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Chapter 5 Thermochemistry Introduction/Definitions 5.1 The Nature of Energy Thermodynamics is the study of energy and its transformations. Thermochemistry is the study of the relationships between chemical reactions and energy changes involving heat. Definitions: Energy is the capacity to do work or to transfer thermal energy (heat). Work is energy used to cause an object with mass to move (ie. Atoms, molecules, electrons, etc.) Heat is the thermal energy used to cause the temperature of an object to increase. A force, F is any kind of push of pull exerted on an object (ie. Atoms, molecules, electrons, etc.) Kinetic and Potential Energies Kinetic Energy is the energy of motion: Ek = ½ m v2 Potential Energy is the energy an object possesses by virtue of its position or composition. (stored, potential to do “work”) Examples: interaction between magnets, ball on top of a hill. Energy Units: SI unit is the joule, J. 1 J = 1kg x m2/s2 Derived from on Ek = ½ mass (velocity)2, So, kg(m/s)2 = kg (m2/s2) Traditionally, we use the calorie, cal as a unit of heat energy. 1 cal = 4.184 J by definition. 1 calorie is the energy to heat 1 gram water, 1 oCelcius. (**We apply this definition when trapping thermal energy “into” or “out of” a water sample. We call such experimentation: “calorimetry”**) Nutritional Calories = Cal = 1 kcal = 1000 calories = 4184 Joules = 4.184 kJ Systems A system is the part of the universe we are focused on studying. Surroundings are the rest of the universe (i.e. the surroundings are the portions of the universe that are not invoved in the system). Example: If we are interested in the interaction between hydrogen and oxygen in a cylinder, then the H2 and O2 in the cylinder form a system and the surroundings are the __________________. Transferring Energy: Work and Heat1 From physics (next year): Force is a push or pull on an object. Work is the energy used to move an object against a force. W = F x d. Heat is the thermal energy transferred from a hotter object to a colder one. Energy is the capacity to do work or to transfer thermal energy (heat something).