The AH for the reaction is -137 kJ
Enthalpy of formation is the energy required by reactants to convert into products . It value can be positive or negative depending on the type of reaction, Endothermic or Exothermic.
In the question 3 step reaction is given
C₂H4 (g) + 3 O₂ (g) → 2 CO₂ (g) + 2 H₂O (1) AH = - 1411. KJ
2 C₂H6 (g) + 7 O₂ (g) → 4 CO₂ (g) + 6 H₂O (1) AH = - 3120. KJ
2 H₂ (g) + O₂ (g) → 2 H₂O (1) AH = -571.6 kJ
To determine the enthalpy of formation for
C₂H4 (9) + H₂ (9)→ C₂H6 (g)
Adding reaction 1 and (1/2) 3 will give
C₂H4 (g) + 7/2 O₂ (g) + H₂ (g) ----> 2 CO₂ (g) + 3H₂O AH = -1696.8
For the second reaction
We will reverse the reaction and multiply it by 1/2
2 CO₂ (g) + 3 H₂O (1) --> C₂H6 (g) + 7/2 O₂ (g) AH = +1560
Adding the two new reactions.
C₂H4 (9) + H₂ (9)→ C₂H6 (g) AH = -137 kJ
Therefore The AH for the reaction is -137 kJ
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