Question #21cf8
1 Answer
Explanation:
So, the three reactions that you need to use in order to get the target reaction looks like this
#"H"_text(2(g]) + "F"_text(2(g]) -> 2"HF"_text((g])" "color(blue)((1))" "DeltaH_1 = -"537 kJ"#
#"C"_text((s]) + 2"F"_text(2(g]) -> "CF"_text(4(g]) " "color(blue)((2))" "DeltaH_2 = -"680 kJ"#
#2"C"_text((s]) + 2"H"_text(2(]) -> "C"_2"H"_text(4(g]) " "color(blue)((3))" "DeltaH_3 = +"52.3 kJ"#
According to Hess' Law, the enthalpy change of reaction for a particular chemical reaction is independent of the steps needed to get from the reactans to the products.
Your target reaction is
#"C"_2"H"_text(4(g]) + 6"F"_text(2(g]) -> 2"CF"_text(4(g]) + 4"HF"_text((g])#
Notice that you need 2 moles of
You also need to have
This will get you
#2"H"_text(2(g]) + 2"F"_text(2(g]) -> 4"HF"_text((g])" "color(blue)((1^'))"#
#2"C"_text((s]) + 4"F"_text(2(g]) -> 2"CF"_text(4(g]) " "color(blue)((2^'))"#
#DeltaH_2^' = 2 * DeltaH_2 = 2 * (-"680 kJ") = -"1360 kJ"#
and
#"C"_2"H"_text(4(g]) -> 2"C"_text((s]) + 2"H"_text(2(])" "color(blue)((3^'))#
#DeltaH_3^' = -DeltaH_3 = -"52.3 kJ"#
Now if you add these three reactions,
#color(red)(cancel(color(black)(2"H"_text(2(g])))) + 2"F"_text(2(g]) + color(red)(cancel(color(black)(2"C"_text(2(g])))) + 4"F"_text(2(g]) + "C"_2"H"_text(4(g]) -> 4"HF"_text((g]) + 2"CF"_text(4(g]) + color(red)(cancel(color(black)(2"C"_text((g])))) + color(red)(cancel(color(black)(2"H"_text(2(g]))))#
#"C"_2"H"_text(4(g]) + 6"F"_text(2(g]) -> 2"CF"_text(4(g]) + 4"HF"_text((g])#
The enthalpy change of reaction will thus be
#DeltaH_"rxn" = DeltaH_1^' + DeltaH_2^' + DeltaH_3^'#
#DeltaH_"rxn" = -1074 - 1360 + (-52.3) = color(green)(-"2490 kJ")#
I'll leave the answer rounded to three sig figs.