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Catalyst

Catalyst. Given the following equations and H o values, determine the heat of reaction (kJ) at 298 K for the reaction: B 2 H 6 (g) + 6 Cl 2 (g)  2 BCl 3 (g) + 6 HCl(g) BCl 3 (g) + 3 H 2 O(l)  H 3 BO 3 (g) + 3 HCl(g) ΔH = -112.5 kJ

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Catalyst

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  1. Catalyst • Given the following equations and Ho values, determine the heat of reaction (kJ) at 298 K for the reaction: • B2H6(g) + 6 Cl2(g)  2 BCl3(g) + 6 HCl(g) • BCl3(g) + 3 H2O(l) H3BO3(g) + 3 HCl(g) ΔH = -112.5 kJ • B2H6(g) + 6 H2O(l)  2 H3BO3(s) + 6 H2(g ΔH = -493.4 kJ • 1/2 H2(g) + 1/2 Cl2(g)  HCl(g) Δ H = -92.3 kJ

  2. Lecture 5.3 – Enthalpies of Formation and Bond Enthalpies

  3. Today’s Learning Targets • LT 5.6 – For a given chemical bond, I can characterize and calculate its bond enthalpy. Furthermore, I can apply this value to the type and length of bond that the molecule contains. • LT 5.8 – For a phase change, I can characterize the enthalpy of formation and what this value indicates about a given phase change. • LT 5.9 – I can apply Hess’s Law to a chemical reaction and calculate the enthalpy for a combination of chemical reactions.

  4. Enthalpies of Formation (ΔHf) • The enthalpy change for creating a compound from its constituent elements is known ΔH of formation (ΔHf) 2 C (s)+ H2 (g)  C2H2ΔHf =226.7 kJ/mol • The value of ΔHf depends on temperature, pressure, and state of products/reactants

  5. Standard Enthalpies • In order to compare ΔHf of different reactions, we define standard state of 1 atm and 298 K • Don’t confuse with STP! (1 atm and 273 K) • Standard Enthalpies (ΔHo) are ΔH values calculated under these standard conditions • ΔHfo is the change in enthalpy to produce one mole of the compound from the constituent elements • All elements that are stable at room temperature have a ΔHfo of 0 kJ/mol

  6. Class Example • Write the equation corresponding to the standard enthalpy of formation of liquid carbon tetrachloride (CCl4).

  7. Table Talk • Write the equation corresponding to the standard enthalpy of formation of gaseous propane (C3H8).

  8. Using ΔHf to calculate ΔHrxn • We can use known ΔHfo and Hess’s Law to calculate the ΔHrxno for a given reaction of interest. • For the combustion of propane gas, C3H8 (g), to CO2 (g) and H2O (l) under standard conditions: C3H8 (g) + 5 O2 (g)  3 CO2 (g) + 4H2O (l) ΔHrxno = ? You know that: 3 C (s) + 4H2 (g)  C3H8 (g) ΔHfo = -103.85 kJ/mol C (s) + O2 (g)  CO2 (g) ΔHfo = -393.5 kJ/mol H2 (g) + ½ O2 (g)  H2O (l) ΔHfo = -285.8 kJ/mol

  9. Shortcut for Using ΔHf to calculate ΔHrxn • We can quickly calculate the ΔHrxn by using the equation: • Note that n and m represent the coefficient from the balanced chemical reaction.

  10. Class Example • Calculate the standard enthalpy change for the combustion of 1 mole of benzene (C6H6) to CO2 (g) and H2O (l). • Note: • ΔHfo (CO2) = -393.5 kJ/mol • ΔHfo (C6H6) = 49.0 kJ/mol • ΔHfo (H2O) = -285.8 kJ/mol

  11. Table Talk • Calculate the standard enthalpy change for the combustion of 1 mole of ethanol (C2H5OH) to CO2 (g) and H2O (l). • Note: • ΔHfo (CO2) = -393.5 kJ/mol • ΔHfo (C2H5OH) = -277.7 kJ/mol • ΔHfo (H2O) = -285.8 kJ/mol

  12. Bond Enthalpies and ΔHrxn • We can use bond enthalpies to estimate the ΔHrxn where certain bonds are broken and new bonds are formed. • Allows for a quick determination of exo- or endothermic • NOTE – This is an estimate • This is an application of Hess’s Law • Assume all bonds are broken and then all bonds reform:

  13. Class Example • You run the reaction: 2 C2H6 (g) + 7 O2 (g)  4 CO2 (g) + 6 H2O • Using the following ΔH values, calculate ΔHrxn for the reaction and determine if it is endothermic or exothermic: • ΔHC – H = 413 kJ/mol • ΔHC – C = 348 kJ/mol • ΔHO=O = 495 kJ/mol • ΔHC =O = 799 kJ/mol • ΔHH– O = 463 kJ/mol

  14. Table Talk • You run the reaction: 2 N2H4 (g)  N2 (g) + 2 H2 • Using the following ΔH values, calculate ΔHrxn for the reaction and determine if it is endothermic or exothermic: • ΔHN – H = 391 kJ/mol • ΔHN – N = 163 kJ/mol • ΔHN2 = 941 kJ/mol • ΔHH-H = 436 kJ/mol

  15. Problem Set 5.3 • This is a sampling of free response type questions that have been observed on past AP exams • This will be due next Monday/Tuesday!

  16. Rate Yourself • We have finished almost all of chapter 5! • Rate yourself 1 – 4 on LTs 5.1 to 5.9. • It is your responsibility to study any areas that are a 2 or lower over the long weekend

  17. Happy Thanksgiving!

  18. Closing Time • Read 5.7, 8.8 • Do book problems:

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