Enthalpies of Formation and Reaction Definitions: Standard state –A gas at 1 atm –An aqueous solution with a concentration of 1 M at a pressure of 1 atm.

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Enthalpies of Formation and Reaction Definitions: Standard state –A gas at 1 atm –An aqueous solution with a concentration of 1 M at a pressure of 1 atm –Pure liquids and solids –The most stable form of elements at 1 atm and 25°C allotrope:one or 2 or more forms of an element in the same physical state (diamond and graphite are allotropes of carbon; O 2 (g) and ozone, O 3 (g) are allotropes of oxygen).

Definitions (Cont’d) standard enthalpy of formation (ΔH f °) –enthalpy change for formation of 1 mole of an element or compound from its elements in their standard states (i.e. naturally occurring form at 1 atm and 25˚C) Note: ΔH f ° = 0 for any element in its naturally occurring (most stable) form Enthalpies of Formation and Reaction

ΔH f ° Values Examples of ΔH f ° are as follows: Ag(s) + Cl 2 (g)  AgCl(s)ΔH = –127.1 kJ so ΔH f °(AgCl, s) = kJ N 2 (g) + O 2 (g)  NO 2 (g)ΔH = kJ so ΔH f ° (NO 2, g) = kJ

Which of these are standard enthalpy of formation reactions? a.N 2 (g) + 3 H 2 (g)  2 NH 3 (g) b.C graphite (s) + 2 H 2 (g)  CH 4 (g) c.Ca(s) + Br 2 (g)  CaBr 2 (s)

Example When 50.0 g of nitrogen react with excess hydrogen to form ammonia gas, kJ of heat are liberated (released) at standard state conditions. Calculate the standard enthalpy of formation (in kJ/mol) for ammonia gas.

Direct Method for Calculating ΔH° Calculation of ΔH° (superscript ° denotes standard state conditions, at 1 atm and 25°C) For the reaction: aA + bB  cC + dD where a,b,c,d = stoichiometric coefficients ΔH° = Σ n ΔH f °(products) – Σ m ΔH f °(reactants) = [c (C) + d (D)] – [a (A) + b (B)] = (total energy of products) – (total energy of reactants)

Substance (kJ/mol)Substance (kJ/mol)Substance (kJ/mol) H 2 O (l)-285.8CO (g)-110.5NH 3 (g)-46.1 H 2 O (g)-241.8CO 2 (g)-393.5CH 4 (g)-74.8 CH 3 OH (l)-238.7C 2 H 5 OH (l) NO (g)90.3 Table of Standard Enthalpies of Formation (ΔH f °)

Example Use the table above to calculate ΔH° for the reaction below: C graphite (s) + O 2 (g)  CO 2 (g) ΔH° = (1mol)(CO 2, g) – [(1mol)(C graphite, s) + (1mol) (O 2, g)] ΔH° =

Example Consider the reaction for the combustion of glucose: C 6 H 12 O 6 (s) + 6 O 2 (g)  6 CO 2 (g) + 6 H 2 O(g) DH°= –2537 kJ Use the standard enthalpies of formation provided to calculate the enthalpy of formation for glucose.

Example Consider the following thermochemical equation: 4 NH 3 (g) + 5 O 2 (g)  4 NO(g) + 6 H 2 O(g) DH°= –904 kJ Use the standard enthalpies of formation provided to calculate the enthalpy of formation for NO.