For the incomplete combustion reaction #"C"_6"H"_6(l) + 3/2"O"_2(g) -> 6"C"(s) + 3"H"_2"O"(l)#, answer these questions about enthalpy?
#a)# What is #DeltaH_(rxn)^@# ?
#b)# Is this exothermic or endothermic?
#c)# Where would heat be shown in the reaction with #Delta# ?
#d)# Which participants in the reaction are lower in energy?
1 Answer
Well, you'll have to spend some time looking at the Appendix of your textbook... I'm going to use NIST because I don't necessarily have your book.
#DeltaH_(rxn)^@ = -"906.4 kJ"#
You're also going to have to figure out WHICH solid carbon you formed... graphite? Diamond?
And eventually you should find:
#DeltaH_f^@("C"_6"H"_6(l)) = "49.04 kJ/mol"#
#DeltaH_f^@("H"_2"O"(l)) = -"285.8 kJ/mol"# [Assuming your carbon solid is carbon graphite, why is that all we need?]
The enthalpy of reaction can then be calculated from these enthalpies of formation:
#DeltaH_(rxn)^@ = sum_P n_P DeltaH_(f,P)^@ - sum_R n_R DeltaH_(f,R)^@# where:
#sum_P# is the sum over the products.
#n# is the mols of something.#P# indicates products and#R# indicates reactants.#DeltaH_f^@# is the standard enthalpy of formation for a substance in its particular phase, i.e. the heat involved at constant pressure to form#"1 mol"# of the substance at#25^@ "C"# and#"1 atm"# using its elements in their standard state.You must know how to look these up in your book on your own, or you must be given these in a test question. You must also know why the standard enthalpy of formation of pure elements is ZERO.
And so, the enthalpy of reaction is:
#color(blue)(DeltaH_(rxn)^@) = [("6 mols" cdot "0 kJ/mol") + ("3 mols" cdot -"285.8 kJ/mol")] - [("1 mol" cdot "49.04 kJ/mol") + ("1.5 mols" cdot "0 kJ/mol")]#
#= color(blue)(-"906.4 kJ")#
This is by definition exothermic.
#DeltaH_(rxn)^@ < 0# . Is it intuitive that this incomplete-combustion reaction is exothermic?
#"C"_6"H"_6(l) + 3/2"O"_2(g) -> 6"C"(s) + 3"H"_2"O"(l) + Delta# If I added the heat term on the products side, it means heat energy was released...
....The products are thus lower in energy relative to the reactants.
Just from part
#a)# , you can tell that#sum_P n_P DeltaH_(f,P)^@ < sum_R n_R DeltaH_(f,R)^@# . To make sure you understand this notation, what did I just say in plain English?
