chapter_05au_1

chapter_05au_1 - Thermochemistry Exam 1 Distribution Ave F...

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Unformatted text preview: Thermochemistry Exam 1 Distribution Ave F D C B A Thermochemistry Chapter 5 Thermochemistry Thermochemistry Energy The ability to do work or transfer heat. Work : Energy used to cause an object that has mass to move. Work is movement against an opposing force Heat : Energy used to cause the temperature of an object to rise. Thermochemistry Two kinds of energy: Potential Energy Kinetic energy Energy an object possesses by virtue of its position or chemical composition . h Energy an object possesses by virtue of its motion. 1 2 KE = mv 2 E = mgh Thermochemistry When the child is at points (a) and (c) they have only PE; at point (b) only KE. Total energy is conserved (PE + KE = constant). Pendulum (position) Transfer of energy Thermochemistry A Spring (arrangement) When fully compressed or extended only PE; at natural length only KE. Total energy is conserved. Thermochemistry How does Potential energy relate to Atom Work is required to pull the negatively charged electron away from the positively charged nucleus (movement against restoring force) e- Coul. Force + Work = loss of PE Thermochemistry PE (Bond energy in a chemical reaction) Movie Reactions: weaker bonds stronger bonds + heat Stronger bonds give lower potential energy which is converted to thermal (kinetic energy) 2NI 3 N 2 + 3I 2 + heat Weak strong strong Thermochemistry Changes in Internal Energy ( Movie ) E < endergonic exergonic Thermochemistry Units of Energy The SI unit of energy is the joule (J) . 1 Joule = 1 Newton-meter 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 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). Thermochemistry Conservation of energy Energy is neither created nor destroyed. In other words, the total energy of the universe is a constant; if the system loses energy, it must be gained by the surroundings, and vice versa. 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. By definition, the change in internal energy, E , is the final energy of the system minus the initial energy of the system: E = E final E initial Use Fig. 5.5 Thermochemistry Two ways to exchange energy When energy is exchanged between the system and the surroundings, it is exchanged as either heat ( q ) or work ( w ). Ignore sign for the moment Thermochemistry Work Energy used to move an object over some distance. w = F d , where w is work, F is the force, and d is the distance over which the force is exerted. Thermochemistry Work by gas can be expressed as PV work....
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chapter_05au_1 - Thermochemistry Exam 1 Distribution Ave F...

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