ChemActivity 35 - Addendum Model 3: Standard Molar Enthalpy of Formation. Enthalpies of atom combination may be used to estimate enthalpy changes for chemical reactions, but they are not commonly used because they are not widely available. Instead, tables of the standard molar enthalpy of formation Δ ! ° are used since they are available from many sources. The standard molar enthalpy of formation is for the formation of one mole of a compound from its elements in their standard states. The following are examples: ½ H 2 (g) + ½ I 2 (g) HI (g); Δ ! ° = 26.48 / H 2 (g) + ½ O 2 (g) H 2 O (l); Δ ! ° = −285.830 / Hg (l) + 1/8 S 8 (s) HgS (s); Δ ! ° = −58.2 / The standard enthalpy of formation for elements in their standard states are zero. Standard state means 298 K and 1 atm. Critical Thinking Questions 12. What are the standard states of the following elements? a) Hydrogen b) Oxygen c) Mercury d) Sulfur 13. The reaction for the formation of liquid bromine from the element in its standard state is Br 2 (l) Br 2 (l). What is Δ ! ° for this reaction? 14. Which of the following reactions correspond to the Δ ! ° for the product? Explain your choices. a) 2 H 2 (g) + O 2 (g) 2 H 2 O (g) b) MnS (s) Mn (s) + 1/8 S 8 (s) c) Pb (s) + 2 Cl 2 (g) PbCl 4 (s)