Calculating H using molar heats of formation Chem

Calculating ΔH using molar heats of formation Chem 12

• If 1 mol of compound is formed from its constituent elements, then the enthalpy change for the reaction is called the enthalpy of formation, Hof. • Standard conditions (standard state): 1 atm and 25 o. C (298 K). • Standard enthalpy of formation: 1 mol of compound is formed from substances in their standard states.

Examples – write formation reactions for each: *Remember the compounds are formed directly from their elements. 1. H 2 SO 4 2. NH 4 Cl

• • You want only one mole of the product being formed. Look up the ΔHf on the table 1. H 2 + S + 2 O 2 H 2 SO 4 ΔHf = -814 k. J/ mol 2. ½ N 2 + 2 H 2 + ½ Cl 2 NH 4 Cl ΔHf = -314. 4 k. J/ mol

Enthalpies of Formation

Using Enthalpies of Formation for Calculating Enthalpies of Reaction • For a reaction

By definition, the enthalpy of formation of an element in its standard state is zero. Example, oxygen (O 2) and chlorine (Cl 2) both have Hof of zero.

Sample Problem 1 • Calculate ΔH for the following reaction using standard molar heats of formation, ΔH°f. • 2 NH 3(g) + 3 Cl 2(g) → N 2(g) + 6 HCl(g) ΔH = ?

• ΔH°f for NH 3(g) = -45. 9 k. J/mol • ΔH°f for HCl(g) = -92. 3 k. J/mol • ΔH°f for Cl 2(g) and N 2(g) is 0 • ΔHrxn = Σ nΔH°f(product) - Σ nΔH°f(reactant) • ΔHrxn= (0 + 6(-92. 3 k. J)) - (2(-45. 9 k. J) + 0) = (-553 k. J) - (-91. 8 k. J) = -461 k. J

Examples Use the tables in the back of your book to calculate ΔH for the following reactions: 1. 4 Cu. O (s) 2 Cu 2 O (s) + O 2 (g) 2. C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O (l) 3. NH 3 (g) + HCl (g) NH 4 Cl (s) 1. +292 k. J 2. -2220. k. J 3. -176. 2 k. J

Practice: • Page 685 #15 • Page 687 #19 -21 • Page 691 # 5