Question

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

Answer 1

The molecular mass of one molecule of carbon dioxide is 44 grams. 44 grams of $C{O_2}$ releases -393.5 kJ/mol heat. Using this, the heat released for the formation of 35.2 g of $C{O_2}$ can be calculated.

Complete step by step answer:
We can represent the formation of carbon dioxide from carbon and dioxygen by the equation:
$C(s) + {O_2}(g) \to C{O_2}(g)$

The data given in the question is:
${\Delta _f}H$= -393.5 kJ/mol that is 1 mole of $C{O_2}$ releases -393.5 kJ/mol heat.
To find the heat released upon formation of 35.2 g of $C{O_2}$​ from carbon and oxygen gas, we have to first find the mass of one molecule of carbon dioxide.
One molecule of carbon dioxide consists of 1 atom of carbon and 2 atoms of oxygen. We can calculate the mass of one molecule of carbon dioxide by adding the masses of 1 atom of carbon and 2 atoms of oxygen.

Mass of carbon atom = 12 grams.
Mass of oxygen atom = 16 grams.
Mass of one molecule of carbon dioxide = 12 + 2(16) = 44 grams.
Heat released by,
1 mole of $C{O_2}$ $\to$ -393.5 kJ/mol
44 grams of $C{O_2}$ $\to$ -393.5 kJ/mol
1 gram of $C{O_2}$ $\to$ $\dfrac{{ - 393.5}}{{44}}$ kJ/mol
35.2 g of $C{O_2}$ $\to$ $\dfrac{{ - 393.5}}{{44}} \times 35.2$g = -315 KJ/mole
So, the correct answer is “Option C”.

Note: Combustion of carbon to $C{O_2}$ is an exothermic reaction. It is denoted by a negative sign which indicates that the heat is released. In contrast, the positive sign denotes absorption of heat.