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Formation Reactions

Formation Reactions. Examples: C (s) + O 2(g)  CO 2(g) Na (s) + ½ Br 2(l)  NaBr (s) 6C (s) + 6H 2(g) + 3O 2(g)  C 6 H 12 O 6(s). Formation Reactions.

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Formation Reactions

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

  2. 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)

  3. 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)

  4. 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

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

  6. 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?

  7. CH4(g) + 2O2(g) CO2(g) + 2H2O(g) C(s) + 2H2(g) CH4(g) x (-1) ΔHfox (-1) O2(g)  O2(g) x(-1) ΔHfox(-1) C(s) + O2(g) CO2(g) x1 ΔHfox1 H2(g) + ½ O2 H2O(g) x2 ΔHfox2

  8. CH4(g) + 2O2(g) CO2(g) + 2H2O(g) ΔHrxn = (1)ΔHfoCO2 + (2)ΔHfoH2O + (-1)ΔHfoCH4 + (-1)ΔHfoO2 ΔHrxn = [(1)ΔHfoCO2+ (2)ΔHfoH2O] –[(1)ΔHfoCH4+ [(1)ΔHfoO2] ΔHrxn = [(n)ΔHfoCO2 + (n)ΔHfoH2O] –[(n)ΔHfoCH4 + [(n)ΔHfoO2] ΔHrxn = Σn ΔHfo Products - Σn ΔHfo Reactants ΔHrxn= [-393.5 + 2(-242)] – [-75 + 2(0)] = -802.5kJ

  9. 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= -2220.5kJ

  10. 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

  11. 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

  12. 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.

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