# University of North Carolina, Wilmington

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University of North Carolina, Wilmington
Thermochemistry David P. White University of North Carolina, Wilmington Chapter 5 Copyright 1999, PRENTICE HALL Chapter 5 1 1 1 1

The Nature of Energy Kinetic and Potential Energy From Physics:
Force is a push or pull on an object. Work is the product of force applied to an object over a distance: w = F  d Energy is the work done to move an object against a force. Kinetic energy is the energy of motion: Copyright 1999, PRENTICE HALL Chapter 5

The Nature of Energy Kinetic and Potential Energy
Potential energy is the energy an object possesses by virtue of its position. Potential energy can be converted into kinetic energy. Example: a ball of clay dropping off a building. Copyright 1999, PRENTICE HALL Chapter 5

The Nature of Energy Energy Units SI Unit for energy is the joule, J:
We sometimes use the calorie instead of the joule: 1 cal = J (exactly) A nutritional Calorie: 1 Cal = 1000 cal = 1 kcal Copyright 1999, PRENTICE HALL Chapter 5

The Nature of Energy Systems and Surroundings
System: part of the universe we are interested in. Surroundings: the rest of the universe. Copyright 1999, PRENTICE HALL Chapter 5

First Law of Thermodynamics
Internal Energy Internal Energy: total energy of a system. Cannot measure absolute internal energy. Change in internal energy, DE = Efinal - Einitial Copyright 1999, PRENTICE HALL Chapter 5

First Law of Thermodynamics
Relating DE to Heat and Work Energy cannot be created or destroyed. Energy of (system + surroundings) is constant. Any energy transferred from a system must be transferred to the surroundings (and vice versa). From the first law of thermodynamics: when a system undergoes a physical or chemical change, the change in internal energy is given by the heat added to or absorbed by the system plus the work done on or by the system: DE = q + w Copyright 1999, PRENTICE HALL Chapter 5

First Law of Thermodynamics
Relating DE to Heat and Work Copyright 1999, PRENTICE HALL Chapter 5

First Law of Thermodynamics
Relating DE to Heat and Work Copyright 1999, PRENTICE HALL Chapter 5

First Law of Thermodynamics
Endothermic and Exothermic Processes Endothermic: absorbs heat from the surroundings. Exothermic: transfers heat to the surroundings. An endothermic reaction feels cold. An exothermic reaction feels hot. Copyright 1999, PRENTICE HALL Chapter 5

First Law of Thermodynamics
State Functions State function: depends only on the initial and final states of system, not on how the internal energy is used. Copyright 1999, PRENTICE HALL Chapter 5

First Law of Thermodynamics
State Functions Copyright 1999, PRENTICE HALL Chapter 5

DH = Hfinal - Hinitial = qP
Enthalpy Enthalpy, H: Heat transferred between the system and surroundings carried out under constant pressure. Can only measure the change in enthalpy: DH = Hfinal - Hinitial = qP Copyright 1999, PRENTICE HALL Chapter 5

Enthalpies of Reaction
For a reaction DHrxn = H(products) - H (reactants) Enthalpy is an extensive property (magnitude DH is directly proportional to amount): CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) DH = -802 kJ 2CH4(g) + 4O2(g)  2CO2(g) + 4H2O(g) DH = kJ When we reverse a reaction, we change the sign of DH: CO2(g) + 2H2O(g)  CH4(g) + 2O2(g) DH = +802 kJ Change in enthalpy depends on state: H2O(g)  H2O(l) DH = -88 kJ Copyright 1999, PRENTICE HALL Chapter 5

q = (specific heat)  (grams of substance)  T.
Calorimetry Heat Capacity and Specific Heat Calorimetry = measurement of heat flow. Calorimeter = apparatus that measures heat flow. Heat capacity = the amount of energy required to raise the temperature of an object (by one degree). Molar heat capacity = heat capacity of 1 mol of a substance. Specific heat = specific heat capacity = heat capacity of 1 g of a substance. q = (specific heat)  (grams of substance)  T. Be careful of the sign of q. Copyright 1999, PRENTICE HALL Chapter 5

Calorimetry Heat Capacity and Specific Heat
Copyright 1999, PRENTICE HALL Chapter 5

Calorimetry Constant-Pressure Calorimetry
Atmospheric pressure is constant! DH = qP qrxn = -qsoln = -(specific heat of solution)  (grams of solution)  DT. Copyright 1999, PRENTICE HALL Chapter 5

Calorimetry Bomb Calorimetry (Constant-Volume Calorimetry)
Reaction carried out under constant volume. Use a bomb calorimeter. Usually study combustion. Copyright 1999, PRENTICE HALL Chapter 5

Calorimetry Bomb Calorimetry (Constant-Volume Calorimetry)
qrxn = -CcalorimeterT. Copyright 1999, PRENTICE HALL Chapter 5

Hess’s Law Hess’s law: if a reaction is carried out in a number of steps, H for the overall reaction is the sum of H for each individual step. For example: CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) H = -802 kJ 2H2O(g)  2H2O(l) H = -88 kJ CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) H = -890 kJ Copyright 1999, PRENTICE HALL Chapter 5

Hess’s Law In the above enthalpy diagram note that H1 = H2 + H3
Copyright 1999, PRENTICE HALL Chapter 5

Enthalpies of Formation
If 1 mol of compound is formed from its constituent elements, then the enthalpy change for the reaction is called the enthalpy of formation, Hof . Standard conditions (standard state): 1 atm and 25 oC (298 K). Standard enthalpy, Ho, is the enthalpy measured when everything is in its standard state. Standard enthalpy of formation: 1 mol of compound is formed from substances in their standard states. If there is more than one state for a substance under standard conditions, the more stable one is used. Copyright 1999, PRENTICE HALL Chapter 5

Enthalpies of Formation
Standard enthalpy of formation of the most stable form of an element is zero. Copyright 1999, PRENTICE HALL Chapter 5

Enthalpies of Formation
Using Enthalpies of Formation to Calculate Enthalpies of Reaction We use Hess’ Law to calculate enthalpies of a reaction from enthalpies of formation. Hrxn = H1 + H2 + H3 Copyright 1999, PRENTICE HALL Chapter 5

Enthalpies of Formation
Using Enthalpies of Formation to Calculate Enthalpies of Reaction For a reaction: Copyright 1999, PRENTICE HALL Chapter 5

Foods and Fuels Foods Fuel value = energy released when 1 g of substance is burned. 1 nutritional Calorie, 1 Cal = 1000 cal = 1 kcal. Energy in our bodies comes from carbohydrates and fats (mostly). Intestines: carbohydrates converted into glucose: C6H12O6 + 6O2  6CO2 + 6H2O, DH = kJ Fats break down as follows: 2C57H110O O2  114CO H2O, DH = -75,520 kJ Fats: contain more energy; are not water soluble, so are good for energy storage. Copyright 1999, PRENTICE HALL Chapter 5

Foods and Fuels Foods Copyright 1999, PRENTICE HALL Chapter 5

Foods and Fuels Fuels U.S.: 1.0 x 106 kJ of fuel per day.
Most from petroleum and natural gas. Remainder from coal, nuclear, and hydroelectric. Fossil fuels are not renewable. Copyright 1999, PRENTICE HALL Chapter 5

Foods and Fuels Fuels Fuel value = energy released when 1 g of substance is burned. Hydrogen has great potential as a fuel with a fuel value of 142 kJ/g. Copyright 1999, PRENTICE HALL Chapter 5

Thermochemistry End of Chapter 5 Copyright 1999, PRENTICE HALL
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