Question #7ee50
1 Answer
Explanation:
Your goal here is to figure out the enthalpy change of reaction for
#"D + F " -> " G + M", " "DeltaH_"rxn" = ? color(white)(aaaaa)color(red)("(*)")#
by using the thermochemical equations that describe the following reactions
#"G + C " -> " A + B" color(white)(aaaa)DeltaH_1 = +"277 kJ"" " " "color(blue)((1))#
#"C + F " -> " A" color(white)(aaaaaaaa)DeltaH_2 = +"303 kJ"" " " "color(blue)((2))#
#color(white)(aaa.)"D " ->" B + M" color(white)(aa.a)DeltaH_3 = - "158 kJ"" " " "color(blue)((3))#
Now, Hess' Law tells you that the enthalpy change of a reaction is independent of the number of steps needed to get from the products to the reactants.
Simply put, it doesn't matter how you get from the products to the reactants, the enthalpy change will always be the same for a given reaction.
Notice that in your target equation, you need
When you flip a chemical equation, you must change sign of the enthalpy change of reaction!
In this case,
#"Flip" color(white)(.)color(blue)((1)):" " "A + B " -> " G + C" color(white)(aaaa)DeltaH_"1 flip" = -"277 kJ"#
Now notice what happens when you add equation
#"A + B " -> " G + C" color(white)(aaaa)DeltaH_"1 flip" = -"277 kJ"#
#"C + F " -> " A" color(white)(aaaaaaaaaa)DeltaH_2 = +"303 kJ"#
#color(white)(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa)/color(white)(a)#
#color(red)(cancel(color(black)("A"))) + "B" + color(red)(cancel(color(black)("C"))) + "F " -> " G" + color(red)(cancel(color(black)("C"))) + color(red)(cancel(color(black)("A")))#
which is equivalent to
#"B + F " -> " G" #
Since you've added the two reactions, you must also 8add8 the enthalpy changes of reaction
#DeltaH_"1 flip + 2" = DeltaH_"1 flip" + DeltaH_2#
#DeltaH_"1 flip + 2" = -"277 kJ" + "303 kJ"#
#DeltaH_"1 flip + 2" = +"26 kJ"#
This means that I can get from
Notice that in order to get
#"B + F " -> " G" color(white)(aaaaaaaa)DeltaH_"1 flip + 2" = +"26 kJ"#
#color(white)(aaaa)"D " ->" B + M" color(white)(aaaaaaaa)DeltaH_3 = - "158 kJ"#
#color(white)(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa)/color(white)(a)#
#color(red)(cancel(color(black)("B"))) + "D + F " -> " G" + color(red)(cancel(color(black)("B"))) + "M"#
which is equivalent to equation
#"D + F " -> " G + M"#
The enthalpy change for this reaction will be
#DeltaH_"rxn" = DeltaH_"1 flip + 2" + DeltaH_3#
#DeltaH_"rxn" = + "26 kJ" + (-"158 kJ")#
#color(darkgreen)(ul(color(black)(DeltaH_"rxn" = -"132 kJ")))#