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Please Sit with Your Group

Please Sit with Your Group. Please be sure each member of your team has a copy of Energy Cycles Lecture Notes Ice, Water, Steam Problem Set Today’s reporter is the most talkative person. Energy Cycles. E. A. Mottel Integrated, First-Year Curriculum in Science, Engineering and Mathematics.

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Please Sit with Your Group

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  1. Please Sit with Your Group • Please be sure each member of your team has a copy of • Energy Cycles Lecture Notes • Ice, Water, Steam Problem Set • Today’s reporter is the most talkative person

  2. Energy Cycles E. A. Mottel Integrated, First-Year Curriculum in Science, Engineering and Mathematics

  3. Energy Cycles • This lecture involves the concept of thermodynamic energy cycles and calculation of the heat energy released by these cyclic processes. • The importance of different pathways is exemplified by the Carnot cycle and Hess' Law.

  4. Energy Cycles • Change in Enthalpy (isobaric) • Isothermal • Constant external pressure • Varying external pressure • Adiabatic

  5. Energy Cycle • A series of energy steps following a defined pathway in which the final step returns the system to the original state conditions.

  6. sublimation Sublimation of Water at 0 °C H2O (l, 100 °C) heat capacity vaporization H2O (l, 0 °C) H2O (g, 100 °C) fusion heat capacity H2O (s, 0 °C) H2O (g, 0 °C) What thermodynamic terms are associated with each step?

  7. H2O (l, 0 °C) H2O (g, 100 °C) Determine the Enthalpy ofSublimation of Water at 0 °C H2O (l, 100 °C) -100 cal/g -418 J/g -540 cal/g -2259 j/g -80 cal/g -335 J/g +50 cal/g +209 J/g sublimation H2O (s, 0 °C) H2O (g, 0 °C) +670 cal/g +2803 J/g Assume the heat capacity of water vapor is constant from 0 °C to 200 °C.

  8. Ice, Water, Steam Problem Set • Select various masses of ice, water and steam at temperatures consistent with the phases. • Determine the final temperature of the mixture and the number of grams of each phase present in the final mixture.

  9. ¬1.5 atm ¬1.5 atm ¬1.5 atm Isothermal ExpansionConstant External Pressure The initial and final temperature of the gas in the cylinder is the same. Sketch a graph of this process One liter of a compressed gas in a cylinder causes a piston to expand against a constant external pressure of 1.5 atm until the total volume of the gas in the cylinder is three liters.

  10. 2 Pext (atm) 1 0 0 1 2 3 4 Volume (L) Vf Vf ó ó work = - P dV = - P dV õ õ Vi Vi Isothermal ExpansionConstant External Pressure Work may be represented as the area under the curve. = - P (Vf - Vi) Because pressure is constant, it can be factored out of the integral.

  11. Energy Units • L×atm can be converted to more common energy units (e.g., J or cal) by using the value of R as a conversion factor. work = - (1.5 atm) (3.0 L - 1.0 L) = -3.0 L×atm Determine the work done in calories and joules

  12. 1.987 cal×mol-1×K -1 = -3.0 L×atm × = -73 cal 0.08206 L×atm×mol-1×K -1 8.314 J×mol-1×K -1 = -3.0 L×atm × = -304 J 0.08206 L×atm×mol-1×K -1 Energy Units work = - (1.5 atm) (3.0 L - 1.0 L) = -3.0 L×atm Determine the work done in calories and joules

  13. Engines • An engine is a machine which can perform work. • The expansion of a gas in a piston can do work. • Describe the activities of an expanding gas at constant external pressure.

  14. ¬1.5 atm ¬1.5 atm ¬1.5 atm ¬1.5 atm ¬1.5 atm EnginesConstant External Pressure Compression Stroke Expansion Stroke

  15. 2 Pext (atm) 1 compression cycle 0 0 1 2 3 4 Volume (L) Engines expansion cycle As described this does not represent a practical engine because the final state does not equal the initial state. How much work is done in this overall process?

  16. ¬4.5 atm ¬2.25 atm ¬1.5 atm Isothermal ExpansionDecreasing External Pressure The initial and final temperature of the gas in the cylinder is the same. Sketch a graph of this process One liter of a compressed gas in a cylinder causes a piston to expand against a decreasing external pressure until the total volume of the gas in the cylinder is three liters.

  17. Isothermal ExpansionDecreasing External Pressure 4 3 PV = nRT Work may be represented as the area under the curve. 2 Pext (atm) 1 0 0 1 2 3 4 Volume (L) Vf Vf Vf nRT ó ó æ ö work = - P dV = - dV = - nRT ln õ õ è ø V Vi Vi Vi Isothermal ExpansionDecreasing External Pressure The pressure term is rewritten in terms of volume.

  18. Isothermal ExpansionDecreasing External Pressure 4 3 2 Pext (atm) 1 0 0 1 2 3 4 DE = Cv× DT Volume (L) isothermal Is there any net heat flow in this process? Is heat flowing into or out of the system? DE = q + w = 0 the internal energy of a phase is a function of its temperature

  19. Isothermal ExpansionDecreasing External Pressure 4 3 2 Constant External Pressure Pext (atm) 1 0 0 1 2 3 4 Volume (L) The decreasing external pressure piston performs more work (greater efficiency) than a piston working against a constant external pressure equal to Pfinal.

  20. Isothermal ExpansionDecreasing External Pressure 4 3 2 Pext (atm) 1 0 0 1 2 3 4 Volume (L) If the process is reversed, how much work is done?

  21. Room temperature gas colder gas q = 0 Gas Expansion When a gas expands against a low restraining pressure why does it cool? adiabatic expansion DE = q + w

  22. Gas ExpansionAir Conditioner

  23. gaseous CO2 liquid CO2 Gas ExpansionCarbon Dioxide Fire Extinguisher Why is it dangerous to point a carbon dioxide fire extinguisher at a person?

  24. Adiabatic Expansion • The same process occurs, except there is no heat flow allowed between the system and the surroundings. • On expansion, the gas will cool and follows a non-isothermal PV curve. • PVg = constant • for an ideal diatomic gas, g =1.67

  25. Adiabatic Expansion • In each of the examples, a different pressure change pathway is followed by the gas. • How much work will be done if the process is reversed to complete the cycle?

  26. Carnot Cycle • Consists of two isothermal and two adiabatic steps, occurring alternatively. • One of each type of step is involved in compression and expansion.

  27. Hess' Law Elements DHf,products - DHf,reactants DHrx Reactants Products The enthalpy change of a chemical reaction is equal to the difference of the enthalpy of the products and the enthalpy of the reactants.

  28. Hess' Law • DHrx = S DHf,products - SDHf,reactants • The system is exothermic if the enthalpy change is negative (DHrx < 0) and endothermic if the enthalpy change is positive (DHrx > 0).

  29. Thermodynamic ApplicationsComparison of Liquid Fuels • isooctane • methanol • ethanol

  30. Please Sit with Your Group • Please be sure each member of your team has a copy of • Thermodynamics Applications Lecture Notes • Today’s reporter is the most talkative person • Reading Assignment: Oxidation-Reduction: Zumdahl Chap. 4.10-4.12

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