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5 Thermochemistry - Chapter 5 Thermochemistry...

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1 Chapter 5 Thermochemistry Definitions: - Thermochemistry is the study of heat change in chemical reactions. - Heat is the transfer of thermal energy between two bodies that are at different temperatures. - Work is the energy used to cause an object that has mass to move. 2
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2 - Potential Energy is the energy an object possesses by virtue of its position or chemical composition. Definitions: Example: The electrostatic potential energy between two charged particles is: E el = κ Q 1 Q 2 d Charges of the particules Distance between their centers Proportionality constant (8.99x10 9 jm/C 2 ) 1 2 K E = mv 2 - Kinetic Energy is the energy an object possesses by virtue of its motion. Definitions: m = mass (kg) v = speed (m/s) 4
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3 Units of Energy The SI unit of energy is the joule (J). 1 J = 1  kg m 2 s 2 An older, non-SI unit is still in widespread use: The calorie (cal). 1 cal = 4.184 J System and Surroundings Hydrogen Oxygen Water Represent the system The rest of the universe represents the surroundings The reaction generates heat which is transferred from the system to its surroundings. This reaction is Exothermic Example: Combustion of hydrogen 2H 2 + O 2 2 H 2 O + Energy 6
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4 Work Energy used to move an object over some distance. w = F x d w is work F is the force d is the distance over which the force is exerted 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. 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. 8 First Law of Thermodynamics
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5 Internal Energy (cont) 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 Changes in Internal Energy • If E > 0, E final > E initial – Therefore, the system absorbed energy from the surroundings.
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