Chap5 - Chem 200 Dr Saidane Lecture Notes Chapter 5...

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Chem 200 Dr. Saidane Lecture Notes Chapter 5 Thermochemistry ENERGY AND ITS FORMS Energy is defined as the capacity to do work or to transfer heat. In chemistry, energy can be potential (stored), kinetic (motion), thermal (heat), radiant (light), and electrical (transfer of electrons). Potential energy is possessed by an object by virtue of its position, and is associated with attractions and repulsions. The higher an object is the more potential energy it has. When an object moves, its potential energy is converted into kinetic energy . KE = ½ mv 2 . m is the mass of the object and v its velocity. Heat is the form of energy that is often associated with changing the temperature of an object. An object’s temperature increases because energy is transferred to it. The energy transfer, in the form of heat, happens when two objects of different temperature are brought into contact to establish thermal equilibrium (to equalize their temperatures). Heat is the transfer of energy from regions of high temperature to regions of low temperature . Thermal energy is a synonym of heat. It is associated with the motion of atoms or molecules. The more rapid the motion, the greater the thermal energy. The law of conservation of energy tells us that energy cannot be created or destroyed. It can only be transformed from one form to another or transferred from one place to another. When there is a transfer, the energy moves from the system to the surrounding . The system is the sample or reaction mixture that we are interested in. Outside the system are the surroundings. The system plus the surrounding is called the universe . When energy leaves a system as a result of a temperature difference between the system and the surrounding, we say that the system has lost energy as heat. It is an exothermic process . This transfer of energy stimulates random thermal motion of
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molecules in the surroundings. Inversely, when energy is gained by the system, it is an endothermic process . Thermochemistry is the science of heat or thermal energy transfer and its application to chemical reactions and processes. UNITS OF ENERGY Most forms of energy can be converted into heat. This is why most units of energy are units that measure heat.
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