Chem121-Chapter-5-SH

Chem121-Chapter-5-SH - Chapter 5: Thermochemistry 2009 R....

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Chapter 5: Thermochemistry © 2009 R. Spinney The world’s 1 st calorimeter The Nature of Energy Thermochemistry is the study of the energy changes in chemical reactions. Energy is the capacity to do work or transfer heat . Work is the energy used to move an object. Heat is the energy used to increase the temperature of an object. The Nature of Energy (cont’d) Work: in general work is done by moving an object through a distance by applying a force to the object, i.e. And in SI, force is mass times acceleration, The units are:
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The Nature of Energy (cont’d) There are several basic forms of energy, Kinetic energy is the energy of motion. E k = ½ mv 2 Energy = kg (m/s) 2 = kg m 2 /s 2 = J The Nature of Energy (cont’d) Potential energy is energy stored in an object due to its position in a “field of force” or its “composition”. Position : in a field of force , i.e. gravitational (E p = mgh), electrical (E p = kQ 1 Q 2 /r) The Nature of Energy (cont’d) Composition: chemical energy is stored in the “bonds” between the atoms, i.e. BDE
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The Nature of Energy (cont’d) System : defines the contents that are being studied, i.e. the contents of your beaker. The surroundings is the rest of the universe. The distinction is important as the system can do work on the surrounding and transfer energy (heat) to or from it. The First Law of Thermodynamics Also known as the law of conservation of energy: energy can neither be created or destroyed, only converted from one form to another. if the system loses energy the surroundings must gain the same amount of energy. Energy can be transferred as heat which is transferred from hot objects to colder ones. The First Law of Thermodynamics (cont’d) Internal energy : E is the sum of kinetic & potential energies of the particles that make up the system. -E k p
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The First Law of Thermodynamics (cont’d) E can not be determined directly, but the change in E can be, i.e. Δ E which is given by Δ E = final state – initial state Δ E = E f –E i Sign is important: The First Law of Thermodynamics (cont’d) Relating Δ E to work & heat : during a chemical change in a system, heat energy may be absorbed from or released to the surroundings, and work may be done on or by the system, Δ E = q + w This is the 1 st law of thermodynamics in equation form. The First Law of Thermodynamics (cont’d) Both q (heat) and w (work) refer to the system. The signs are very important. The conventions are: +q -q +w -w
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The First Law of Thermodynamics (cont’d) In ordinary chemical reactions work arises from pressure-volume changes. If the reaction increases the volume, work is done against the pressure of the atmosphere. PV has units of work. P is force/unit area PV = The First Law of Thermodynamics (cont’d) Endothermic & Exothermic Reactions: - if heat is lost to the surroundings it is an
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Chem121-Chapter-5-SH - Chapter 5: Thermochemistry 2009 R....

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