The enthalpy of combustion of acetylene c2h2 is described by

c2h2 (g) + (5/2)o2 (g) > > > > > > > co2 (g) + h2o (l) heat of reaction (rxn) = -1299kj/mol

calculate the enthalpy of formation of accetylene, given the following enthalpies of formation
standard formation [co2 (g)]= -393.5 kj/mol
standard formation [h2o (l)] = -285.8 kj/mol

The enthalpy of formation of accetylene is, 226.2 kJ/mol

Explanation:

According to Hess’s law of constant heat summation, the heat absorbed or evolved in a given chemical equation is the same whether the process occurs in one step or several steps.

The chemical equation for the combustion of acetylene follows:

(1)

The formation of will be,

(2)    

The formation of will be,

(3)    

The formation of will be,

(4)    

Now we are reversing equation 1, multiplying equation 2 by 2 and then adding equation 1, 2 and 3, we get:

Reaction (1) :

Reaction (2) :

Reaction (3) :

Therefore, the enthalpy of formation of accetylene is, 226.2 kJ/mol

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.