Chapter 8
Chapter 8 Thermochemistry: Chemical Energy Energy and Its Conservation Energy: The capacity to supply heat or do work. Kinetic Energy (EK): The energy of motion. Potential Energy (EP): Stored energy. 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 Units: 1 cal = 4.184 J (exactly) 1 Cal = 1000 cal = 1 kcal 8.2 Internal Energy and State Functions First Law of Thermodynamics: The total internal energy E of an isolated system is constant - The sum of all the kinetic and potential energies of particles making up a substance DE = Efinal - Einitial Systems and Surroundings System = the object in question Surrounding(s) = everything outside the system When both system and surrounding at same temperature thermal equilibrium When not Heat transfer to surrounding = exothermic (you feel the heat) hot metal!! Heat transfer to system = endothermic (you feel cold) cold metal!! 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 8.3 Expansion Work Expansion Work: Work done as the result of a volume change in the system C3H8(g) + 5O2(g) 6 mol of gas 3CO2(g) + 4H2O(g) 7 mol of gas w=Fxd = - P∆V 8.4 Energy and Enthalpy DE = q + w q = heat transferred q = DE + PDV Constant Volume (DV = 0): Constant Pressure: w = work = -PDV 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) DH° = -2043 kJ Enthalpies of Physical and Chemical Change 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 2Al(s) + Fe2O3(s) 2Fe(s) + Al2O3(s) exothermic The reverse reaction 2Fe(s) + Al2O3(s) DHo = -852 kJ 2Al(s) + Fe2O3(s) DHo = +852 kJ endothermic Applying Stoichiometry to Heats of Reaction a. A propellant for rockets is obtained by mixing the liquids hydrazine, N2H4, and dinitrogen tetroxide, N2O4. These compounds react to give gaseous nitrogen, N2 and water vapor, evolving 1049 kJ of heat at constant pressure when 1 mol N2O4 reacts. Write the thermochemical equation for this reaction b. Write the thermochemical equation for the reverse of the reaction c. How much heat evolves when 10.0 g of hydrazine reacts according to the reaction described in (a) ? 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 8.6 Calorimetry The process of measuring heat transfer in at constant pressure (ΔH). chemical/physical process qrxn + qsol = 0 qrxn = -qsol Rrxn = system Ssoln = surrounding What you’ll do in lab Heat given off by rxn Measured by thermometter Figure out qrxn indirectly 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 Example How much heat must be added to a 8.21 g sample of gold to increase its temperature by 6.2 oC? The specific heat of gold is 0.13 J/g oC. Example When 1.045 g of CaO is added to 50.0 mL of water at 25.0oC in a calorimeter, the temperature of the water increases to 32.2 oC. Assuming that the specific heat of the solution is 4.18 J/g oC and that the calorimeter itself absorbes a negligible amount of heat, calculate ∆H in kilojoules for the reaction CaO(s) + H2O(l) Ca(OH)2(aq) 8.9 Hess’s Law 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 DH°1 = 95.4 Kj 2H2(g) + N2(g) N2H4(g) N2H4(g) + H2(g) 2NH3(g) DH°2 = -187.6 kJ 3H2(g) + N2(g) 2NH3(g) DH°total = ????? Example Find 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) 2CO(g) + O2(g) 2CO2(g) 2H2 (g) + O2(g) 2H2O (g) ΔHo = -393.5 kJ Δ Ho = -566.0kJ 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 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 Calculate the enthalpy change for the following reaction 3 NO2(g) + H2O(l) 2 HNO3(aq) + NO(g) *Use standard enthalpies of formation An Introduction to Entropy Spontaneous Process: A process that, once started, proceeds on its own without a continuous external influence 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 a. b. H2C=CH2(g) + Br2(g) BrCH2CH2Br(l) Consider the following figures
© Copyright 2024