Parts (b) and (c) pertain to the combustion of propane according to the process outlined below.
C3H8(g) + 5O2(g) —> 3 CO2(g) + 4 H2O(l)
AН°= -2043.9 kJ/mol
b. Calculate the amount of heat that is released when 6.78 grams of propane, C3H8, is burned under standard conditions.
c. Explain why the combustion of C3H8 in the presence of O2, is exothermic in terms of bond energies.
To calculate the amount of heat released when 6.78 grams of propane is burned, we first need to calculate the moles of propane being burned.
Step 1: Calculate the molar mass of propane (C3H8):
C = 12.01 g/mol
H = 1.01 g/mol
Molar mass of C3H8 = 3(12.01 g/mol) + 8(1.01 g/mol) = 44.11 g/mol
Step 2: Calculate the moles of propane being burned:
6.78 g C3H8 x (1 mol / 44.11 g) = 0.154 mol C3H8
Step 3: Use the stoichiometry of the reaction to calculate the amount of heat released:
From the balanced equation, we can see that 1 mole of C3H8 releases 2043.9 kJ of heat. Therefore, 0.154 moles of C3H8 will release:
0.154 mol C3H8 x (-2043.9 kJ / 1 mol C3H8) = -314.6 kJ
Therefore, 6.78 grams of propane burned under standard conditions will release 314.6 kJ of heat.
Explanation for why the combustion of C3H8 in the presence of O2 is exothermic in terms of bond energies:
When propane reacts with oxygen to form carbon dioxide and water, new bonds are formed and energy is released in the form of heat. In the reactants (propane and oxygen), there are high-energy bonds, which are broken. In the products (carbon dioxide and water), new, lower-energy bonds are formed. The energy released from the formation of these new bonds is greater than the energy needed to break the original bonds, leading to a net release of energy. This excess energy released is observed as heat, making the combustion of C3H8 exothermic.