Calculate the average C—C bond strength in cyclopropane (in kJ/mol). Its combust

Question

Calculate the average C—C bond strength in cyclopropane (in kJ/mol). Its combustion and the experimental enthalpy of reaction are:

C3H6(g) + 4.5O2(g) 3CO2(g) + 3H2O(g) Hºrxn = -1,957.7 kJ/mol

Average bond strentgh (kj/mol):

O - H = 463

C - H = 414

C - C = 347

O -- O = 498 (DOUBLE BOND)

C -- O = 745 (DOUBLE BOND)

C -- O (CO2 = 799)

Since all reactants and products are in the gaseous state, bond energies may be used to estimate the enthalpy of reaction. Do not use the standard C—C bond strength, leave the C—C bond strength as an unknown and solve for its value.

Explanation / Answer

on reaction sides bonds are:

C-C : 2 bond

C-H : 6 bond

O=O : 4.5 bond

on product side, bonds are:

C=O : 6 bond

O-H : 6 bond

delta H = Bond energy (reactant) – Bond energy (product)

-1957.7 = 2*(C-C) + 6*(C-H)+4.5*(O=O) – 6*(C=O) -6*(O-H)

-1957.7 = 2*(C-C) + 6*(414) + 4.5*(498) -6(799) -6(463)

2*(C-C) = 889.3

(C-C) = 444.65 KJ/mol

Answer: 444.65 KJ/mol

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