Chapter 6 - Chapter 6 Thermochemistry 6.1 The Nature of...

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Chapter 6 Thermochemistry 6.1 The Nature of Energy Energy is defined as the capacity to do work or to produce heat . In this chapter we will concentrate specifically on the heat transfer that accompanies chemical processes. One of the most important characteristics of energy is that it is conserved. The law of conservation of energy states that “energy can be converted from one form to another but can neither created nor destroyed”. That is the energy of the universe is constant. Energy can be classified as either potential or kinetic . Potential energy : It is due to position . Attractive and repulsive forces also lead to potential energy. Kinetic energy : It is due to motion of the object and depends on the mass of the object in a velocity ( υ ): KE = 1/2 m υ 2
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Energy and Work: Energy is capacity to do work . Work is defined as the force to a distance . State function : It is a property of the system and independent on the pathway taken in going from one state to another (depends only on its state and not on how the system arrived at that state). Energy is a state function, while work and heat are not. It is important to recognize that heat and temperature are different. Chemical Energy : The universe is divided into two parts: 1. The system: It is the part of the universe on which we wish to focus attention (e.g. the reactants and products in a given chemical reaction. 2. The surroundings: It includes every thing else in the universe (e.g. the reaction container). 2
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: 1. Exothermic reactions: When heat is released out of the system CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) + (heat energy) 2. Endothermic reactions : When heat is absorbed into the system N 2 (g) + O 2 (g) + (heat energy) 2NO(g) Since total energy is conserved, the energy gained (or lost) by the surrounding must be equal to the energy lost (or gained) by the system . The study of energy and its interconversion is called thermodynamics . The law of conservation of energy is called the first law of thermodynamics . It stated that the energy of the universe is constant . 3
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Chapter 6 - Chapter 6 Thermochemistry 6.1 The Nature of...

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