Lecture Set 1--Thermochemistry

Lecture Set 1--Thermochemistry - Thermochemistry Energy...

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Energy (U): capacity to supply heat or do work Heat (q): transfer of energy from high to low Temperature (T): measure of thermal motion Work (w): transfer of energy by moving an object against a force * (force x position change) or * (pressure x volume change) Thermochemistry •Divide the world in two: •System: part we study •Surroundings: everything else •open; closed; isolated Thermodynamics Thermo (heat) + dynamics (motion) • Heat and Work are energy transfer functions. • First Law of Thermodynamics U = q + w • Energy is conserved: U sys Any change in energy of the system must be brought about via a transfer of heat or work with the surroundings.

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Forms of Energy • Energy of system can be in many forms: * Kinetic (motion - temperature): E K * Potential (position, composition): E P • Energy of system or surroundings is total: U system = E K,system + E P,system Forms of Energy, cont’d * Units: Joules (J) = kg m 2 /s 2 * Kinetic: energy of motion E K = 1/2 m v 2 m=mass (kg); v=velocity (m/s) * Potential: energy of position (in a force field) or • Position in a gravitational field (at earth’s surface): E p = m g Δ h m=mass (kg); g=accel. (m/s 2 ); h=height (m) Example:Conservation of Energy Consider a 165 lb. person on a 15 ft. diving board Δ E p = m g Δ h m = 165 lb * 1 kg/2.20 lb = 75.0 kg g = 9.81 m/s 2 Δ h = 15 ft * .305 m/ft = 4.6 m Δ E p h e i g t o r a v . p n l
More on Potential Energy • Position: relative to what? * Source of gravitational or electromagnetic field • Distance between two masses • Distance between charged particles • Distance between oriented spinning charges • Composition: relative to what? * Different chemical/physical form •C 8 H 18 (gasoline) + O 2 vs • steam vs water vs ice Why Chemists Care: • Can predict * whether reaction tends to occur * amount of heat or work obtained or consumed * final composition after reaction • Heat Flow During Process: * system absorbs heat: Endothermic * system gives off heat: Exothermic Endothermic Reaction

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Exothermic Reaction T sys time Endothermic T sys time Exothermic q sys > 0 (q surr < 0) q sys < 0 (q surr > 0) rxn starts rxn starts
Chemical Work • Usually only have “PV” work: (PV) *" " always means "final minus initial" * For reactions: "products minus reactants" • System in cylinder at constant Pressure: • If surroundings do work on system (compress) * V of system decreases, so P V < 0 * Energy of system increases ∗Δ U = q + w; let q = 0 (no heat exchanged)

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This note was uploaded on 04/30/2009 for the course CHEM 2B taught by Professor Clemens during the Spring '08 term at UC Davis.

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Lecture Set 1--Thermochemistry - Thermochemistry Energy...

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