ThermochemistryCh9

ThermochemistryCh9 - Thermochemistry Internal Energy...

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Thermochemistry Internal Energy Kinetic energy Potential energy
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Thermochemistry Internal Energy Kinetic energy Potential energy
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Chemical Energy Changes System and Surroundings Exothermic Reaction
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Endothermic Reaction
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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
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Heat
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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!)
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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
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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.
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Change in Energy of System Change in internal energy of system = heat + work Convention: point of view of system
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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?
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Chemical Work ׀ W ׀ = ׀ F x h ׀ P = F/A ׀ W ׀ = ׀ P x A x h ׀ ׀ W ׀ = ׀ PV ׀ Sign Convention: W = -P V
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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?
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State Function State Function – Internal Energy – Pressure – Volume Path Dependent – Work – heat Property depends only on present state
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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
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