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THERMODYNAMICS

THERMODYNAMICS. Courtesy of lab-initio.com. Units for Measuring Heat. The Joule is the SI system unit for measuring heat:. The calorie is the heat required to raise the temperature of 1 gram of water by 1 Celsius degree. 1 L atm = 101.3 J. Definitions #1.

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THERMODYNAMICS

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  1. THERMODYNAMICS Courtesy of lab-initio.com

  2. Units for Measuring Heat The Joule is the SI system unit for measuring heat: The calorie is the heat required to raise the temperature of 1 gram of water by 1 Celsius degree 1 L atm = 101.3 J

  3. Definitions #1 Energy: The capacity to do work or produce heat Potential Energy: Energy due to position or composition Kinetic Energy: Energy due to the motion of the object

  4. Definitions #2 Law of Conservation of Energy: Energy can neither be created nor destroyed, but can be converted between forms The First Law of Thermodynamics: The total energy content of the universe is constant

  5. State Functions depend ONLY on the present state of the system ENERGYIS A STATE FUNCTION A person standing at the top of Mt. Everest has the same potential energy whether they got there by hiking up, or by falling down from a plane  WORKIS NOT A STATE FUNCTION WHY NOT???

  6. E = q + w E= change in internal energy of a system q= heat flowing into or out of the system -qif energy is leaving tothe surroundings +qif energy is entering fromthe surroundings w= work done by, or on, the system -wif work is done bythe systemonthe surroundings +wif work is done onthe system bythe surroundings

  7. Energy Change in Chemical Processes Endothermic: Reactions in which energy flows into the system as the reaction proceeds. + qsystem - qsurroundings

  8. Energy Change in Chemical Processes Exothermic: Reactions in which energy flows out of the system as the reaction proceeds. - qsystem + qsurroundings

  9. Work, Pressure, and Volume Compression Expansion +V(increase) -V(decrease) -wresults +wresults Esystemdecreases Esystemincreases Work has been done on the system by the surroundings Work has been done by the system on the surroundings

  10. Enthalpy • H is a state function – ΔH doesn’t depend on the pathway between the two states H = E + PV, E is internal energy of the system, P is pressure of the system, and V is volume of the system. At constant pressure and knowing only P-V work is allowed (w=-PΔV), we can derive: -ΔH = q, at constant pressure ΔH = Hρroducts - Hreactants

  11. Example6.4 • When 1 mole of methane is burned at constant pressure, 890 kJ of energy is released as heat. Calculate ∆H for a process in which a 5.8 g sample of methane is burned at constant pressure. Given: Find: ∆H of the rxn (KJ) • Mass = 5.8 g CH₄ • Molar mass CH₄ = 16.04 g • q of the product = ΔH = -890 kJ/mol CH₄ Find moles of methane: 5.8 g 1 mole = 0.36 mol 16.04g ∆H = .36 mol -890 kJ = -320 kJ, exothermic 1 mol

  12. Calorimetry The amount of heat absorbed or released during a physical or chemical change can be measured, usually by the change in temperature of a known quantity of water in a calorimeter.

  13. The amount of heat required to raise the temperature of one gram of substance by one degree Celsius. Specific Heat

  14. Catalyst (10 mins) In book: P.277 #49

  15. Calculations Involving Specific Heatat constant pressure q = Heat lost or gained s = Specific Heat Capacity T= Temperature change Assumption: that the density of the solution is the same as pure water, 1 g/mL. Otherwise, a different density will be given.

  16. Example 1 • 1.00 x 10² mL of 0.500 M HCl was mixed with 100. mL of 0.500 M NaOH in a constant pressure calorimeter. Initially, both solutions are at 22.5˚C. Find the heat change on a molar basis for the rxn. NaOH(aq) + HCl(aq) -> NaCl(aq) + H₂O (l) • Qcal (surrounding): qrxn(∆H)(system): 100. mLHClsoln = 100g .05 molsHCl 100 mLNaOHsoln= 100 g .05 molsNaOH s = 4.18 J/g C Ti: 22.50 ˚C Tf: 25.86˚C

  17. Example 2 • A 3.53 g sample of ammonium nitrate (NH₄NO₃) was added to 80.0 mL of water in a coffee cup calorimeter. The water temp decreased from 21.6 C to 18.1 C. Find the ∆Hrxn for: 2NH₄NO₃(s) -> 2NH₄⁺(aq) + 2NO₃⁻(aq) Qcal (surroundings) ∆Hrxn (system) 3.53 g NH₄NO₃ 3.52 g NH₄NO₃ 80.0 mL water = 80.0g Total mass=83.5g S= 4.18 J/g C Ti= 21.6 ˚C Tf= 18.1 ˚C

  18. Classwork • Do #54, then 53, which will be a little different • Review sec 6.2 in packet, then work on 39, 41, 43, 49, 51, 57. • Recommended reading: Calorimetry p.244-248, then 248-251

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