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Thermochemistry

Thermochemistry. Thermodynamics. Energy. potential. kinetic. energy possessed by an object in virtue of its motion. E kin =1/2 m v 2 E kin =3/2 R T. Heat is the energy transferred from one object to another in virtue of T-difference. Never confuse T and heat.

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Thermochemistry

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  1. Thermochemistry Thermodynamics

  2. Energy potential kinetic • energy possessed by an object in virtue of its motion. • Ekin=1/2 mv2 • Ekin=3/2 RT Heat is the energy transferred from one object to another in virtue of T-difference Never confuse T and heat Energy: Ability to do work or produce heat. Work=force x distance force causes the object to move • Gravitational force causes the water to fall. • can generate electricity

  3. Potential energy: • energy possessed by an object due to its presence in a force field i.e. under the effect of external force. • Object attracted/repelled by external force. • stored energy! Attraction causes the ball to fall, h smaller, Epot smaller. Epot=mgh Attraction causes the potential energy to decrease. Repulsion causes the potential energy to increase.

  4. Law of conservation of Energy (Axiome): • Energy can neither be created nor destroyed. • Energy of universe is constant. • Energy can be converted from one form to another. Ekin ↔ Epot Heat ↔ Work • Thermodynamics: the study of energy transformation from one form to another. • First Law of TD.

  5. System Part of universe under investigation. sys sys surroundings surroundings sys + surr = universe

  6. State Function Change in state function depends only on initial and final state. Amman h=900 m Final state Irbid h=650 m Initial state Irbid → Amman Dh=hfinal-hinitial Dh=hamman-hirbid Dh=900 m-650 m=250 m Change doesn’t depend on path Sea level

  7. Examples of state functions: • Temperature • Volume • Pressure • Altitude • Mass • Energy • Concentration

  8. Internal Energy E Sum of Ekin and Epot of all particles in the system. State function First Law of TD DE = Q + W • The internal energy of a system can be changed • by gaining or losing heat, Q • Work, W, done on the system or by the System

  9. surroundings sys Q DE = Q + W • heat transferred from surr to sys. • Surr loses heat, loses E, Esurr↓ • Sys gains heat, gains E, Esys • for Surr: Q < 0 (neg.), DE < 0 • for Sys: Q > 0 (pos.), DE > 0 surroundings sys Q Qsys > 0 : endothermic process • heat transferred from sys to surr. • Sys loses heat, loses E, Esys↓ • Surr gains heat, gains E, Esurr • for Sys: Q < 0 (neg.), DE < 0 • for Surr: Q > 0 (pos.), DE > 0 Qsys < 0 : exothermic process

  10. surr m1 m2 surr m1 sys surr m1 m2 sys sys Work done by surroundings on system (Esys)f > (Esys)i DEsys > 0 wsys > 0 Esurr↓ Esys h of m1 and m2↓ Ep of m1 and m2↓ Ep=mgh h↓, Ep↓

  11. surr surr m1 m1 surr m1 m2 sys sys sys Work done by system on surroundings (Esys)f < (Esys)i DEsys < 0 wsys < 0 Esurr Esys↓ h of m1 Ep of m1 Ep=mgh h, Ep

  12. Ex. 6.1 A system undergoes an endothermic process in which 15.6 kJ of heat flows and where 1.4 kJ work is done on the system. Calculate the total change in the internal Energy of the system. Qsys > 0 Q=+15.6 kJ wsys > 0 w=+1.4 kJ DEsys = Qsys + wsys DEsys = (+15.6 kJ) + (+1.4 kJ) = +17 kJ

  13. surr surr m1 m1 sys sys hf hi final initial -

  14. Q and w are path functions (Depend on path). full empty final initial Path 1 Path 2

  15. Ex. 6.2 Calculate the work associated with the expansion of a gas from 46 L to 64 L at constant external pressure of 15 atm. 15 atm 15 atm 64 L 46 L • Expansion against the external pressure • External pressure opposes the expansion • popp=15 atm = constant

  16. 1 atm 1 atm 4.50x106 L 4.00x106 L • Ex. 6.3 Given a balloon with a volume of 4.00x106 L. It was heatedby 1.3x108 J until the volume became 4.5x106L. Assuming the balloon is expanding against a constant external pressure of 1 atm, calculate the change in the internal energy of the gas confined by the balloon. Vi Q Vf popp

  17. C=O C-H O=O O-H R Ep P Chemical Energy CH4(g) +2O2(g)→ CO2(g) + 2H2O(g) • Chemical reaction: • No change in the number/nature of atoms • Redistribution of Bonds (change in bonding) • Change in attraction & repulsion forces between the atoms • Change in the potential energy Ep of molecules • Energy is conserved! • Energy difference released as heat. • Heat of reaction (Qv, Qp). • Reaction exothermic. DHreaction= Hf – Hi = HP – HR < 0

  18. N=O N≡N O=O P Ep R N2(g) + O2(g)→ 2NO(g) • Energy is conserved! • Energy difference obtained from surroundings as heat. • Heat of reaction (Qv, Qp=DE, DH). • Reaction endothermic. DHreaction= Hf – Hi = HP – HR > 0 DEreaction= Ef – Ei = EP – ER > 0 • Ep(R) > Ep(P), reaction exothermic • Ep(R) < Ep(P), reaction endothermic

  19. Thermochemical equation moles CH4(g) +2O2(g)→ CO2(g) + 2H2O(g) DHo=-802.3 kJ 1 mole of gaseous methane (CH4) reacts with two moles of gaseous molecular oxygen producing 1 mole of gaseous carbon dioxide, 2 moles of water vapor and 802.3 kJ of heat. N2(g) + O2(g)→ 2NO(g) DHo=+180.5 kJ DHo: Standard heat of reaction: Standard conditions: T=25oC, p=1atm.

  20. Calorimetry calorie cal measurement heat unit 1 cal = 4.185 J Calorimetry = heat measurement experiments 1 Cal =1000 cal Problem:- heat (Q) is a path function!!! - Q differs from one way of performing the experiment to another. - details of the experiment must be described!!!!

  21. heat measurement experiments • Heat measured at constant volume: • Equal to DE • Equal to a change in a state function!! • Details of the experiment no more important.

  22. However, heat measurement experiments are usually performed at constant pressure • Heat measured at constant pressure: • Equal to DH • Equal to a change in a state function!! • Details of the exp. no more important.

  23. csp:specific heat (specific heat capacity) heat needed to raise the temperature of 1 gram of substance by 1ºC. C:heat capacity heat needed to raise the temperature of substance (m gram) by 1ºC. csp 1 g T increase by 1ºC: ? = C m gr. Q:heat heat needed to raise the temperature of substance (m gram) by a given temperature difference, DTºC. T increase by 1ºC C m gr. T increase by DTºC ? = Q

  24. Bomb Calorimeter

  25. 0.5269 g of octane (C8H18) were placed in a bomb calorimeter with aheat capacity of 11.3 kJ/ºC. The octane sample was ignited in presence of excess oxygen. Thetemperatureof the calorimeter was found toincrease by 2.25ºC. CalculateDEof thecombustionreaction of octane. DE defined for the reaction as written!!!!!!!!!!! C8H18(g) +12.5O2(g)→ 8CO2(g) + 9H2O(g) DE defined for the combustion of 1 mole octane (114.2 g)!! DE=-(114.2 g x 25.4 kJ)/0.5269g=-5505 kJ QV 0.5269 g ? = DE 114.2 g

  26. When 1.5 g of methane (CH4) was ignited in a bomb calorimeter with 11.3 kJ/ºC heat capacity, the temperature rised by 7.3ºC. When 1.15 g hydrogen (H2) was ignited in the same calorimeter, the temperature rised by 14.3ºC. Which one of the two substances has a higher specific heat of combustion (i.e. heat evolved upon the combustion of 1 g of substance)? QV=83 kJ 1.5 g =55 kJ/g ? 1 g QV=162 kJ 1.15 g =141 kJ/g ? 1 g

  27. Coffee-Cup Calorimeter 50 mL of 1.0 M HCl at 25ºC were added to 50 mL of 1.0 M NaOH at 25ºC in a coffee-cup calorimeter. The tempe-rature was found to rise to 31.9ºC. Calculate the heat of the neutraliza-tion reaction! Was caused the temperature to increase? Exothermic Reaction HCl(aq) +NaOH(aq)→ NaCl(aq) + H2O(l) H+(aq) +OH-(aq)→ H2O(l)

  28. heat evolved = heat gained + heat gained by reaction by solution by calorimeter • Assumptions: • Ccal=0 (very small mass) • Solution ≈ water • (csp)solution=(csp)water=4.18 Jg-1ºC-1 • (density)solution=(density)water=1 g/mL

  29. nHCl=MHClxVHCl = 1 mol/L x 0.050 L = 0.050 mol nNaOH=MNaOHxVNaOH = 1 mol/L x 0.050 L = 0.050 mol HCl(aq) +NaOH(aq)→ NaCl(aq) + H2O(l) 0.050 mol 0.050 mol 0.050 mol 0.050 mol 0.050 mol H2O 2884.2 J mol ? Qp=57,684 J/molH2O 1 mol H2O DH= -57,684 J/molH2O DH= -57.7 kJ/molH2O

  30. Hess’s Law initial final N2(g) + 2O2(g) 2NO2(g) path 1 O2(g) O2(g) 2NO(g) path 2

  31. Hess’s Law: N2(g) + O2(g)→ 2NO(g)DH2a 2NO(g) + O2(g)→ 2NO2(g)DH2b N2(g) + 2O2(g)→ 2NO2(g)DH1 DH1=DH2a+DH2b The enthalpy of a given chemical reaction is constant, regardless of the reaction happening in one step or many steps. If a chemical equation can be written as the sum of several other chemical equations (steps), the enthalpy change of the first chemical equation equals the sum of the enthalpy changes of the other chemical equations (steps).

  32. Rules for manipulating thermochemical equations - If equation is multiplied by a factor, multiply DH by this factor. N2(g)+3H2(g)→ 2NH3(g)DH=-92 kJ 2x (N2(g)+3H2(g)→ 2NH3(g)DH=-92 kJ) 2N2(g)+6H2(g)→ 4NH3(g)DH=-184 kJ 1/2x (N2(g)+3H2(g)→ 2NH3(g)DH=-92 kJ) 1/2N2(g)+3/2H2(g)→ NH3(g)DH=-46 kJ - If equation is reversed, change the sign of DH 2NH3(g)→ N2(g) + 3H2(g)DH=+92 kJ

  33. The enthalpy of combustion of graphite is -394 kJ/mol. The enthalpy of combustion of diamond is -396 kJ/mol. Calculate DH for the reaction: Cgraphite → Cdiamond • Solving Strategy • Write the given data in form of thermochemical equations: • CG + O2(g)→ CO2(g)DH=-394 kJ • CD + O2(g)→ CO2(g)DH=-396 kJ • Construct the equation of interest from the given data: • 1 mole cgraphite is needed as reactant. Take the equation in the given data that contains cgraphite. Check the number of moles and whether it is on the reactant side. Manipulate if necessary. • CG + O2(g) → CO2(g)DH=-394 kJ • 1 mole cdiamond is needed as product. Take the equation in the given data that contains cdiamond. Check the number of moles and whether it is on the product side. Manipulate if necessary. • CO2(g) → CD + O2(g) DH=+396 kJ

  34. Sum the resulting equations and their DH values: • CG + O2(g) → CO2(g)DH=-394 kJ • CO2(g) → CD + O2(g) DH=+396 kJ Cgraphite → Cdiamond DH=+2 kJ

  35. Given: 2B(s)+3/2O2(g)→ B2O3(s)DH=-1273 kJ B2H6(g)+3O2(g)→ B2O3(s) + 3H2O(g)DH=-2035 kJ H2(g)+1/2O2(g)→ H2O(l)DH=-286 kJ H2O(l) → H2O(g)DH=+44 kJ Calculate DH for 2B(s) + 3H2(g)→ B2H6(g) 2B(s)+3/2O2(g)→ B2O3(s)DH=-1273 kJ 3H2(g)+3/2O2(g)→ 3H2O(l)DH=3x(-286) kJ B2O3(s) + 3 H2O(g)→B2H6(g)+3 O2(g)DH=+2035 kJ 2B(s) + 3 H2O(g)+ 3 H2(g)→B2H6(g)+3 H2O(l)DH=-96 kJ 3H2O(l)→ 3H2O(g)DH=3x(+44) kJ 2B(s) + 3 H2(g)→B2H6(g DH=+36 kJ

  36. Heat of Formation Formation reaction: reaction of forming 1 mole of product from the elements in their stable form at 25ºC and 1 atm. Heat of formation = DH of formation reaction = DFH Standard heat of formation = DHº of formation reaction = DFHº ½N2(g)+½O2(g)→NO(g)DHº DFHº(NO(g)): Cgraphite(s)+½O2(g)→CO(g)DHº DFHº(CO(g)): ½O2(g)→O(g)DHº DFHº(O(g)): Cgraphite(s)→Cdiamond(s)DHº DFHº(Cdiamond(s)): O2(g)→O2(g)DHº=0 DFHº(O2(g)): Cgraphite(s)→Cgraphite(s)DHº=0 DFHº(Cgraphite(s)):

  37. CH4(g) +2O2(g)→ CO2(g) + 2H2O(g) DH=? CG(s)+O2(g)→ CO2(g)DFH(CO2) 2x(H2(g)+1/2O2(g)→H2O(g))2xDFH(H2O) CH4(g)→ CG(s) + 2 H2(g)-DFH(CH4) O2(g) → O2(g)-DFH(O2)=0 CH4(g) +2O2(g)→ CO2(g) + 2H2O(g) DH=DFH(CO2)+ 2xDFH(H2O) - DFH(CH4) - DFH(O2) DH=DFH(CO2)+ 2xDFH(H2O) – [DFH(CH4) +DFH(O2)]

  38. 4NH3(g) +7O2(g)→ 4NO2(g) + 6H2O(l) DH=? DH= 4xDFH(NO2)+ 6xDFH(H2O) – 4xDFH(NH3) DH=? 2Al(s) +Fe2O3(s)→ Al2O3(s) + 2Fe(s) DH= DFH(Al2O3)+ 2xDFH(Fe) – [DFH(Fe2O3)+ 2xDFH(Al)] DH= DFH(Al2O3) – DFH(Fe2O3)

  39. Calculate the heat of combustion of methanol (CH3OH(l)) in kJ/g and compare its value with that of octane (C8H18(l)). 2CH3OH(l) +3O2(g)→ 2CO2(g) + 4H2O(l) DH=? DH= 2xDFH(CO2)+ 4xDFH(H2O) – 2xDFH(CH3OH) DH= 2x(-394 kJ)+ 4x(-286 kJ) – 2x(-239 kJ)=-1454 kJ 2 mol CH3OH -1454 kJ 2x32 g -1454 kJ = -22.7 kJ/g 1 g ?

  40. C8H18(l) +12.5O2(g)→ 8CO2(g) + 9H2O(l) DH= 8xDFH(CO2)+ 9xDFH(H2O) – DFH(C8H18) DH= 8x(-394 kJ)+ 9x(-286 kJ) – (-276 kJ)=-5450 kJ -5450 kJ 1 mol C8H18 114 g -5450 kJ = -47.8 kJ/g 1 g ?

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