ThermochemistryCh9

ThermochemistryCh9 - Thermochemistry Internal Energy...

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Unformatted text preview: Thermochemistry Internal Energy Kinetic energy Potential energy Thermochemistry Internal Energy Kinetic energy Potential energy Chemical Energy Changes System and Surroundings Exothermic Reaction Endothermic Reaction Thermochemistry Thermochemisty is the study of the relationship between heat and chemical reactions. 1. Kinetic energy is energy possessed by matter because it is in motion Thermal energy-- random motion of the particles in any sample above 0 K Heat-- causes a change in the thermal energy of a sample. Flows from hot to cold Heat Potential Energy 2. Potential energy is energy possessed by matter because of its position or condition. A brick on top of a building has potential energy that is converted to kinetic energy when it is dropped on your head Chemical energy is energy possessed by atoms as a result of forces which hold the atoms together (Boxes!) Where is the Energy? Definitions we will use: System: Reaction (bonds) Surrounding: solvent, reaction vessel, air, etc. An everyday example: burning wood Initially, much energy stored as potential in C-H bonds, little kinetic energy in the air Finally, lower potential energy in the C=O bonds, higher kinetic energy in the air Total Energy Total Energy = kinetic + potential Law of Conservation of Energy- The total energy of universe is constant Internal Energy- E- the sum of all the kinetic and potential energies of all the atoms and molecules in a sample. Change in Energy of System Change in internal energy of system = heat + work Convention: point of view of system Change in Internal Energy E = q + w Work = Force x distance What happens to your internal energy when you push a boulder? What happens to your internal energy when you push a boulder on a rough surface? Chemical Work W = F x h P = F/A W = P x A x h W = PV Sign Convention: W = -P V Test Your Understanding For the following three reactions: Are they performed under constant pressure or not? What is the sign of work in each case? State Function State Function Internal Energy Pressure Volume Path Dependent Work heat Property depends only on present state Enthalpy Most reactions are done in open containers, so P is constant Need a term for constant pressure where only work is PV At constant pressure, qp E = qp PV qp = E + PV H = E + PV (definition) H = qp Enthalpy...
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ThermochemistryCh9 - Thermochemistry Internal Energy...

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