# Production of one mole of a material from the

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production of one mole of a material from the elements in their standard states at 25 C and 1 bar. I Important: Enthalpy of formation is different from enthalpy of lattice formation. I Example of a formation reaction: H 2(g) + 1 2 O 2(g) H 2 O (l) I By definition, Δ f H = 0 for an element in its standard state. I For a reaction, Δ H is the difference between the enthalpies of formation of products and reactants. I Enthalpy changes are path independent .

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Born-Haber cycle H in gas phase ionize to form cation accept electron(s) to form anion lattice formation Δ H f o elements ionic crystal vaporize atoms in gas phase dissociate ions in gas phase elements
Calculation of the enthalpy of lattice formation from the Born-Haber cycle for NaCl Na (s) Na (g) Δ H 1 = Δ f H Na (g) = 107 . 3 kJ / mol Na (g) Na + (g) + e - Δ H 2 = I 1 , Na = 495 . 6 kJ / mol 1 2 Cl 2(g) Cl (g) Δ H 3 = Δ f H Cl (g) = 121 . 3 kJ / mol Cl (g) + e - Cl - (g) Δ H 4 = Δ ea H Cl = - 348 . 8 kJ / mol Na + (g) + Cl - (g) NaCl (s) Δ lf H NaCl is our unknown. Na (s) + 1 2 Cl 2(g) NaCl (s) Δ f H NaCl = - 411 . 2 kJ / mol Δ f H NaCl = Δ H 1 + Δ H 2 + Δ H 3 + Δ H 4 + Δ lf H NaCl - 411 . 2 kJ / mol = 107 . 3 + 495 . 6 + 121 . 3 + - 348 . 8 kJ / mol + Δ lf H NaCl Δ lf H NaCl = - 786 . 6 kJ / mol

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Calculation of Δ lf H for MgCl 2 Mg (s) Mg (g) Δ H 1 = Δ f H Mg (g) = 147 . 1 kJ / mol Mg (g) Mg 2+ (g) + 2e - Δ H 2 = I 1 , Mg + I 2 , Mg = 2189 kJ / mol Cl 2(g) 2Cl (g) Δ H 3 = 2Δ f H Cl (g) = 242 . 6 kJ / mol 2Cl (g) + 2e - 2Cl - (g) Δ H 4 = 2Δ
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• Fall '06
• Marc
• Enthalpy, Sodium chloride, Standard enthalpy change of formation, Marc R. Roussel, Lattice formation

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