NH3(g) 3H2(g) + N2(g) 2NH3(g) DH°1 = 95.4 Kj DH°2 = -187.6 kJ DH°total = DH°1 + DH°2 Hess’s Law Example Find ΔHorxn for the following reaction: 3H2(g) + O3(g) 3 H2O(g) ΔHorxn = ?? Use the following reactions with known ΔH’s 2H2 (g) + O2(g) 2 H2O(g) Δ Ho = -483.6 kJ 3O2(g) 2 O3 (g) Δ Ho = +285.4 kJ Example Fin ΔHorxn for the following reaction C(s) + H2O(g) CO(g) + H2(g) Horxn = ? Use the following reactions with known H’s C(s) + O2(g) CO2(g) ΔHo = -393.5 kJ 2CO(g) + O2(g) 2CO2(g) Δ Ho = -566.0kJ 2H2 (g) + O2(g) 2H2O (g) Δ Ho = -483.6 kJ Standard Heats of Formation Standard Heat of Formation (DHof ): The enthalpy change for the formation of 1 mol of a substance in its standard state from its constituent elements in their standard states Standard states C(s) + 2H2(g) CH4(g) 1 mol of 1 substance DHof = -74.8 kJ Standard Heats of Formation Standard Heats of Formation DHo = DHof (Products) - DHof (Reactants) aA + bB cC + dD DHo = [c DHof (C) + d DHof (D)] - [a DHof (A) + b DHof (B)] Products Reactants Standard Heats of Formation Using standard heats of formation, calculate the standard enthalpy of reaction for the photosynthesis of glucose (C6H12O6) and O2 from CO2 and liquid H2O. 6CO2(g) + 6H2O(l) C6H12O6(s) + 6O2(g) DHo = ? Example Use the information in Table 8.2 to calculate ΔHo (in kJ) for the reaction of ammonia with oxygen gas to yield nitric oxide (NO) and water vapor, a step in the Ostwald process for the commercial production of nitric acid An Introduction to Entropy Spontaneous Process: A process that, once started, proceeds on its own without a continuous external influence An Introduction to Entropy Entropy (S): The amount of molecular randomness in a system An Introduction to Entropy Spontaneous processes are • favored by a decrease in H (negative DH). • favored by an increase in S (positive DS). Nonspontaneous processes are • favored by an increase in H (positive DH). • favored by a decrease in S (negative DS). Example Predict whether ΔSo is likely to be positive or negative for each of the following reactions H2C=CH2(g) + Br2(g) BrCH2CH2Br(l) 8.14 An Introduction to Free Energy Gibbs Free Energy Change (DG) DG = DH - T DS Enthalpy of reaction Temperature (Kelvin) Copyright © 2008 Pearson Prentice Hall, Inc. Entropy change Chapter 8/38 An Introduction to Free Energy Gibbs Free Energy Change (DG) DG = DH - T DS DG < 0 Process is spontaneous DG = 0 Process is at equilibrium (neither spontaneous nor nonspontaneous) DG > 0 Process is nonspontaneous Example Is the Haber process for the industrial synthesis of ammonia spontaneous or nonspontaneous under standard conditions at 25.0oC. At what temperature (oC) does the changeover occur? An Introduction to Free Energy Gibbs Free Energy Change (DG) DG = DH - T DS Enthalpy of reaction Temperature (Kelvin) Entropy change An Introduction to Free Energy Gibbs Free Energy Change (DG) DG = DH - T DS DG < 0 Process is spontaneous DG = 0 Process is at equilibrium (neither spontaneous nor nonspontaneous) DG > 0 Process is nonspontaneous © 2012 Pearson Education, Inc. Chapter 8/42