I know I posted this question before, but can you clarify it??
MY QUESTION IS AFTER YOU SWITCHED THE DELTA H1 THE CHANGE IN THE ENTHALPY IS NEGATIVE.... BUT THAT DOES NOT MAKE SENSE BECAUSE THE CHANGE IN ENTHALPY OF THE ORIGINAL DELTA H1 IS NEGATIVE(ACCORDING TO THE CALCULATION OF THE ENTHALPY OF FORMATION YOU TOLD ME TO DO EARLIER...) AND SO AFTER SWITCHING IT WOULDN'T IT BE POSITIVE?????AND PLUS THIS IS PUTING THINGS TOGETHER WHICH IS EXOTHERMIC WHICH HAS A NEGATIVE H......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
You said....
Reverse H1 (make - dH) and add to H2.
See your post above.
To clarify, you are given the enthalpy changes for two hydrogenation reactions:
1) C2H2 + H2 --> C2H4 (ΔH1)
2) C2H2 + 2H2 --> C2H6 (ΔH2)
You are asked to determine the enthalpy change for the reaction:
C2H4 + H2 --> C2H6
To get the enthalpy change for this reaction, you need to use the given enthalpy changes for the two hydrogenation reactions.
The strategy you were told is to reverse the sign of ΔH1 (-ΔH1) and add it to ΔH2, which means (-ΔH1) + ΔH2.
The reasoning behind reversing ΔH1 is that when you switch the direction of a reaction, the sign of the enthalpy change also switches. So, if ΔH1 is negative (exothermic) for the forward reaction, when you reverse it, it becomes positive (endothermic). The change in sign ensures that the enthalpy change is correctly accounted for.
By adding (-ΔH1) to ΔH2, you are essentially combining the two reactions to get the overall enthalpy change for the desired reaction.
Now, to match this with the given options:
A. ΔH1 + ΔH2 - This is not the correct expression because ΔH1 should be reversed before adding.
B. ΔH1 - ΔH2 - This is also not the correct expression because ΔH1 should be reversed before subtraction.
C. ΔH2 - ΔH1 - This is the correct expression because it represents the sum of reversed ΔH1 and ΔH2.
D. -ΔH1 - ΔH2 - This is not the correct expression because it subtracts both ΔH1 and ΔH2 without reversing ΔH1.
Therefore, the correct expression for the enthalpy change of the reaction C2H4 + H2 --> C2H6 is C. ΔH2 - ΔH1.