calcuting enthalpy when given bond dissociation energy

[Smooth Operater]

calculate the enthalpy change for the following reaction:

C (s) + 2 H2 (g) --> CH4 (g)
(bond dissociation energy of H-H and C-H bonds are 436 KJ/mol and 415 KJ/mol, respectively. dHf of C (g) = 715 KJ/mol)

My solution:

CH4 - 2*H2 - C =
4*C-H - 2*H-H - C =
4*(415) - (715) - 2*2*(436) =


Am I doing it right?

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[aranjuez]

calculate the enthalpy change for the following reaction:

C (s) + 2 H2 (g) --> CH4 (g)
(bond dissociation energy of H-H and C-H bonds are 436 KJ/mol and 415 KJ/mol, respectively. dHf of C (g) = 715 KJ/mol)

My solution:

CH4 - 2*H2 - C =
4*C-H - 2*H-H - C =
4*(415) - (715) - 2*2*(436) =


Am I doing it right?

Almost... note that they give you the bond DISSOCIATION energy. The 4 CH bongs are being formed, not dissociated, so that would get the negative sign in front of it. Products (negative) while reactants (positive). The bonds in the reactants are the ones that are dissociating.
-4*(415) + [(715) + 2*(436)]

By the way... in the H2 bond... there's only one bond between the two H's... but we have two of those types of molecules... so it's only 2*436.

aranjuez

 

[Smooth Operater]

Hello, thanks for the reply. To find the enthaly, we go

dH of product - dH of reactant

So, isn't it -4*415 - [ -2*436 + 715]

dH of C should be postive right?

 

[Manyak222]

An easier way of thinking of it is this:
bond breaking - bond making cuz you might have a product that released OR absorbed heat. But breaking bonds is ALWAYS endothermic and making bonds is always exothermic, thus the positve and negative delta H's, respectively.

Hello, thanks for the reply. To find the enthaly, we go

dH of product - dH of reactant

So, isn't it -4*415 - [ -2*436 + 715]

dH of C should be postive right?

 

[Smooth Operater]

but, how does that explain the -ve value of C. C is C, we are not break or making it. Sry that I am kinda slow. Thanks

 

[Manyak222]

but, how does that explain the -ve value of C. C is C, we are not break or making it. Sry that I am kinda slow. Thanks

that part was given. it could've been positive or negative. in problems like taht, going from one state (eg liquid, gas) to another has to be given . in this case it was given as positive.

 

[GRAD]

yea im totally confused about these kinds of problems....why is the 415 negative?

 

[Smooth Operater]

C (s) + 2 H2 (g) --> CH4 (g)
(bond dissociation energy of H-H and C-H bonds are 436 KJ/mol and 415 KJ/mol, respectively. dHf of C (g) = 715 KJ/mol)

yeah, I am kinda confused too..

Don't we alwasy go "dH of products - dH of the reactants" to find the dH of one step rxn?
So,
-4*415 - [ 715 + 2*436] should be the equation?

 

[Smooth Operater]

bump