Prentice Hall © 2003Chapter 5 Chapter 5 Thermochemistry CHEMISTRY The Central Science 9th Edition David P. White

Prentice Hall © 2003Chapter 5 Kinetic Energy and Potential Energy Kinetic energy is the energy of motion: Potential energy is the energy an object possesses by virtue of its position. Potential energy can be converted into kinetic energy. Example: a bicyclist at the top of a hill. The Nature of Energy

Prentice Hall © 2003Chapter 5 Kinetic Energy and Potential Energy Electrostatic potential energy, E d, is the attraction between two oppositely charged particles, Q 1 and Q 2, a distance d apart: The constant  = 8.99  10 9 J-m/C 2. If the two particles are of opposite charge, then E d is the electrostatic repulsion between them. The Nature of Energy

Prentice Hall © 2003Chapter 5 Units of Energy 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 The Nature of Energy

Prentice Hall © 2003Chapter 5 Systems and Surroundings System: part of the universe we are interested in. Surroundings: the rest of the universe. The Nature of Energy

Prentice Hall © 2003Chapter 5 Transferring Energy: Work and Heat Force is a push or pull on an object. Work is the product of force applied to an object over a distance: Energy is the work done to move an object against a force. Heat is the transfer of energy between two objects. Energy is the capacity to do work or transfer heat. The Nature of Energy

Prentice Hall © 2003Chapter 5 Internal Energy Internal Energy: total energy of a system. Cannot measure absolute internal energy. Change in internal energy, The First Law of Thermodynamics

Prentice Hall © 2003Chapter 5 Relating  E 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: The First Law of Thermodynamics

Prentice Hall © 2003Chapter 5 Exothermic and Endothermic Processes Endothermic: absorbs heat from the surroundings. Exothermic: transfers heat to the surroundings. An endothermic reaction feels cold. An exothermic reaction feels hot. The First Law of Thermodynamics

Prentice Hall © 2003Chapter 5 State Functions State function: depends only on the initial and final states of system, not on how the internal energy is used. The First Law of Thermodynamics

State Functions

Prentice Hall © 2003Chapter 5 Chemical reactions can absorb or release heat. However, they also have the ability to do work. For example, when a gas is produced, then the gas produced can be used to push a piston, thus doing work. Zn(s) + 2H + (aq)  Zn 2+ (aq) + H 2 (g) The work performed by the above reaction is called pressure-volume work. When the pressure is constant, Enthalpy

Enthalpy

Prentice Hall © 2003Chapter 5 Enthalpy, H: Heat transferred between the system and surroundings carried out under constant pressure. Enthalpy is a state function. If the process occurs at constant pressure, Enthalpy

Prentice Hall © 2003Chapter 5 Since we know that We can write When  H, is positive, the system gains heat from the surroundings. When  H, is negative, the surroundings gain heat from the system. Enthalpy

Prentice Hall © 2003Chapter 5 Enthalpy

Prentice Hall © 2003Chapter 5 For a reaction: Enthalpy is an extensive property (magnitude  H is directly proportional to amount): CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(g)  H = -802 kJ 2CH 4 (g) + 4O 2 (g)  2CO 2 (g) + 4H 2 O(g)  H =  1604 kJ Enthalpies of Reaction

Prentice Hall © 2003Chapter 5 When we reverse a reaction, we change the sign of  H: CO 2 (g) + 2H 2 O(g)  CH 4 (g) + 2O 2 (g)  H = +802 kJ Change in enthalpy depends on state: 2H 2 O(g)  2H 2 O(l)  H = -88 kJ Enthalpies of Reaction

Prentice Hall © 2003Chapter 5 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. Calorimetry

Prentice Hall © 2003Chapter 5 Constant Pressure Calorimetry Atmospheric pressure is constant! Calorimetry

Prentice Hall © 2003Chapter 5 Calorimetry Constant Pressure Calorimetry

Prentice Hall © 2003Chapter 5 Calorimetry Reaction carried out under constant volume. Use a bomb calorimeter. Usually study combustion. Bomb Calorimetry (Constant Volume Calorimetry)

Prentice Hall © 2003Chapter 5 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: CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(g)  H = -802 kJ 2H 2 O(g)  2H 2 O(l)  H = -88 kJ CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(l)  H = -890 kJ Hess’s Law

Prentice Hall © 2003Chapter 5 Note that:  H 1 =  H 2 +  H 3 Hess’s Law

Prentice Hall © 2003Chapter 5 If 1 mol of compound is formed from its constituent elements, then the enthalpy change for the reaction is called the enthalpy of formation,  H o f. Standard conditions (standard state): 1 atm and 25 o C (298 K). Standard enthalpy,  H o, 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. Enthalpies of Formation

Prentice Hall © 2003Chapter 5 If there is more than one state for a substance under standard conditions, the more stable one is used. Standard enthalpy of formation of the most stable form of an element is zero. Enthalpies of Formation

Prentice Hall © 2003Chapter 5 Enthalpies of Formation

Prentice Hall © 2003Chapter 5 Using Enthalpies of Formation of Calculate Enthalpies of Reaction We use Hess’ Law to calculate enthalpies of a reaction from enthalpies of formation. Enthalpies of Formation

Prentice Hall © 2003Chapter 5 Using Enthalpies of Formation of Calculate Enthalpies of Reaction For a reaction Enthalpies of Formation

Prentice Hall © 2003Chapter 5 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: C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O,  H = kJ Fats break down as follows: 2C 57 H 110 O O 2  114CO H 2 O,  H = -75,520 kJ Foods and Fuels

Prentice Hall © 2003Chapter 5 Foods Fats: contain more energy; are not water soluble, so are good for energy storage. Foods and Fuels

Prentice Hall © 2003Chapter 5 Fuels In 2000 the United States consumed 1.03  kJ of fuel. Most from petroleum and natural gas. Remainder from coal, nuclear, and hydroelectric. Fossil fuels are not renewable. Foods and Fuels

Prentice Hall © 2003Chapter 5 Foods and Fuels

Prentice Hall © 2003Chapter 5 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. Foods and Fuels

Prentice Hall © 2003Chapter 5 End of Chapter 5: Thermochemistry