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CH 6: Thermochemistry

CH 6: Thermochemistry. Renee Y. Becker Valencia Community College CHM 1045. Energy. Energy : is the capacity to do work, or supply heat. Energy = Work + Heat Kinetic Energy : is the energy of motion. E K = 1 / 2 mv 2 (1 Joule = 1 kg m 2 /s 2 ) (1 calorie = 4.184 J)

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CH 6: Thermochemistry

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  1. CH 6: Thermochemistry Renee Y. Becker Valencia Community College CHM 1045

  2. Energy • Energy: is the capacity to do work, or supply heat. Energy = Work + Heat • Kinetic Energy: is the energy of motion. EK = 1/2mv2(1 Joule = 1 kgm2/s2) (1 calorie = 4.184 J) • Potential Energy: is stored energy.

  3. Ek & Ep

  4. Example 1: KE Which of the following has the greatest kinetic energy? • A 12 kg toy car moving at 5 mph? • A 12 kg toy car standing at the top of a large hill?

  5. Energy • Thermal Energy is the kinetic energy of molecular motion • Thermal energy is proportional to the temperature in degrees Kelvin. Ethermal T(K) • Heat is the amount of thermal energy transferred between two objects at different temperatures.

  6. In an experiment:Reactants and products are the system; everything else is the surroundings. • Energy flow from the system to the surroundings has a negative sign (loss of energy). (-E or - H) • Energy flow from the surroundings to the system has a positive sign (gain of energy). (+E or +H)

  7. The law of the conservation of energy: Energy cannot be created or destroyed. • The energy of an isolated system must be constant. • The energy change in a system equals the work done on the system + the heat added. DE = Efinal – Einitial = E2 – E1 = q + w q = heat, w = work

  8. Pressure is the force per unit area. (1 N/m2 = 1 Pa) (1 atm = 101,325 Pa) • Work is a force (F) that produces an object’s movement, times the distance moved (d): Work = Force x Distance

  9. The expansion in volume that occurs during a reaction forces the piston outward against atmospheric pressure, P. Work = -atmospheric pressure * area of piston * distance piston moves

  10. Example 2: Work How much work is done (in kilojoules), and in which direction, as a result of the following reaction?

  11. The amount of heat exchanged between the system and the surroundings is given the symbolq. q = DE + PDV At constant volume (DV = 0): qv = DE At constant pressure: qp = DE + PDV = DH Enthalpy change: DH = Hproducts – Hreactants

  12. Example 3: Work The explosion of 2.00 mol of solid TNT with a volume of approximately 0.274 L produces gases with a volume of 489 L at room temperature. How much PV (in kilojoules) work is done during the explosion? Assume P = 1 atm, T = 25°C. 2 C7H5N3O6(s)  12 CO(g) + 5 H2(g) + 3 N2(g) + 2 C(s)

  13. Enthalpies of Physical Change: Enthalpy is a state function, the enthalpy change from solid to vapor does not depend on the path taken between the two states. Hsubl = Hfusion + Hvap

  14. Enthalpies of Chemical Change:Often called heats of reaction (DHreaction). Endothermic:Heat flows into the system from the surroundings and DH has a positive sign. Exothermic:Heat flows out of the system into the surroundings and DH has a negative sign.

  15. Reversing a reaction changes the sign of DH for a reaction. C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l) DH = –2219 kJ 3 CO2(g) + 4 H2O(l)  C3H8(g) + 5 O2(g) DH = +2219 kJ • Multiplying a reaction increases DH by the same factor. 3 [C3H8(g) + 15 O2(g)  9 CO2(g) + 12 H2O(l)] DH = 3(-2219) kJ DH = -6657 kJ

  16. Example 4: Heat • How much heat (in kilojoules) is evolved or absorbed in each of the following reactions? a) Burning of 15.5 g of propane: C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l) DH = –2219 kJ/mole b) Reaction of 4.88 g of barium hydroxide octahydrate with ammonium chloride: Ba(OH)2·8 H2O(s) + 2 NH4Cl(s)  BaCl2(aq) + 2 NH3(aq) + 10 H2O(l) DH = +80.3 kJ/mole

  17. Thermodynamic Standard State:Most stable form of a substance at 1 atm pressure and 25°C; 1 M concentration for all substances in solution. • These are indicated by a superscript ° to the symbol of the quantity reported. • Standard enthalpy changeis indicated by the symbol DH°.

  18. Example 5: Is an endothermic reaction a favorable process thermodynamically speaking? • Yes • No

  19. Hess’s Law • Hess’s Law:The overall enthalpy change for a reaction is equal to the sum of the enthalpy changes for the individual steps in the reaction.(not a physical change, chemical change) 3 H2(g) + N2(g)  2 NH3(g) DH° = –92.2 kJ

  20. Reactants and products in individual steps can be added and subtracted to determine the overall equation. (1) 2 H2(g) + N2(g)  N2H4(g) DH°1 = ? (2) N2H4(g) + H2(g)  2 NH3(g) DH°2 = –187.6 kJ (3) 3 H2(g) + N2(g)  2 NH3(g) DH°3 = –92.2 kJ DH°1 + DH°2 = DH°reaction Then DH°1 = DH°reaction - DH°2 DH°1 = DH°3 – DH°2 = (–92.2 kJ) – (–187.6 kJ) = +95.4 kJ

  21. Example 6: Hess’s Law • The industrial degreasing solvent methylene chloride (CH2Cl2, dichloromethane) is prepared from methane by reaction with chlorine: CH4(g) + 2 Cl2(g) CH2Cl2(g) + 2 HCl(g) Use the following data to calculate DH° (in kilojoules) for the above reaction: CH4(g) + Cl2(g)  CH3Cl(g) + HCl(g) DH° = –98.3 kJ CH3Cl(g) + Cl2(g)  CH2Cl2(g) + HCl(g) DH° = –104 kJ

  22. Standard Heats of Formation (DH°f): The enthalpy change for the formation of 1 mole of substance in its standard state from its constituent elements in their standard states. • The standard heat of formation for any element in its standard state is defined as being ZERO. DH°f = 0 for an element in its standard state

  23. Standard Heats of Formation • Calculating DH° for a reaction: DH° = DH°f (Products) – DH°f (Reactants) • For a balanced equation, each heat of formation must be multiplied by the stoichiometric coefficient. aA + bB  cC + dD DH° = [cDH°f(C) + dDH°f(D)] – [aDH°f(A) + bDH°f(B)]

  24. CO(g) -111 C2H2(g) 227 Ag+(aq) 106 CO2(g) -394 C2H4(g) 52 Na+(aq) -240 H2O(l) -286 C2H6(g) -85 NO3-(aq) -207 NH3(g) -46 CH3OH(g) -201 Cl-(aq) -167 N2H4(g) 95.4 C2H5OH(g) -235 AgCl(s) -127 HCl(g) -92 C6H6(l) 49 Na2CO3(s) -1131 Standard Heats of Formation Some Heats of Formation, Hf° (kJ/mol)

  25. Example 7: Standard heat of formation Calculate DH° (in kilojoules) for the reaction of ammonia with O2 to yield nitric oxide (NO) and H2O(g), a step in the Ostwald process for the commercial production of nitric acid.

  26. Example 8: Standard heat of formation Calculate DH° (in kilojoules) for the photosynthesis of glucose and O2 from CO2 and liquid water, a reaction carried out by all green plants.

  27. Example 9: Which of the following would indicate an endothermic reaction? Why? • -H • + H

  28. Heat of Phase Transitions from Hf Calculate the heat of vaporization, Hvap of water, using standard enthalpies of formation Hf H2O(g) -241.8 kJ/mol H2O(l) -285.8 kJ/mol

  29. Calorimetry and Heat Capacity • Calorimetry is the science of measuring heat changes (q) for chemical reactions. There are two types of calorimeters: • Bomb Calorimetry: A bomb calorimeter measures the heat change at constant volume such that q = DE. • Constant Pressure Calorimetry: A constant pressure calorimeter measures the heat change at constant pressure such that q = DH.

  30. Constant Pressure Bomb

  31. Calorimetry and Heat Capacity • Heat capacity (C)is the amount of heat required to raise the temperature of an object or substance a given amount. Specific Heat:The amount of heat required to raise the temperature of 1.00 g of substance by 1.00°C. q = s x m x t q = heat required (energy) s = specific heat m = mass in grams t = Tf - Ti

  32. Calorimetry and Heat Capacity • Molar Heat:The amount of heat required to raise the temperature of 1.00 mole of substance by 1.00°C. q = MH x n x t q = heat required (energy) MH = molar heat n = moles t = Tf - Ti

  33. Example 10: Specific Heat What is the specific heat of lead if it takes 96 J to raise the temperature of a 75 g block by 10.0°C?

  34. Example 11: Specific Heat How much energy (in J) does it take to increase the temperature of 12.8 g of Gold from 56C to 85C?

  35. Example 12: Molar Heat • How much energy (in J) does it take to increase the temperature of 1.45 x104 moles of water from 69C to 94C?

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