Thermodynamics (IIT question)

Compute the heat of formation of liquid methyl alchohol in kilo joule per mole using the following data. Heat of vaporisation of liquid methyl alcohol=38 kJ/mol. Heat of formation of gaseous atoms from the elements in their standard states: H - 218 kJ/mol, O - 249 kJ/mol.
Average bond energies: C-H, 415 kJ/mol; C-O, 356 kJ/mol and O-H, 463 kJ/mol.

5 Answers

11
Khyati ·

CH3OH (l)--->CH3OH ΔH = 38 kJ/mol

1/2H2(g)------------->H(g) ΔH = 218 kJ/mol

C(graphite)-->C(g) ΔH = 715 kj/mol

1/2 O2---------->O(g) ΔH = 249 kJ/mol

CH(g)---------------> C(g) +H(g) ΔH= 415kJ/mol

C-O(g)---------->C(g) + O(g) ΔH= 356kJ/mol

O-H(g)--------------->O(g) + H(g) ΔH=463kJ/mol

For the calculation of ΔHf of CH3OH, the thermochemical equation is written as

C(s) + 2H2 +1/2 O2(g)----->CH3OH, ΔH=?-----(a)

On th basis of bond enthalpy concept,

ΔH = - {sum of bond enthalpy of all bonds of products - sum of bond enthalpies of all bonds of reactants}

ΔH = {(3 x EC-H + EC-O + EO-H) - (EC(s)----->C(g)+ E1/2 H-H(g) + E1/2O2(g)}

= - {(3 x 415 +356 +463) - (715 + 4 x 218 +249)}

ΔH = -228 kJ/mol -------- (answer of equation (a))

CH3OH (g) ------> CH3OH(l)--------- (b)

On adding two thermochemical equation (a) and (b),

ΔHf = -266kJ/mol,

Hence the answer.

11
Khyati ·

Oye Talwar Machhli [3][3], Tell here whether the answer is correct or not.

1
chessenthus ·

Hello Khyati,

Yes, This is the correct answer. -266kj/mol.!

1
swordfish ·

@Chessenthus- Tu Talwaar Machhli hai kya?? :p
@Khyati- Its absolutely right :)

11
Khyati ·

Oye Swordfish, talwar Machhli tumhara naam hai, janbhujkar anjaan mat bano [3]

Your Answer

Close [X]