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bond enthalpy question

the question is:
this question is about hydrazine, N2H4, and ammonia, NH3. These are both compounds of nitrogen and hydrogen.
the equation is : N2H4 + O2 ----> N triple bond N + 2H2O
some average bond enthalpies are given below:
N-N = +163
N triple bond N = +945
N-H = +390
O=O = +497
O-H = +463

Use the data to calculate the enthalpy change for the reaction of hydrazine with oxygen, as shown.

I got -577, but the answer is +577??
I'm not sure why it's positive; pls help!!!!!!!
(edited 1 year ago)
Original post by d_burr
the question is:
this question is about hydrazine, N2H4, and ammonia, NH3. These are both compounds of nitrogen and hydrogen.
the equation is : N2H4 + O2 ----> N triple bond N + 2H2O
some average bond enthalpies are given below:
N-N = +163
N triple bond N = +945
N-H = +390
O=O = +497
O-H = +463

Use the data to calculate the enthalpy change for the reaction of hydrazine with oxygen, as shown.

I got -577, but the answer is +577??
I'm not sure why it's positive; pls help!!!!!!!


did you do bonds made-bonds broken or the other way round?
Reply 2
Original post by ihateschool968
did you do bonds made-bonds broken or the other way round?


I did bonds broken - bonds made

normally when i do it like that the answer is right, which is why I'm confused with this one
(edited 1 year ago)
Original post by d_burr
the question is:
this question is about hydrazine, N2H4, and ammonia, NH3. These are both compounds of nitrogen and hydrogen.
the equation is : N2H4 + O2 ----> N triple bond N + 2H2O
some average bond enthalpies are given below:
N-N = +163
N triple bond N = +945
N-H = +390
O=O = +497
O-H = +463

Use the data to calculate the enthalpy change for the reaction of hydrazine with oxygen, as shown.

I got -577, but the answer is +577??
I'm not sure why it's positive; pls help!!!!!!!


You are correct - it is negative not positive. Where did the so-called answer come from? It is wrong. (Think about it - this is a combustion reaction. When did you ever see a combustion reaction that is endothermic?)
(edited 1 year ago)
Reply 4
Original post by Pinkelephant1954
You are correct - it is negative not positive. Where did the so-called answer come from? It is wrong. (Think about it - this is a combustion reaction. When did you ever see a combustion reaction that is endothermic?)

I thought so too, I just wanted to make sure - thanks !!
It was from a question paper from physicsandmathstutor
(edited 1 year ago)

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