lecture 5

# lecture 5 - Application of Hesss Law Determine the enthalpy...

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1 Application of Hess’s Law: Determine the enthalpy change for reaction R3 from the enthalpy changes for reactions R1 and R2. All are in the gas phase. R1) ½ N 2 + O 2 NO 2 H o f1 = 33 kJ/mol R2) ½ N 2 + ½ O 2 NO H o f2 = 90 kJ/mol R3) NO + ½O 2 NO 2 H o 3 = ?? kJ/mol Combine reactions R1 and R2 using algebra to get R3, then do the same algebra on H o f1 and H o f2 to get H o 3 . 2 R1) ½ N 2 + O 2 NO 2 H o f1 = 33 kJ/mol R2) ½ N 2 + ½ O 2 NO H o f2 = 90 kJ/mol R3) NO + ½O 2 NO 2 H o 3 = ?? kJ/mol A) +123 kJ/mol B) +57 kJ/mol C) –57 kJ/mol D) – 123 kJ/mol

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3 How about subtracting R2 from R1? Which means reversing R2 and adding! NO + ½ N 2 + O 2 NO 2 + ½ N 2 + ½ O 2 NO + ½ O 2 NO 2 Whatever arithmetic is done on R1 and R2 to get R3 needs to be done on the numbers: H o f1 H o f2 = H o 3 33 kJ/mol – 90 kJ/mol = 57 kJ/mol R1) ½ N 2 + O 2 NO 2 H o f1 = 33 kJ/mol R2) ½N 2 + ½O 2 NO H o f2 = 90 kJ/mol R3) NO + ½O 2 NO 2 H o 3 = ?? kJ/mol 4 A diagram illustrating how Hess’s Law is used to determine reaction enthalpies. . NO 2 NO N 2 and O 2 90 kJ ?? kJ 33 kJ Two paths for the formation of NO 2 , the enthalpy change is independent of the path. Values for the reaction enthalpies are given. Visualize this diagram, but working with equations is necessary to get the stoichiometry!
5 Today’s To Do Use Hess’s law in calculations of reaction enthalpies. Calculate the fuel value, energy density, and

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lecture 5 - Application of Hesss Law Determine the enthalpy...

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