[Jules18]

Hey guys, this is my question.
I have almost no idea how to tackle it, so if you can help me at all I'll be forever in your debt.

An unknown gas, X₂(g), which behaves much like N₂(g), is analysed and the following enthalpies of formation are obtained;

Gas         delta(Hf)

X(g)        441 kJ/mol
H(g)        216 kJ/mol
X₂H₄(g)     321 kJ/mol

We also know that the X-H bond energy is 323 kJ/mol. Use this information to estimate the X-X single-bond energy. (ans in kJ/mol.)

(is it a typo that the first two gases don't have a subscript of 2, like they do in the preamble?)

[sjb]

...
(is it a typo that the first two gases don't have a subscript of 2, like they do in the preamble?)

No, I don't think so - recall that for N₂ (and indeed any element in its natural state at standard conditions) the  :delta: Hf is 0, so this most likely refers to the reaction 1/2 X₂ (g) X (g) having a heat of formation as quoted.

Can you write a series of equations that lead you from X₂ and H₂ in terms of known and unknown bond energies?

[Jules18]

mmm... I tried one equation:

X₂ + H₂ --> X₂H₄

... do I need more?  am I sounding totally clueless right now?   

I'm having trouble understanding why the question gives me information about enthalpies of formation when it wants me to give it a bond enthalpy.  How do I use H_f to get an Hº of a rxn?

please help ?

[sjb]

mmm... I tried one equation:

X₂ + H₂ --> X₂H₄

... do I need more?  am I sounding totally clueless right now?   

I'm having trouble understanding why the question gives me information about enthalpies of formation when it wants me to give it a bond enthalpy.  How do I use H_f to get an Hº of a rxn?

please help ?

Well, for starters your equation needs to be balanced, so that will be X₂ + 2 H₂  X₂H₄.

So you're breaking 1 x X~X bond (I have indicated the bond here as a ~ as you don't have, or need the actual bond order), and 2 H-H bonds, which will form 2 X and 4 H, and you know that from X₂ and 2H₂ you have the energy for the process that forms your product. How many X~X bonds are there in your product? How many X-H bonds?

[savy2020]

First recall the structure of N₂H₄{which would be same as that of X₂H₄} . What I mean by structure is how many N-H bonds and how many H-H bonds.
Now after that just recall that a molecule is formed from atoms due to formation of chemical bonds between the atoms-a process with releases energy. The energy released in forming a bond is called bond energy.
In the reverse process breaking a bond requires energy.

I'm having trouble understanding why the question gives me information about enthalpies of formation when it wants me to give it a bond enthalpy.  How do I use Hf to get an Hº of a rxn?

If you just take 2 X atoms and 4 H atoms and then form (?) X-H bonds and (?)X-X bonds then the energy change for such an equation is same as the total of the energy released by all the bond formations.

(?) It's for you to find out how many bonds are there 

[Jules18]

Well, for starters your equation needs to be balanced, so that will be X₂ + 2 H₂  X₂H₄.

So you're breaking 1 x X~X bond (I have indicated the bond here as a ~ as you don't have, or need the actual bond order), and 2 H-H bonds, which will form 2 X and 4 H, and you know that from X₂ and 2H₂ you have the energy for the process that forms your product. How many X~X bonds are there in your product? How many X-H bonds?

Okay ... I've drawn that out.  There are 4 X-H bonds and one X-X bond in X2H4.  I get that.  but I'm still stuck

[sjb]

OK, so from X₂H₄ to 2X + 4H you're breaking 1 X-X bond, and 4 X-H bonds. How much energy is that?

You can also do this via X₂H₄  X₂ + 2H₂  2X + 4H, for which you have the energies in your question. Hess' law says that these two ways are the same energy, so equate the two expressions, and solve for the energy of the X-X bond.