Chapter 5 - Thermochemistry

Chapter 5 - Thermochemistry - Chapter 5 Thermochemistry...

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1 Chapter 5 Chapter 5 Thermochemistry Thermochemistry Thermochemistry is an aspect of thermodynamics involving the study of the relationships between chemical reactions and energy changes involving heat . Heat Energy used to cause the temperature of an object to increase. Work Energy used to cause an object that has mass to move. w = F x d
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2 Energy Energy - the capacity to do work or transfer heat . Kinetic Energy, E k - Energy an object possesses by virtue of its motion. 1 2 E k = mv 2 Potential Energy, E p: Energy an object possesses by virtue of its position or chemical composition. Thermal energy is kinetic energy in the form of random motion of the particles in any sample of matter. As temperature increases, thermal energy increases. Chemical energy is energy possessed by atoms as a result of their state of chemical combination. The energy of 2 mol of H 2 and 1 mol of O 2 is different than that of 2 mol H 2 O. Units of Energy 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 kcal = 1 Calorie (food “calorie”) 1 J = 1 ⎯⎯ kg m 2 s 2
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3 System and Surroundings System and Surroundings •The system includes the molecules we want to study (e.g. hydrogen and oxygen molecules). surroundings are everything else (here, the cylinder and piston). • In thermochemistry we study the exchange of energy between the system and surroundings. Transferal of Energy Transferal of Energy a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall.
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4 Transferal of Energy Transferal of Energy a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall. b) As the ball falls, its potential energy is converted to kinetic energy. Transferal of Energy a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall. b) As the ball falls, its potential energy is converted to kinetic energy. c) When it hits the ground, its kinetic energy falls to zero (since it is no longer moving); some of the energy does work on the ball, the rest is dissipated as heat.
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5 Internal Energy 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. Δ E = E final E initial Changes in Internal Energy Changes in Internal Energy • When energy is exchanged between the system and the surroundings, it is exchanged as either heat ( q ) or work ( w ). Δ E = q + w First Law of Thermodynamics is obeyed
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6 Δ E , , q , , w , and sign convention , and sign convention Δ E = q + w (greater than zero) (less than zero) System absorbed energy from the surroundings System released energy to the surroundings.
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Chapter 5 - Thermochemistry - Chapter 5 Thermochemistry...

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