Question

The heat of combustion of carbon to $ C{O_2} $ is $ - 393.5KJ/mol $ . The heat released upon formation of $ 35.2g $ $ C{O_2} $ from carbon and oxygen gas will be.
(A) $ - 315\;KJ $
(B) $ + 315\;KJ $
(C) $ - 630\;KJ $
(D) $ + 31.5\;KJ $

Answer 1

Hint: The heat of combustion of a substance is defined as the amount of heat released when the given amount of a substance undergoes a combustion reaction. The enthalpy change for the combustion reaction is always negative. With the help of the reaction enthalpies we can easily calculate the heat released on the formation of carbon dioxide.

Complete step by step answer:
Now we are aware about the basic concept of heat of combustion or the enthalpy of combustion both represents the amount of energy required for the combustion of substance. Now we will get back to our question. So , the question says that the heat of combustion of carbon to $ C{O_2} $ is $ - 393.5KJ/mol $ . So, now we will understand the given quantities so we have given the heat or the enthalpy of combustion that is $ (\Delta H) = - 393.5KJ/mol $ . Now we will write the reaction for the heat of combustion of carbon to carbon dioxide. So according to the question the reaction will be $ C + {O_2} \to C{O_2} $ . So according to the reaction we have given the heat of combustion for $ 1 $ mole of . $ C{O_2} $ . Now if we calculate the number of moles in $ 35.2g $ of $ C{O_2} $ we can easily calculate the heat released. So, the moles of $ C{O_2} = \dfrac{{35.2}}{{44}} = 0.8mole $ the molar mass of $ C{O_2} $ is $ 44g/mol $ . Now we can easily calculate the heat of combustion for $ 0.8mole $ of $ C{O_2} $ which is $ - 393.5 \times 8 = - 315KJ $ .
Therefore, the correct option is (A).

Note:
The heat of combustion is always negative as they are the exothermic reactions. Exothermic reactions are the reactions in which heat is released therefore the calculated heat is negative. The temperature change affects the heat of combustion.