Final answer:
The properly balanced complete combustion reactions among the given options are I: CH₄ (g) + 2 O₂ (g) --> CO₂ (g) + 2H₂O and II: C₃H₈ + 5 O₂ --> 3 CO₂ + 4 H₂O.
Step-by-step explanation:
The chemical equation I: CH₄ (g) + 2 O₂ (g) --> CO₂ (g) + 2H₂O represents the combustion of methane (CH₄). For this equation to be properly balanced, the number of atoms of each element on the reactant side must equal the number of atoms of the same element on the product side. In this case, we have 1 carbon (C) on both sides, 4 hydrogen (H) on both sides, and 4 oxygen (O) on both sides, satisfying the law of conservation of mass.
Similarly, for equation II: C₃H₈ + 5 O₂ --> 3 CO₂ + 4 H₂O, the combustion of propane (C₃H₈) is represented. Balancing the equation ensures that the number of atoms for each element is conserved. In this case, we have 3 carbon (C) atoms, 8 hydrogen (H) atoms, and 10 oxygen (O) atoms on both sides, maintaining mass balance.
On the other hand, equation III: C₂H₅OH + 3O₂ --> 2 CO₂ + 3H₂O represents the combustion of ethanol. However, it is not properly balanced as the number of oxygen atoms on the reactant side is not equal to the number on the product side, violating the law of conservation of mass.
In summary, equations I and II are properly balanced complete combustion reactions, adhering to
the principles of conservation of mass. The balanced equations ensure that the total number of atoms for each element is the same on both the reactant and product sides. For example, in equation I, the balanced coefficients ensure that 1 carbon (C), 4 hydrogen (H), and 4 oxygen (O) atoms are present on both sides of the reaction.
In equation II, representing the combustion of propane, the coefficients are adjusted to have 3 carbon (C), 8 hydrogen (H), and 10 oxygen (O) atoms on both sides. This balance guarantees that no atoms are created or destroyed during the chemical reaction, aligning with the fundamental principle of the conservation of mass.
Equation III, however, is not properly balanced as the number of oxygen atoms on the reactant side is insufficient to match the number on the product side. This inconsistency violates the conservation of mass, indicating an incomplete combustion reaction.
In conclusion, equations I and II represent properly balanced complete combustion reactions, meeting the criteria of mass conservation, while equation III does not fulfill these requirements and is therefore not properly balanced.