What is the heat of reaction (δh°rxn) for the combustion of acetone (c3h6o) given the following thermochemical equations? 1. 3 c(s) + 3 h2(g) + ½ o2(g) → c3h6o(ℓ) δhf° = −285.0 kj 2. c(s) + o2(g) → co2(g) δhf° = −394.0 kj 3. h2(g) + ½ o2(g) → h2o(ℓ) δhf° = −286.0 kj

Answer : The enthalpy of combustion of will be -1775 kJ

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.

According to this law, the chemical equation can be treated as ordinary algebraic expression and can be added or subtracted to yield the required equation. That means the enthalpy change of the overall reaction is the sum of the enthalpy changes of the intermediate reactions.

The combustion of will be,

The intermediate balanced chemical reaction will be,

(1)    

(2)    

(3)    

Now we are reversing the reaction 1, multiplying reaction 2 and 3 by 3 and then adding all the equations, we get :

(1)    

(2)    

(3)    

The expression for enthalpy of combustion of will be,

Therefore, the enthalpy of combustion of will be -1775 kJ

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