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Understanding Thermodynamics: Flow of Energy in Chemistry

Explore the principles of thermodynamics, including kinetic and potential energy, heat transfer, and enthalpy changes. Learn about the First Law of Thermodynamics, calorimetry, Hess's Law, and enthalpy of formation.

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Understanding Thermodynamics: Flow of Energy in Chemistry

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  1. AP CHEMISTRY CHAPTER 5 Thermodynamics- the flow of heat

  2. 5.1 Thermochemistry • A. kinetic NRG- NRG of motion KE = ½ mv2 • B. Potential NRG – stored NRG PE= mgh • D. Chemical NRG- due to PE • E. thermal NRG- due to temperature

  3. 5.1 Con. • F. units Joules= kg-m2/s2 1 calorie = 4.184 J (exactly. EXACTLY, I say!) 1000 cal = 1Cal = 1kcal = 1 food calorie • G. system- what we are studying • H. surroundings- everything else • I. Universe = system + surroundings

  4. 5.1, Still • J.force- push or pull • K.work = force x distance • L.heat- NRG transfer from hotter to colder • M.energy- is the capacity to do work OR to transfer heat

  5. 5.2 First Law of ThermodynamicsIt is observed in this house! NRG is conserved OR NRG cannot be created or destroyed • A. internal NRG, E= KE + PE 1.) +Δ E = NRG gained by system 2.) - Δ E = NRG lost to surroundings • B. E = q + w 1.) E = internal NRG 2.) q= heat 3.) w = work

  6. 5.2 Con. • C.endothermic - absorbs heat ( ΔH= +) • D.exothermic – gives off heat (ΔH= -) • E.state function - does not depend on path, only initial and final values.

  7. 5.3 + 5.4 • 5.3 enthalpy- heat content of a system. A state function • 5.4 enthalpy of reaction (heat of reaction), an extensive property ΔH = products - reactants TABLE on p.189 Appendix C p. 1112

  8. Sample 5.3 p. 157 (Mass - heat Prob.) CH4 + 2O2 CO2 + 2H2O(l) + heat -74.8 0 -393.5 -285.8 kJ/mol ΔH= [(-393.5) + 2(-285.8)] – [-74.8 + 0] = -890.3 kJ/mol (exo) 4.50g CH4 x 1mol CH4 x –890.3 kJ 16.0 g CH4 1mol CH4 = -250 kJ

  9. 5.5 Calorimetry • A. calorimeter- apparatus that measures heat flow. • B. heat capacity- temp. change when substance absorbs heat. (extensive) • C. Molar heat capacity- heat capacity of 1 mol • D. specific heat- heat needed to raise 1 g of a substance to 1degreeC. (Cp) (intensive) between 15.5 and 16.5 degrees C Water = 1 cal = 4.18J/g-deg.C = 4.18J/g-K

  10. Calorimeter

  11. 5.6 • Hess’s Law – if a reaction is carried out in a series of steps, H for the reaction will be equal to the sum of enthalpy change for the individual steps. ( a state function)

  12. 5.7 Enthalpy of Formation (Heat of Formation) Sample p. 187 ∆Hf f= formed from 1mol of it’s elements • A. standard state 1 atm & 25 degrees C (298K) • B. standard enthalpy of formation, Hf, for any element is zero. H time

  13. Energy Profile Diagram ΔH = positive #

  14. ΔH = negative #

  15. q = m Cp ΔT • q = heat in Joules (J) • m = mass in grams • Cp = specific heat in J/g-oC • ΔT = change in temp in oC

  16. Slope = Δy/Δx

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