Upload
alexander-atkins
View
226
Download
8
Tags:
Embed Size (px)
Citation preview
Copyright 1999, PRENTICE HALL Chapter 5 1
ThermochemistryThermochemistry
Chapter 5Chapter 5
David P. WhiteDavid P. White
University of North Carolina, WilmingtonUniversity of North Carolina, Wilmington
Copyright 1999, PRENTICE HALL Chapter 5 2
The Nature of EnergyThe Nature of Energy
Kinetic and Potential EnergyKinetic and Potential EnergyFrom 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:
221 mvEk
Copyright 1999, PRENTICE HALL Chapter 5 3
The Nature of EnergyThe Nature of Energy
Kinetic and Potential EnergyKinetic 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 4
The Nature of EnergyThe Nature of Energy
Energy UnitsEnergy UnitsSI Unit for energy is the joule, 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
J 1
s/m kg 1
m/s 1kg 2
22
2212
21
mvEk
Copyright 1999, PRENTICE HALL Chapter 5 5
The Nature of EnergyThe Nature of Energy
Systems and SurroundingsSystems and SurroundingsSystem: part of the universe we are interested in.Surroundings: the rest of the universe.
Copyright 1999, PRENTICE HALL Chapter 5 6
First Law of ThermodynamicsFirst Law of Thermodynamics
Internal EnergyInternal Energy• Internal Energy:
total energy of a system.
• Cannot measure absolute internal energy.
• Change in internal energy, E = Efinal - Einitial
Copyright 1999, PRENTICE HALL Chapter 5 7
Relating Relating EE to Heat and Work to Heat and WorkEnergy 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 when a system undergoes a physical or chemical change, the change in internal energy is given by the heat added to or change in internal energy is given by the heat added to or absorbed by the system plus the work done on or by the absorbed by the system plus the work done on or by the system:system:
EE = = qq + + ww
First Law of ThermodynamicsFirst Law of Thermodynamics
Copyright 1999, PRENTICE HALL Chapter 5 8
Relating Relating EE to Heat and Work to Heat and Work
First Law of ThermodynamicsFirst Law of Thermodynamics
Copyright 1999, PRENTICE HALL Chapter 5 9
Relating Relating EE to Heat and Work to Heat and Work
First Law of ThermodynamicsFirst Law of Thermodynamics
Copyright 1999, PRENTICE HALL Chapter 5 10
Endothermic and Exothermic ProcessesEndothermic and Exothermic ProcessesEndothermic: absorbs heat from the surroundings.Exothermic: transfers heat to the surroundings.An endothermic reaction feels cold.An exothermic reaction feels hot.
First Law of ThermodynamicsFirst Law of Thermodynamics
Copyright 1999, PRENTICE HALL Chapter 5 11
State FunctionsState FunctionsState function: depends only on the initial and final states of system, not on how the internal energy is used.
First Law of ThermodynamicsFirst Law of Thermodynamics
Copyright 1999, PRENTICE HALL Chapter 5 12
State FunctionsState Functions
First Law of ThermodynamicsFirst Law of Thermodynamics
Copyright 1999, PRENTICE HALL Chapter 5 13
Enthalpy, H: Heat transferred between the system and surroundings carried out under constant pressure.Can only measure the change in enthalpy:
H = Hfinal - Hinitial = qP
EnthalpyEnthalpy
Copyright 1999, PRENTICE HALL Chapter 5 14
For a reaction
Hrxn = H(products) - H (reactants)
Enthalpy is an extensive property (magnitude H is
directly proportional to amount):
CH4(g) + 2O2(g) CO2(g) + 2H2O(g) H = -802 kJ
2CH4(g) + 4O2(g) 2CO2(g) + 4H2O(g) H = -1604 kJ
When we reverse a reaction, we change the sign of H:
CO2(g) + 2H2O(g) CH4(g) + 2O2(g) H = +802 kJ
Change in enthalpy depends on state:
H2O(g) H2O(l) H = -88 kJ
Enthalpies of Reaction Enthalpies of Reaction
Copyright 1999, PRENTICE HALL Chapter 5 15
Heat Capacity and Specific HeatHeat Capacity and Specific HeatCalorimetry = 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.
Calorimetry Calorimetry
Copyright 1999, PRENTICE HALL Chapter 5 16
Heat Capacity and Specific HeatHeat Capacity and Specific HeatCalorimetry Calorimetry
Copyright 1999, PRENTICE HALL Chapter 5 17
Constant-Pressure Constant-Pressure CalorimetryCalorimetryAtmospheric pressure is constant!
H = qP
qrxn = -qsoln = -(specific heat of solution) (grams of solution)
T.
Calorimetry Calorimetry
Copyright 1999, PRENTICE HALL Chapter 5 18
Bomb Calorimetry (Constant-Volume Bomb Calorimetry (Constant-Volume Calorimetry)Calorimetry)Reaction carried out under constant volume.Use a bomb calorimeter.Usually study combustion.
Calorimetry Calorimetry
Copyright 1999, PRENTICE HALL Chapter 5 19
qrxn = -CcalorimeterT.
Bomb Calorimetry (Constant-Volume Bomb Calorimetry (Constant-Volume Calorimetry)Calorimetry)
Calorimetry Calorimetry
Copyright 1999, PRENTICE HALL Chapter 5 20
• Hess’s lawHess’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
Hess’s Law Hess’s Law
Copyright 1999, PRENTICE HALL Chapter 5 21
In the above enthalpy diagram note that H1 = H2 + H3
Hess’s Law Hess’s Law
Copyright 1999, PRENTICE HALL Chapter 5 22
Enthalpies of FormationEnthalpies 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, Ho
f .
• 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 23
Enthalpies of FormationEnthalpies of Formation• Standard enthalpy of formation of the most stable
form of an element is zero.
Copyright 1999, PRENTICE HALL Chapter 5 24
Hrxn = H1 + H2 + H3
Enthalpies of FormationEnthalpies of Formation
Using Enthalpies of Formation to Calculate Using Enthalpies of Formation to Calculate Enthalpies of ReactionEnthalpies of ReactionWe use Hess’ Law to calculate enthalpies of a reaction from enthalpies of formation.
Copyright 1999, PRENTICE HALL Chapter 5 25
reactants products rxnf fH m H n H
Enthalpies of FormationEnthalpies of Formation
Using Enthalpies of Formation to Calculate Using Enthalpies of Formation to Calculate Enthalpies of ReactionEnthalpies of ReactionFor a reaction:
Copyright 1999, PRENTICE HALL Chapter 5 26
Foods and FuelsFoods and Fuels
FoodsFoods •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
•Fats break down as follows:2C57H110O6 + 163O2 114CO2 + 110H2O, H = -75,520 kJ
•Fats: contain more energy; are not water soluble, so are good for energy storage.
Copyright 1999, PRENTICE HALL Chapter 5 27
Foods and FuelsFoods and Fuels
FoodsFoods
Copyright 1999, PRENTICE HALL Chapter 5 28
Foods and FuelsFoods and FuelsFuelsFuels 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 29
Foods and FuelsFoods and FuelsFuelsFuels 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 30
End of Chapter 5End of Chapter 5
ThermochemistryThermochemistry