Thermochemistry Thermodynamics = study of energy and its transformations Thermochemistry = study of...
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Transcript of Thermochemistry Thermodynamics = study of energy and its transformations Thermochemistry = study of...
Thermochemistry
Thermochemistry
Thermodynamics = study of energy and its transformations
Thermochemistry = study of chemical reactions involving changes in heat energy
© 2009, Prentice-Hall, Inc.
Thermochemistry
© 2009, Prentice-Hall, Inc.
Energy
Energy = the ability to do work or transfer heat energy.
Work = energy used to cause an object with mass to move (w = f x d)
Heat = energy used to cause the temperature of an object to increase
Thermochemistry
© 2009, Prentice-Hall, Inc.
Major Types of EnergyPotential energy = energy an object possesses by
virtue of its position or chemical composition.
Kinetic energy = energy an object possesses by virtue of its motion.
Thermochemistry
Kinetic Energy
m = mass in kilograms (kg)
v = velocity in meters per second (m/s)
KE = kinetic energy in joules (J)
1 Joule = 1 kg-m2/s2
A mass of 2 kg moving at a speed of one meter per second possesses a kinetic energy of 1 Joule.
© 2009, Prentice-Hall, Inc.
12KE = m v 2
Thermochemistry
Potential Energy
m = mass in kilograms (kg)
g = acceleration due to gravity (9.8 m/s2)
h = height (m)
PE = potential energy in joules (J)
1 Joule = 1 kg-m2/s2
© 2009, Prentice-Hall, Inc.
PE = m g h
Thermochemistry
© 2009, Prentice-Hall, Inc.
Units of Energy• The SI unit of energy is the joule (J).
• An older, non-SI unit is still in widespread use: the calorie (cal).
1 cal = 4.184 J
• 1000 calories = one nutritional Calorie
Thermochemistry
© 2009, Prentice-Hall, Inc.
First Law of Thermodynamics• Energy is neither created nor destroyed, but it can undergo a transformation from
one type to another. (Law of Conservation of Energy)
• The total energy of the universe is a constant.
• The energy lost by a system must equal the energy gained by its surroundings, and vice versa.
Thermochemistry
© 2009, Prentice-Hall, Inc.
System and Surroundings
System = the molecules we want to study (here, the hydrogen and oxygen molecules).
Surroundings = everything else (here, the cylinder and piston).
Thermochemistry
© 2009, Prentice-Hall, Inc.
Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system; we call it E.
E = Efinal − Einitial (It’s a state function)
• If E is positive, the system absorbed energy from the surroundings.
• If E is negative, the system released energy to the surroundings.
Thermochemistry
© 2009, Prentice-Hall, Inc.
E = q + w
• When energy is exchanged between the system and the surroundings, it is exchanged as either heat (q) or work (w).
• That is, E = q + w.
Thermochemistry
q, w, and their signs+ q = system gains or takes in heat
- q = system loses or gives off heat
+ w = work is done on the system by the surroundings (piston pushed in)
- w = work is done by the system on its surroundings (piston moves out)
© 2009, Prentice-Hall, Inc.
Thermochemistry
ExampleAs hydrogen and oxygen gas are ignited in a cylinder,
the system loses 550 J of heat to its surroundings. The expanding gases move a pistion to do 240 J of work on its surroundings. E for system = ?
Answer: E = q + w
E = (-550 J) + (-240 J)E = - 790 J
What does it mean?The system gave off 790 J of energy to its surroundings
© 2009, Prentice-Hall, Inc.
Thermochemistry
© 2009, Prentice-Hall, Inc.
Enthalpy & H• The symbol for enthalpy is H.• Enthalpy is the internal energy plus the product of
pressure and volume:
• At constant pressure:H =E = q
• So at constant pressure, heat lost or gained by the system.
H = E + PV
Thermochemistry
© 2009, Prentice-Hall, Inc.
Recall: Endothermic• When heat is absorbed (taken in) by the system from the surroundings, the process
is endothermic.
H = Hfinal − Hinitial
H = Hproducts − Hreactants
H = positive value for endothermic
Thermochemistry
© 2009, Prentice-Hall, Inc.
Recall: Exothermic When heat is released (given off) by the system into the surroundings, the process is
exothermic.
H = Hfinal − Hinitial
H = Hproducts − Hreactants
H = negative value for exothermic