Problem: We can use Hess' law to calculate enthalpy changes that cannot be measured. One such reaction is the conversion of methane to ethylene: 2 CH4(g) → C2H4(g) + 2 H2(g)Calculate the ΔH° for this reaction using the following thermochemical data:ΔH°rxn = - 890.3CH4(g) + 2 O2(g)⟶ CO2(g)+2 H2O(l)ΔH°rxn = - 136.3C2H4(g )+ H2(g)⟶ C2H6(g)ΔH°rxn = - 571.62 H2(g) + O2(g) ⟶ 2 H2O(l)ΔH°rxn = - 3120.82 C2H6(g) + 7 O2(g)⟶4 CO2(g)+ 6 H2O(l)Delta {H}^circ = - 3120.8;{ m kJ}

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We are asked to calculate the ΔH° for 2 CH4(g) → C2H4(g) + 2 H2(g)  using the thermochemical data.


Modify the reactions such that when they add up, it will result to the target equation. 


Reaction                                                                                                Modification

2 CH4(g) +4 O2(g)2 CO2(g) +H2O(l                                          2Eqn1 (multiplied by 2)

C2H6(g) →  C2H4(g )+ H2(g)                                                               -Eqn2  (reversed)

 H2O(l→  H2(g) + 1/2 O2(g)                                                                - (1/2)Eqn3  (reversed and multiplied by 1/2)

2 CO2(g)+ H2O(lC2H6(g)  + 7/2 O2(g)                                      - (1/2)Eqn4 (reversed and multiplied by 1/2) 

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2 CH4(g) → C2H4(g) + 2 H2(g)


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Problem Details

We can use Hess' law to calculate enthalpy changes that cannot be measured. One such reaction is the conversion of methane to ethylene: 2 CH4(g) → C2H4(g) + 2 H2(g)

Calculate the ΔH° for this reaction using the following thermochemical data:


ΔH°rxn = - 890.3CH4(g) + 2 O2(g)⟶ CO2(g)+2 H2O(l)
ΔH°rxn = - 136.3C2H4(g )+ H2(g)⟶ C2H6(g)
ΔH°rxn = - 571.62 H2(g) + O2(g) ⟶ 2 H2O(l)
ΔH°rxn = - 3120.82 C2H6(g) + 7 O2(g)⟶4 CO2(g)+ 6 H2O(l)



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