Chapter 8 Thermochemistry: Chemical Energy Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 7:13:46 PM Chapter 8 Thermochemistry: Chemical Energy Copyright © 2011 Pearson Prentice Hall, Inc.

Energy and Its Conservation Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy Energy and Its Conservation 4/19/2017 7:13:46 PM 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. This is one way to state the first law of thermodynamics. Units: 1 cal = 4.184 J (exactly) 1 Cal = 1000 cal = 1 kcal Copyright © 2011 Pearson Prentice Hall, Inc.

Energy and Its Conservation Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 7:13:46 PM Energy and Its Conservation Copyright © 2011 Pearson Prentice Hall, Inc.

Energy and Its Conservation Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy Energy and Its Conservation 4/19/2017 7:13:46 PM 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 Heat transfers from hot to cold. Hot and cold are relative terms. Copyright © 2011 Pearson Prentice Hall, Inc.

Internal Energy and State Functions Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy Internal Energy and State Functions 4/19/2017 7:13:46 PM First Law of Thermodynamics: The total internal energy E of an isolated system is constant DE = Efinal - Einitial The first law is essentially the law of conservation of energy. Copyright © 2011 Pearson Prentice Hall, Inc.

Internal Energy and State Functions Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 7:13:46 PM Internal Energy and State Functions CO2(g) + 2H2O(g) + 802 kJ energy CH4(g) + 2O2(g) 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. Copyright © 2011 Pearson Prentice Hall, Inc.

Internal Energy and State Functions Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 7:13:46 PM 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 Internal energy, E, is a state function. You can also think of climbing a hill. Regardless of the path taken to climb the hill, the height difference is simply the height of the hill. Copyright © 2011 Pearson Prentice Hall, Inc.

Expansion Work w = F x d 3CO2(g) + 4H2O(g) C3H8(g) + 5O2(g) Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 7:13:46 PM Expansion Work w = F x d 3CO2(g) + 4H2O(g) C3H8(g) + 5O2(g) 6 mol of gas 7 mol of gas The production of gas (net change is 1 mole of gas from reactants to products) causes the piston to move up. Copyright © 2011 Pearson Prentice Hall, Inc.

Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 7:13:46 PM Expansion Work Expansion Work: Work done as the result of a volume change in the system Copyright © 2011 Pearson Prentice Hall, Inc.

Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 8.5 Energy and Enthalpy DE = q + w q = heat transferred w = work = -PDV q = DE + PDV Constant Volume (DV = 0): qV = DE Constant Pressure: qP = DE + PDV This book uses the definition: DE = Efinal - Einitial “U” is also used for internal energy. Copyright © 2008 Pearson Prentice Hall, Inc.

Heat of reaction (at constant pressure) Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy Energy and Enthalpy 4/19/2017 7:13:46 PM qP = DE + PDV = H Enthalpy change or Heat of reaction (at constant pressure) DH = Hfinal - Hinitial 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. Enthalpy is path-independent. = Hproducts - Hreactants Copyright © 2011 Pearson Prentice Hall, Inc.

8.6The Thermodynamic Standard State Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 8.6The Thermodynamic Standard State 3CO2(g) + 4H2O(g) C3H8(g) + 5O2(g) DH= -2043 kJ 3CO2(g) + 4H2O(l) C3H8(g) + 5O2(g) 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 The conversion from liquid to gas phases requires energy. Therefore, it’s important that the states are specified. Putting the “°” superscripted next to the DH means standard state. The standard pressure is actually 1 bar. The enthalpy difference is usually small, however. 3CO2(g) + 4H2O(g) C3H8(g) + 5O2(g) DH° = -2043 kJ Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

Enthalpies of Physical and Chemical Change Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 7:13:46 PM 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 Physical changes. Copyright © 2011 Pearson Prentice Hall, Inc.

Enthalpies of Physical and Chemical Change Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 7:13:46 PM Enthalpies of Physical and Chemical Change Copyright © 2011 Pearson Prentice Hall, Inc.

Enthalpies of Physical and Chemical Change Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 7:13:46 PM Enthalpies of Physical and Chemical Change 2Fe(s) + Al2O3(s) 2Al(s) + Fe2O3(s) DHo = -852 kJ exothermic 2Al(s) + Fe2O3(s) 2Fe(s) + Al2O3(s) The enthalpy of reaction is for the reaction in the direction written. DHo = +852 kJ endothermic Copyright © 2011 Pearson Prentice Hall, Inc.

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 Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 7:13:46 PM Calorimetry and Heat Capacity Measure the heat flow at constant pressure (DH). Calorimetry = study of heat flow. This is sometimes done in undergraduate labs in styrofoam cups. Copyright © 2011 Pearson Prentice Hall, Inc.

Calorimetry and Heat Capacity Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy Calorimetry and Heat Capacity 4/19/2017 7:13:46 PM Measure the heat flow at constant volume (DE). Bomb calorimeter. Copyright © 2011 Pearson Prentice Hall, Inc.

Calorimetry and Heat Capacity Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 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. DT is the temperature change. Heat capacity is an extensive property. Specific heat is an intensive property. q = (specific heat) x (mass of substance) x DT Copyright © 2008 Pearson Prentice Hall, Inc.

Calorimetry and Heat Capacity Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 7:13:46 PM 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 Copyright © 2011 Pearson Prentice Hall, Inc.

Calorimetry and Heat Capacity Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 7:13:46 PM Calorimetry and Heat Capacity Note that the values for water are considerably higher than most other substances. Interesting discussion points could be how large bodies of water affect weather, the human body is approximately 60% water and the role it plays in homeostasis, etc. © 2012 Pearson Education, Inc. Copyright © 2011 Pearson Prentice Hall, Inc.

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

Chapter 8: Thermochemistry: Chemical Energy 8.9 Hess’s Law 4/19/2017 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: 2NH3(g) 3H2(g) + N2(g) DH°total = ??? Multiple-Step Process - Given N2H4(g) 2H2(g) + N2(g) DH°1 = 95.4 Kj 2NH3(g) N2H4(g) + H2(g) DH°2 = -187.6 kJ Hydrazine, N2H4, is a reactive intermediate (more on that in Ch 12: Chemical Kinetics). Chemical reactions are far more complicated than we discuss in Ch 3 (stoichiometry). The overall reaction and step 2 can be measured, but step 1 cannot be measured easily. 2NH3(g) 3H2(g) + N2(g) DH°total = DH°1 + DH°2 Copyright © 2008 Pearson Prentice Hall, Inc.

Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 Hess’s Law Copyright © 2008 Pearson Prentice Hall, Inc.

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 Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy Standard Heats of Formation 4/19/2017 7:13:46 PM 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 CH4(g) C(s) + 2H2(g) DHof = -74.8 kJ 1 mol of 1 substance Students need to remember solid, liquid, gases, and diatomics learned earlier. Copyright © 2011 Pearson Prentice Hall, Inc.

Standard Heats of Formation Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 7:13:46 PM Standard Heats of Formation Note the lack of anything that is 0. Elements (standard state) are defined to have a standard enthalpy of formation of 0. Copyright © 2011 Pearson Prentice Hall, Inc.

Standard Heats of Formation Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy Standard Heats of Formation 4/19/2017 7:13:46 PM Ho = Hof (Products) - Hof (Reactants) cC + dD aA + bB Ho = [c Hof (C) + d Hof (D)] - [a Hof (A) + b Hof (B)] Using table values. Products Reactants Copyright © 2011 Pearson Prentice Hall, Inc.

Standard Heats of Formation Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 7:13:46 PM 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. C6H12O6(s) + 6O2(g) 6CO2(g) + 6H2O(l) DHo = ? This is Problem 8.16 in the text. Copyright © 2011 Pearson Prentice Hall, Inc.

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 Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy An Introduction to Entropy 4/19/2017 7:13:46 PM Spontaneous Process: A process that, once started, proceeds on its own without a continuous external influence Copyright © 2011 Pearson Prentice Hall, Inc.

An Introduction to Entropy Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy An Introduction to Entropy 4/19/2017 7:13:46 PM Entropy (S): The amount of molecular randomness in a system Copyright © 2011 Pearson Prentice Hall, Inc.

An Introduction to Entropy Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy An Introduction to Entropy 4/19/2017 7:13:46 PM 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). Copyright © 2011 Pearson Prentice Hall, Inc.

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 Chapter 8: Thermochemistry: Chemical Energy 8.14 An Introduction to Free Energy 4/19/2017 Gibbs Free Energy Change (DG) DG = DH - T DS Enthalpy of reaction Temperature (Kelvin) Entropy change Copyright © 2008 Pearson Prentice Hall, Inc. Copyright © 2008 Pearson Prentice Hall, Inc.

An Introduction to Free Energy Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 An Introduction to Free Energy Gibbs Free Energy Change (DG) DG = DH DS - T DG < 0 Process is spontaneous DG = 0 Process is at equilibrium (neither spontaneous nor nonspontaneous) DG > 0 Process is nonspontaneous Figure 8.13 nicely demonstrates how enthalpy, entropy, and free energy relate to each other for solid water (ice) and liquid water. Copyright © 2008 Pearson Prentice Hall, Inc.

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 Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy An Introduction to Free Energy 4/19/2017 7:13:46 PM Gibbs Free Energy Change (DG) DG = H - T S Temperature (Kelvin) Enthalpy of reaction Entropy change Copyright © 2011 Pearson Prentice Hall, Inc.

An Introduction to Free Energy Chemistry: McMurry and Fay, 6th Edition Chapter 8: Thermochemistry: Chemical Energy 4/19/2017 7:13:46 PM An Introduction to Free Energy Gibbs Free Energy Change (DG) DG = H - T S DG < 0 Process is spontaneous DG = 0 Process is at equilibrium (neither spontaneous nor nonspontaneous) DG > 0 Process is nonspontaneous © 2012 Pearson Education, Inc. Copyright © 2011 Pearson Prentice Hall, Inc.