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# chapter 6 slides - 1 THERMODYNAMICS The First Law Chapter 6...

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Unformatted text preview: 1 THERMODYNAMICS: The First Law Chapter 6 CHEM 6B Dr. DiPasquale CHEMICAL PRINCIPLES: A Quest for Insight – Atkins & Jones THERMODYNAMICS: The First Law DON’T TALK ABOUT THERMODYNAMICS! 2 THERMODYNAMICS: The First Law c Thermodynamics – the study of the transformation of energy c First Law of Thermodynamics – Law of Conservation of Energy states: c Energy is neither created nor destroyed, but transferred or converted from one form to another. c Total amount of energy in the universe is constant. Systems c The system is the sample or reaction mixture of interest. Outside the system are the surroundings. Both together are the universe. c Open System – matter and energy exchange c Closed System – only energy exchange c Isolated or Insulated System – no exchange 3 Work and Energy c Work = force × distance c 1 joule (J) = 1 kg m 2 s-2 = 1 N m c Capacity of a system to do work = internal energy ( U ) – not measurable c We can measure changes in internal energy, designated as ∆ U (where ∆ U = U f – U i ). c Work done on a system increases the internal energy of that system. Types of Work c Expansion: – P ex ∆ V c Extension: f ∆ l c Elevation: mg ∆ h c Electrical: φ ∆ q c Surface Expansion: γ ∆ A 4 Example Problem c If an ideal gas expands by 750 mL (0.750 L) against a pressure of 2.00 atm, how much work is done in the expansion? c w = – P ex ∆ V = –(2.00 atm) × (0.750 L) c w = – 1.50 L·atm c 1 L·atm = 101.325 J c w = – 152 J Reversibility c reversible process – a process that can be reversed by an infinitesimal change in a variable; equilibrium reactions c irreversible process – a process that can not be reversed by an infinitesimal change in a variable; precipitation reactions c reversible processes are of greatest importance to thermodynamics; irreversible processes - kinetics 5 Reversible, Isothermal Expansion of an Ideal Gas c 1.00 mol of an ideal gas expands reversibly from 1.00L to 22.4L at STP. How much work is done? ln final initial V w nRT V = − Reversible, Isothermal Expansion of an Ideal Gas-1-1 22.4 (1.00 mol)(8.31447 J K mol )(298.15 K) ln 1.00 w = − 7707.235 J 7.71 kJ w w = − = − 6 Heat c Heat ( q ) – energy transferred as a result of a temperature difference c Change of internal energy by transfer of heat: c ∆ U = q c 1 cal (calorie) = 4.184 J (exactly) c The energy needed to raise the temperature of 1 g H 2 O by 1 ° C c 1 Cal (nutritional calorie) = 1 kcal Heat c Sign of q indicates whether heat is entering or leaving the system. c q = positive: energy enters as heat, internal energy increases: endothermic → feels cold to the touch c q = negative; energy leaves as heat, internal energy decreases: exothermic → feels warm to the touch 7 Measurement of Heat c Heat capacity – the ratio of the heat supplied to the temperature rise produced of a given object: c C = q / ∆T (or rewritten as q = C∆T ) c specific heat (capacity): C s = C / m c molar heat capacity:...
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chapter 6 slides - 1 THERMODYNAMICS The First Law Chapter 6...

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