Hess's Law Practice

Hessʼs Law (Page 509-511)
A theoretical way to determine ΔH for a chemical reaction is provided by Hessʼs law, which states
that if two or more thermochemical equations can be added to produce a final equation for a
reaction, then the enthalpy change for the final reaction equals the sum of the enthalpy changes
for the individual reactions.
Practice Problems
Use the given thermochemical equations a and b to determine ΔH for each of the the following
reactions.
1.) # # # # CuO(s)# +# H2(g)# # ---># Cu(s)# # +! H2O(g) , !! ΔH =
# # # a. # # # # # # CuO(s)# ---># Cu(s) +! ! ½ O2(g)! , ! ΔH = 155 kJ
! ! ! b. !! H2(g)# +# ½ O2(g)# ---># H2O(g) , ! ! ! ! ! ΔH = -242 kJ
2.) # # # 2CO(s)# +# O2(g)# # ---># 2CO2(g), !! ΔH =
# # # a. # # # # # CO(g)#---># C(s)# +! ! ½ O2(g) , ! ΔH = 110. kJ
! ! ! b. !! C(s)# +# O2(g)# # ---># CO2(g) , ! ! ! ! ! ! ΔH = -394 kJ
(over)

3.) # # # # S(s)# +# O2(g)# # # ---># SO2(g), ! ! ΔH =
# # # a. # # 2SO2(g)# +# O2(g)# # ---># 2SO3(g), !! ! ! ΔH = -196. kJ
! ! ! b. !! 2S(s)# +# 3O2(g)# # ---># 2SO3(g), !! ! ! ΔH = -790. kJ
4.) # # # # CS2(l)## +# 2H2O(l)# # ---># CO2(g)# +! 2H2S(g) , ! ΔH =
3 2
# # # a. ## H2S(g)# +# # O2(g)# # ---># H2O(l)## +! SO2(g),! ! ΔH = -563 kJ
! ! ! b. !! CS2(l)## +# 3O2(g)# # ---># CO2(g)# +! 2SO2(g),! ! ΔH = -1075 kJ