Chapter 5 Lecture Notes

Chapter 5 Lecture Notes - Chapter 5 Thermochemistry The...

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Thermochemistry Chapter 5 Thermochemistry The energy of chemical reactions. How do you keep track of it? Where does it come from?
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Thermochemistry Energy • The ability to: • do work • transfer heat. Work: Energy used to cause an object that has mass to move. Heat: Energy used to cause the temperature of an object to rise. We will see that both are due to molecular motion.
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Thermochemistry Potential Energy Energy an object possesses by virtue of its position or chemical composition. More potential E Less P.E. as bike goes down.
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Thermochemistry Kinetic Energy Energy an object possesses by virtue of its motion. 1 2 KE = mv 2
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Thermochemistry 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 1 J = 1  kg m 2 s 2
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Thermochemistry Energy accounting • Must identify where different types of Energy go. • Therefore, must identify the places.
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Thermochemistry System and Surroundings • The system includes the molecules we want to study (here, the hydrogen and oxygen molecules). • The surroundings are everything else (here, the cylinder and piston).
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Thermochemistry Work • Energy used to move an object over some distance. w = F d , w = work, F = force d = distance over which the force is exerted.
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Thermochemistry Heat • Energy can also be transferred as heat. • Heat flows from warmer objects to cooler objects.
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Thermochemistry Transfer of Energy a) Add P.E. to a ball by lifting it to the top of the wall
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Thermochemistry Transferal of Energy a) Add P.E. to a ball by lifting it to the top of the wall b) As the ball falls, P.E ------> K. E. (1/2mv 2 )
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Thermochemistry Transferal of Energy Ball hits ground, K.E. =0, but E has to go somewhere. So 1. Ball gets squashed 2. Heat comes out.
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Thermochemistry First Law of Thermodynamics • Energy is conserved. • In other words, the total energy of the universe is a constant; Δ E System = - Λ E surroundings
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Thermochemistry Internal Energy The 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 internal, Total = E K.E. + E P.E. + E electrons + E nuclei + . ....... almost impossible to calculate total internal energy. Instead, we are always looking at change in energy ( Δ E)
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Thermochemistry Internal Energy By definition, the change in internal energy, Δ E , is the final energy of the system minus the initial energy of the system: Δ E = E final E initial Use Fig. 5.5
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Thermochemistry Changes in Internal Energy • If Δ E > 0, E final > E initial Therefore, the system absorbed energy from the surroundings. This energy change is called endergonic .
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Thermochemistry Changes in Internal Energy • If Δ E < 0, E final < E initial Therefore, the system released energy to the surroundings.
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This note was uploaded on 05/25/2011 for the course CHEM 151 taught by Professor Jamesgeiger during the Fall '09 term at Michigan State University.

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Chapter 5 Lecture Notes - Chapter 5 Thermochemistry The...

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