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Standard Enthalpies of Formation. Standard Enthalpy of Formation. the quantity of energy associated with the formation of one mole of a substance from its elements in their standard states (at SATP) Symbol: Δ H ° f Units: kJ/mol For example:
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Standard Enthalpy of Formation • the quantity of energy associated with the formation of one mole of a substance from its elements in their standard states (at SATP) • Symbol: ΔH°f Units: kJ/mol • For example: C(s) + ½ O2 (g) + 2 H2 (g) CH3OH (l) Δ H°f = -239.1 kJ/mol 2 Al (s) + 3/2 O2 (g) Al2O3 (s) Δ H°f = -1675.7 kJ/mol • All of the elements on the left sides of the equations are in their standard states (SATP)
Writing Formation Equations • Step 1: Write one mole of the product in the state that has been specified. Example: FeSO4 (s) • Step 2: Write the reactant elements in their standard states (reference periodic table. Pg 838). Fe (s) + S (s) + O2 (g)FeSO4 (s) • Step 3: Write coefficients for the reactants to give a balanced equation yielding one mole of product. Fe (s) + S (s) + 2 O2 (g)FeSO4 (s)
Calculating ΔH • A third way of calculating ΔH is possible using standard enthalpies of formation. • Table C.6 pg 799 gives standard enthalpies of formation for common molecules. • According to Hess’s Law, the enthalpies of known equations may be used to calculate the enthalpy of an unknown reaction. • For example: The formation of hydrogen and carbon monoxide gas from water and graphite: H2O (g) + C (s) H2 (g) + CO (g) ΔH = ? • Use the formation equations for each of the products and reactants to create the target equation.
Calculating ΔH0, continued H2O (g) + C (s) H2 (g) + CO (g) Reactants H2(g) + 1/2 O2(g) --> H2O(g) H0f H2O = - 241.8 kJ/mol C (s) – graphite in standard state H0f = 0 kJ/mol Products C(s) + 1/2 O2(g) --> CO(g) H0f CO = - 110.5 kJ/mol H2 (g)– hydrogen in standard state H0f = 0 kJ/mol H2O(g) --> H2(g) + 1/2 O2(g) H0 = 241.8 kJ/mol C(s) + 1/2 O2(g) --> CO(g) H0 = - 110.5 kJ/mol H2O (g) + C (s) H2 (g) + CO (g) H0 = 131.3 kJ/mol
In general, when all enthalpies of formation are known, the enthalpy of a reaction is equal to the sum of the enthalpies of formation of the products minus the sum of the enthalpies of formation of the reactants. where n = the amount in moles of each product or reactant
Sample Problem Calculate the heat of combustion of methanol. CH3OH (l) + 3/2 O2 (g) --> CO2 (g) + 2 H2O (g)H0comb = ?
Multistep Energy Calculations Several energy calculations may be required to solve multistep problems • Heat flow q = mcΔT • Enthalpy changes ΔH = n ΔHx • Hess’s Law ΔHtarget = ∑ΔHknown • Enthalpies of Formation
