Central CHEMISTRY
The Science 9th Edition
Chapter 5 Thermochemistry
David P. White
Prentice Hall 2003 Chapter 5
The Nature of Energy
Kinetic Energy and Potential Energy Kinetic energy is the energy of motion:
1 2 Ek = mv 2 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.
Prentice Hall 2003 Chapter 5
The Nature of Energy
Kinetic Energy and Potential Energy Electrostatic potential energy, Ed, is the attraction between two oppositely charged particles, Q1 and Q2, a distance d apart: Q1Q2 Ed = d The constant = 8.99 109 J-m/C2. If the two particles are of opposite charge, then Ed is the electrostatic repulsion between them.
Prentice Hall 2003 Chapter 5
The Nature of Energy
Units of Energy SI Unit for energy is the joule, J:
1 2 1 Ek = mv = (2 kg )(1 m/s)2 2 2 = 1 kg - m/s2 = 1 J
We sometimes use the calorie instead of the joule: 1 cal = 4.184 J (exactly) A nutritional Calorie: 1 Cal = 1000 cal = 1 kcal Prentice Hall 2003 Chapter 5
The Nature of Energy
Systems and Surroundings System: part of the universe we are interested in. Surroundings: the rest of the universe.
Prentice Hall 2003
Chapter 5
The Nature of Energy
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: w=F d 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.
Prentice Hall 2003
Chapter 5
The First Law of Thermodynamics
Internal Energy Internal Energy: total energy of a system. Cannot measure absolute internal energy. Change in internal energy,
E = Efinal Einitial
Prentice Hall 2003
Chapter 5
The First Law of Thermodynamics
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: E =q+w
Prentice Hall 2003 Chapter 5
The First Law of Thermodynamics
The First Law of Thermodynamics
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.
Prentice Hall 2003
Chapter 5
The 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.
Prentice Hall 2003
Chapter 5
State Functions
Enthalpy
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) Zn2+(aq) + H2(g) The work performed by the above reaction is called pressure-volume work. When the pressure is constant, w= P V
Prentice Hall 2003 Chapter 5
Enthalpy
Enthalpy
Enthalpy, H: Heat transferred between the system and surroundings carried out under constant pressure. H = E + PV Enthalpy is a state function. If the process occurs at constant pressure, H = (E + PV ) = E+P V
Prentice Hall 2003 Chapter 5
Enthalpy
Since we know that We can write
w= P V
H = E+P V = qP + w When H, is positive, the system gains heat from the surroundings. When H, is negative, the surroundings gain heat from the system.
Prentice Hall 2003 Chapter 5
Enthalpy
Prentice Hall 2003
Chapter 5
Enthalpies of Reaction
For a reaction:
H = H final H initial = H products H reactants Enthalpy is an extensive property (magnitude is
directly H proportional to amount): CH4(g) + 2O2(g) CO2(g) + 2H2O(g) 2CH4(g) + 4O2(g) 2CO2(g) + 4H2O(g)
Prentice Hall 2003 Chapter 5
H = -802 kJ H = 1604 kJ
Enthalpies of Reaction
When we reverse a reaction, we change the sign of H: H = +802 kJ CO2(g) + 2H2O(g) CH4(g) + 2O2(g) Change in enthalpy depends on state: H2O(g) H2O(l) H = -88 kJ
Prentice Hall 2003
Chapter 5
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
Prentice Hall 2003 Chapter 5
Calorimetry
Constant Pressure Calorimetry Atmospheric pressure is constant! H = qP
qrxn = qsoln =
(specific heat of solution ) (grams of solution ) T
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Chapter 5
Calorimetry
Constant Pressure Calorimetry
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Chapter 5
Calorimetry
Bomb Calorimetry (Constant Volume Calorimetry) Reaction carried out under constant volume. Use a bomb calorimeter. Usually study combustion.
qrxn = Ccal T
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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 H = -88 kJ 2H2O(g) 2H2O(l) CH4(g) + 2O2(g) CO2(g) + 2H2O(l) H = -890 kJ
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Hess's Law
Note that: H1 = H2 + H3
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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.
Prentice Hall 2003 Chapter 5
Enthalpies of Formation
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.
Prentice Hall 2003
Chapter 5
Enthalpies of Formation
Prentice Hall 2003
Chapter 5
Enthalpies of Formation
Using Enthalpies of Formation of Calculate Enthalpies of Reaction We use Hess' Law to calculate enthalpies of a reaction from enthalpies of formation.
Prentice Hall 2003
Chapter 5
Enthalpies of Formation
Using Enthalpies of Formation of Calculate Enthalpies of Reaction For a reaction
H rxn = n H f (products ) m H f (reactants)
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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, H = -2816 kJ 2C57H110O6 + 163O2
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Fats break down as follows:
114CO2 + 110H2O, H = -75,520 kJ
Chapter 5
Foods and Fuels
Foods Fats: contain more energy; are not water soluble, so are good for energy storage.
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Chapter 5
Foods and Fuels
In 2000 the United States consumed 1.03 fuel. Most from petroleum and natural gas. Remainder from coal, nuclear, and hydroelectric. Fossil fuels are not renewable. Fuels 1017 kJ of
Prentice Hall 2003
Chapter 5
Foods and Fuels
Prentice Hall 2003
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.
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Chapter 5
End of Chapter 5: Thermochemistry
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Chapter 5