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Lecture 17

Lecture 17. Thermochemistry – work and heat 6 .1-6.7 4 -October Assigned HW 6.4, 6.6, 6.8, 6.10, 6.12, 6.16, 6.18, 6.20, 6.24 Due: Monday 11-Oct. Review. Intermolecular Forces play an important role in liquid and solid phases Ion-ion H-bond Dipole-dipole London Forces

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Lecture 17

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  1. Lecture 17 Thermochemistry – work and heat 6.1-6.7 4-October Assigned HW 6.4, 6.6, 6.8, 6.10, 6.12, 6.16, 6.18, 6.20, 6.24 Due: Monday 11-Oct

  2. Review • Intermolecular Forces play an important role in liquid and solid phases • Ion-ion • H-bond • Dipole-dipole • London Forces • Liquids and solids with stronger intermolecular forces have higher boiling and melting points

  3. Conservation of Energy Total energy must be conserved. What type of energy is present in each case?

  4. Thermodynamics The study of how energy is transformed from one form to another and transferred from one place to another. Values refer to the SYSTEM Positive or negative? Everything Else Region of Interest Universe: System and Surroundings

  5. Thermo Nomenclature – Types of Systems Free transfer of energy AND matter Free transfer of energy ONLY Nothing in, nothing out

  6. Work and Energy The total energy stored within a system Internal Energy (U) The capacity of a system to do work

  7. Work and Energy work = opposing force * distance moved Work is done by the SYSTEM when it expands against an external pressure Work is always negative when a system expands

  8. Work and Energy Calculate the work done when 100g of water freezes and bursts a pipe that exerts a pressure of 1070 atm. Density of liquid water is 1 g/cm3 and ice is 0.92 g/cm3.

  9. Work and Variable External Pressure Consider the isothermal expansion of a gas with varying Pex. Irreversible Process: Expansion against a finite value. If a system is at 2 atm and we are expanding against an atmospheric pressure of 1 atm, a small change in the system pressure will NOT influence the direction of the piston. Reversible Process: A process that can be reversed by an INFINITELY small change in the pressure.

  10. Work – Reversible Isothermal Expansion Consider the isothermal expansion of a gas with varying Pex. External pressure = pressure of gas at every stage of a reversible process Expansion of a gas

  11. Work and Energy When expanding a gas, is there only one way to get from point A to point B? A By definition, ΔU = 0 for an isothermal expansion B

  12. Work and Energy A 2.00 L cylinder with 1mol He(g) at 30 °C. Calculate the work done by: Compressing it isothermally to 1.00L against a constant external pressure of 5.00 atm. Compressing it reversibly and isothermally.

  13. Heat What happens when a hot metal is placed in cold water? In a thermodynamic sense, heat is the ENERGY transferred when there is a change in temperature.

  14. Heat Capacity (C) The amount of energy needed to change the temperature of a substance by 1 °C. The ability of a material to ‘store’ energy. 1 cal = 4.184 J  amount of heat needed to raise 1 g of H2O by 1 K. Which of these require the most energy to change temperature?

  15. Heat Capacity (C) Calculate the heat required to raise the temperature of 10.0g of KClO4 from 25 to 900 °C. Specific heat capacity is 0.8111 J K-1 g-1

  16. Application of Heat Capacities What was the starting temperature of the lead?

  17. Application of Heat Capacities - Calorimetry

  18. The 1st Law of Thermodynamics The total change in internal energy of a closed system is the sum of heat and work This energy WILL NOT CHANGE if the system is isolated If this system loses energy by doing work or transferring heat, this energy cannot be spontaneously regenerated. State functions: A property that depends ONLY on the current state of the system and is independent of how it got there.

  19. The 1st Law of Thermodynamics Consider these 2 pathways with the same starting point (3atm, 8L) and ending point (1.2 atm, 20 L). Calculate w, q and ΔU for each pathway. Constant Volume cooling. Constant Pressure heating. Isothermal expansion

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