Formation Reactions Examples: C(s) + O2(g)  CO2(g) Na(s) + ½ Br2(l)  NaBr(s) 6C(s) + 6H2(g) + 3O2(g)  C6H12O6(s)

Formation Reactions The formation of 1 mole of a substance from its elements in their standard state – i.e. their most stable form at SATP – (what they are on the periodic table)

Write the formation reaction for: Sodium hydroxide Na(s) + ½ O2(g) + ½ H2(g)  NaOH(s) Aluminum phosphate Al(s) + P(s) + 2O2(g)  AlPO4(s) Potassium bromate K(s) + Br2(l) + 3/2 O2(g)  KBrO3(s)

Standard Enthalpy of Formation ΔHfo The energy associated with a formation reaction Units: kJ/mol Enthalpy of formation of an element already in its standard state i.e. O2(g) = zero

Look up the ΔHfo for NaOH KClO3 O2(g) O(g) I2(s) I2(g) I-(aq)

CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) Hess’s Law Using ΔHfo CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) Write the formation reactions for each substance in the above reaction. How would you rearrange each equation to make the target equation?

CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) C(s) + 2H2(g)  CH4(g) x (-1) ΔHfo x (-1) O2(g)  O2(g) x(-1) ΔHfo x(-1) C(s) + O2(g)  CO2(g) x1 ΔHfo x1 H2(g) + ½ O2  H2O(g) x2 ΔHfo x2

CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) ΔHrxn = (1)ΔHfo CO2 + (2)ΔHfo H2O + (-1)ΔHfo CH4 + (-1)ΔHfo O2 ΔHrxn = [(1)ΔHfo CO2 + (2)ΔHfo H2O] –[(1)ΔHfo CH4 + [(1)ΔHfo O2] ΔHrxn = [(n)ΔHfo CO2 + (n)ΔHfo H2O] –[(n)ΔHfo CH4 + [(n)ΔHfo O2] ΔHrxn = Σn ΔHfo Products - Σn ΔHfo Reactants ΔHrxn = [-393.5 + 2(-242)] – [-75 + 2(0)] = -802.5kJ

ΔHrxn = Σn ΔHfo Products - Σn ΔHfo Reactants Try: CH3OH(l) + 3O2(g)  2CO2(g) + 4H2O(l) ΔHrxn = -1692 kJ 2) 4NH3(g) + O2(g)  4NO2(g) + 6H2O(l) ΔHrxn = -1396 kJ 3) What is the molar enthalpy of combustion of propane? ΔHrxn = -2220.5 kJ

4) The standard enthalpy of combustion of liquid cyclohexane, C6H12, to liquid water and carbon dioxide is -3824 kJ. What is the standard enthalpy of formation of cyclohexane? ΔHfo = -252 kJ/mol

Hess’s Law and Calorimetry 5) What mass of methane undergoing complete combustion is required to heat 100.0 kg of water from 5.0oC to 70.0oC in a gas water heater? m = 480 g

Hess’s Law and Calorimetry A 0.092 g sample of acetone is burned in 50.0 g aluminum calorimeter containing 100.0 g of water. The temperature rises from 20.0 oC to 25.0oC. What is the molar enthalpy of combustion of acetone, C3H8O? Calculate the percent error.