Since your question is a little vague to begin with, I”ll assume that you must determine the standard enthalpy change of formation of nitroglicerine, #”C”_3″H”_5″N”_3″O”_9#.
The idea here is that you need to use the standard enthalpy change of reaction, #DeltaH^
You are watching: Calculate the standard enthalpy of formation (δh∘for nitroglycerin
#, and the standard enthalpy changes of formation of the products to find the standard enthalpy change of formation of interest.
You can find the standard enthalpy changes of formation of carbon dioxide, water, and nitrogen gas here
https://en.wikipedia.org/wiki/Standard_enthalpy_change_of_formation_%28data_table%29
To find the standard enthalpy change of formation of nitroglicerine, use Hess” Law, which tells you that the enthalpy change of a reaction is independent of the path taken and the number of steps needed for that reaction to take place.
This means that you can express the standard enthalpy change of reaction by using the standard enthalpy changes of formation of the reactant and of the products
#DeltaH_”rxn”^
= sum(n xx DeltaH_”f prod”^
) – sum(m xx DeltaH_”f react”^
)” “#, where
#n#, #m# – the stoichiometric coefficients of the species that take part in the reaction.
So, the standard enthalpy changes of formation for one mole of carbon dioxide, water, and nitrogen gas are
#”CO”_2: -“393.51 kJ/mol”#
#”H”_2″O”: -“241.82 kJ/mol”#
#”N”_2: ” 0 kJ/mol”#
So, your reaction produces
12 moles of carbon dioxide10 moles of water6 moles of nitrogen gas
and requires
4 moles of nitroglicerine
Notice that the enthalpies of formation are given in kilojoules per mole, so convert the enthalpy change of reaction to kilojoules
#-5678color(red)(cancel(color(black)(“J”))) * “1 kJ”/(10^3color(red)(cancel(color(black)(“J”)))) = -“5.678 kJ”#
Plug in your values and solve for #DeltaH_”f nitro”^
#
#-“5.678 kJ” = <12color(red)(cancel(color(black)("moles"))) * (-393.51"kJ"/color(red)(cancel(color(black)("mol")))) + 10color(red)(cancel(color(black)("moles"))) * (-241.82"kJ"/color(red)(cancel(color(black)("mole")))) + 6color(red)(cancel(color(black)("moles"))) * 0"kJ"/color(red)(cancel(color(black)("mole")))> – (4 * DeltaH_”f nitro”^
)#
Rearrange to get
#4DeltaH_”f nitro”^
= -“7140.32 kJ” + “5.678 kJ”#
#DeltaH_”f nitro”^
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= (-“7134.642 kJ”)/4 = color(green)(-“1784 kJ/mol”)#
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