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, q p • Δ E = q p – P Δ V • q p = Δ E + PV • Δ H = Δ E + P Δ V (definition) • Δ H = q p Enthalpy...
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This note was uploaded on 01/17/2012 for the course S 117 taught by Professor Stephenjacobson during the Fall '11 term at Indiana.

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ThermochemistryCh9 - Thermochemistry • Internal Energy...

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