Please explain.
The enthalpy changes for two different hydrogenation reactions of C2H2 are:
C2H2+H2---->C2H4 Delta H 1
C2H2+2H2---->C2H6 Delta H 2
Which expression represents the enthalpy change for the reaction below?
C2H4+H2---->C2H6 Delta H = ?
A. Delta H 1 + Delta H 2
B. Delta H 1 - Delta H 2
C. Delta H 2 - Delta H 1
D. -Delta H 1- Delta H 2
To determine the enthalpy change for the reaction C2H4 + H2 --> C2H6, we can use Hess's Law. Hess's Law states that if a reaction can be expressed as the sum of two or more other reactions, then the enthalpy change of the overall reaction is equal to the sum of the enthalpy changes of the individual reactions.
In this case, we can see that the given reactions are hydrogenation reactions of C2H2, and we want to find the enthalpy change of the reaction involving C2H4.
We can find C2H4 by combining the given reactions:
1. C2H2 + H2 --> C2H4 (Delta H1)
2. C2H4 + H2 --> C2H6 (Delta H2)
To obtain C2H4 + H2 --> C2H6, we can subtract reaction 1 from reaction 2:
C2H4 + H2 = (C2H2 + H2) + (C2H4 + H2) – C2H2
C2H4 + H2 = (C2H4 + H2) + C2H4 – C2H2
C2H4 + H2 = C2H4 + C2H4 – C2H2
The delta H for this reaction, Delta H, can be obtained by rearranging the terms:
Delta H = C2H4 + C2H4 – C2H2
In this case, the enthalpy change for the reaction C2H4 + H2 --> C2H6 is equal to -Delta H1 - Delta H2.
Therefore, the correct expression for the enthalpy change for this reaction is option D: -Delta H1 - Delta H2.