Standard Enthalpies of Formation Learning Goal: You will be able to write formation equations, find the enthalpies of formation and use them & Hess’ Law to find the enthalpy of a given reaction. You will be able to write formation equations, find the enthalpies of formation and use them & Hess’ Law to find the enthalpy of a given reaction.

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) 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 Symbol: ΔH° f Units: kJ/mol For example: For example: C(s) + ½ O 2 (g) + 2 H 2 (g)  CH 3 OH (l) Δ H° f = kJ/mol 2 Al (s) + 3/2 O 2 (g)  Al 2 O 3 (s) Δ H° f = kJ/mol All of the elements on the left sides of the equations are in their standard states (SATP) All of the elements on the left sides of the equations are in their standard states (SATP)

Formation Equations Are equations that show the production of a compound from its elements. Are equations that show the production of a compound from its elements. How to Write Formation Equations: Step 1: Write one mole of the product in the state that has been specified. Step 1: Write one mole of the product in the state that has been specified. Example: FeSO 4 (s) Step 2: Write the reactant elements in their standard states (HOFBrINCl should be diatomic – all others are monatomic) (reference periodic table. Pg 838). Step 2: Write the reactant elements in their standard states (HOFBrINCl should be diatomic – all others are monatomic) (reference periodic table. Pg 838). Fe (s) + S (s) + O 2 (g)  FeSO 4 (s) Step 3: Write coefficients for the reactants to give a balanced equation yielding one mole of product – you may have to use fractions ! Step 3: Write coefficients for the reactants to give a balanced equation yielding one mole of product – you may have to use fractions ! Fe (s) + S (s) + 2 O 2 (g)  FeSO 4 (s)

Let’s Practice: Write the formation equation for solid sodium chloride. Write the formation equation for solid sodium chloride. Write the formation equation for solid magnesium carbonate Write the formation equation for solid magnesium carbonate

Calculating ΔH A third way of calculating ΔH is possible using standard enthalpies of formation. 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. 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. 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: For example: The formation of hydrogen and carbon monoxide gas from water and graphite: H 2 O (g) + C (s)  H 2 (g) + CO (g) ΔH = ? Use the formation equations for each of the products and reactants to create the target equation. Use the formation equations for each of the products and reactants to create the target equation.

Calculating ΔH 0, continued H 2 O (g) + C (s)  H 2 (g) + CO (g) Reactants H 2 (g) + 1/2 O 2 (g) --> H 2 O(g)  H 0 f H2O = kJ/mol C (s) – graphite in standard state  H 0 f = 0 kJ/mol Products C(s) + 1/2 O 2 (g) --> CO(g)  H 0 f CO = kJ/mol H 2 (g) – hydrogen in standard state  H 0 f = 0 kJ/mol H 2 O(g) --> H 2 (g) + 1/2 O 2 (g)  H 0 = kJ/mol C(s) + 1/2 O 2 (g) --> CO(g)  H 0 = kJ/mol H 2 O (g) + C (s)  H 2 (g) + CO (g)  H 0 = kJ/mol

Using ΔH° f to calculate enthalpy change: where n = the amount in moles of each product or reactant Example: Calculate the standard change in enthalpy for the thermite reaction: Example: Calculate the standard change in enthalpy for the thermite reaction: Al(s) + Fe 2 O 3 (s)  Al 2 O 3 (s) + Fe (s)

Sample Problem Calculate the heat of combustion of methanol. CH 3 OH (l) + 3/2 O 2 (g) --> CO 2 (g) + 2 H 2 O (g)  H 0 comb = ?

Multistep Energy Calculations Several energy calculations may be required to solve multistep problems – –Heat flow q = mcΔT – –Enthalpy changes ΔH = n ΔH x – –Hess’s Law ΔH target = ∑ΔH known – –Enthalpies of Formation

Sample Problem, pg 336

Sample problem cont’d