Question

Use average bond enthalpies to estimate the enthalpy change when 6.5 g of RDX decomposes.

Answer 1

Decomposition balanced reaction is as follows

C₃H₆N₆O₆$\rightarrow$ 3CO + 3N₂ + 3H₂O

So enthalpy change

Hrxn = [ bond enthalpy of brocken bonds ] - [bond enthapy of bonds form]

= [6x (C-N) + 6 x (C-H) + 3 x (N-N) + 3 x (N-O) + 3 x ( N=O) ] - [ 3 x (N $\equiv$ N) + 6 x (H-O) + 3x (C$\equiv$O)]

= [ 6 x 293 + 6 x 413 + (3 x 163)(3x 201) + 3 x 607 ] - [3 x 941+6 x 463 + 3 x 1072)]

= -1668 KJ/mol

Now moles of C₃H₆N₆O₆ = 6.5g/222.09g x 1 mole = 0.02926 moles

so

Hrxn = 0.02926 moles x (-1668 KJ/mol)

= -48.818 KJ