WHS AP Chemistry Thermochemistry...WHS ! AP Chemistry 6 • Thermochemistry HEATS OF FORMATION...
Transcript of WHS AP Chemistry Thermochemistry...WHS ! AP Chemistry 6 • Thermochemistry HEATS OF FORMATION...
WHS w AP Chemistry 6 • Thermochemistry
H E A T S O F F O R M A T I O N I n d i r e c t M e t h o d : H e s s ’ s L a w 1. Calculate the standard enthalpy of formation of carbon disulfide (CS2) from its elements, given that Hint: first identify the target equation! C !"#$!!"# + O! (!) → CO! (!) ∆𝐻°!"# = −393.5 kJ/mol S !!!"#$% + O! (!) → SO! (!) ∆𝐻°!"# = −296.4 kJ/mol CS! ! + 3O! (!) → CO! (!) + 2SO! (!) ∆𝐻°!"# = −1073.6 kJ/mol
2. From the following data, calculate the enthalpy change for the reaction 2 C(graphite) + 3 H2(g) → C2H6(g).
C !"#$!!"# + O! (!) → CO! (!) ∆𝐻°!"# = −393.5 kJ/mol H! (!) +
!!O! (!) → H!O(!) ∆𝐻°!!" = −285.8 kJ/mol
2C!H! ! + 7O! (!) → 4CO! (!) + 6H!O(!) ∆𝐻°!"# = −3119.6 kJ/mol 3. Use the thermochemical equations shown below to determine the enthalpy for the reaction: H2CO + O2 à H2CO3 ∆𝑯°𝒓𝒙𝒏 = ???
H2CO3 à H2O + CO2 ∆𝐻°!"#= 77.5 kJ H2CO + O2 à H2O + CO2 ∆𝐻°!"# = -‐62.5 kJ
4. Use the thermochemical equations shown below to determine the enthalpy for the reaction:
N2(g) + 2O2(g) à N2O4(g) ∆𝑯°𝒓𝒙𝒏 =??? 2NO2(g) à N2(g) + 2O2(g) ∆𝐻°!"#=-‐101.7 kJ
N2O4(g) à 2NO2(g) ∆𝐻°!"#= 87.3 kJ 5. Use the thermochemical equations shown below to determine the enthalpy for the reaction:
PCl5(g) àPCl3(g) + Cl2(g) ∆𝑯°𝒓𝒙𝒏 = ??? 4PCl3(g) àP4(s) + 6Cl2(g) ∆𝐻°!"#=2168 kJ 4PCl5(g) àP4(s) + 10Cl2(g) ∆𝐻°!"#=3056 kJ
6. Use the thermochemical equations shown below to determine the enthalpy for the reaction:
NO2(g) + 7/2H2(g) à 2H2O(l) + NH3(g) ∆𝑯°𝒓𝒙𝒏 =??? N2(g) + 3H2(g) à 2NH3(g) ∆𝐻°!"#= -‐46 kJ NO2(g) + 2H2(g) à 1/2N2(g) + 2H20(l) ∆𝐻°!"#= -‐85 kJ
WHS w AP Chemistry D i r e c t M e t h o d 7. Explain why reactions involving reactant compounds with positive ∆𝐻°! values are generally more exothermic than those with negative ∆𝐻°! values.
8. Calculate the heat of decomposition for the process below at constant pressure and 25°C: CaCO! ! → CaO(!) + CO! (!) look in Appendix 3 of book
9. Calculate the heats of combustion for the following reactions from the standard enthalpies of formation listed in Appendix 3 of your book: a. C!H! (!) + 3O! (!) → 2CO! ! + 2H!O(!) b. 2H!S(!) + 3O! (!) → 2SO! ! + 2H!O(!)
10. Which of the following standard enthalpy of formation values is not zero at 25°C? a. Na(s) b. Ne(g)
c. CH4(g) d. S8(s)
e. Hg(l) f. H(g)
11. Benzene (C6H6) burns in air to produce carbon dioxide and liquid water. Calculate the heat released (in kilojoules) per gram of the compound reacted with oxygen. The standard enthalpy of formation of benzene is 49.04 kJ/mol.
12. Pentaborane-‐9, B5H9, is a colorless, highly reactive liquid that will burst into flame when exposed to oxygen. The reaction is 2B!H! (!) + 12 O! (!) → 5B!O! ! + 9H!O(!) Calculate the kilojoules of heat released per gram of the compound reacted with oxygen. The standard enthalpy of formation of B5H9 is 73.2 kJ/mol.