Question

Which of the following statements is true of combustion reactions?

A. Hydrogen gas (H2) must be one of the reactants

B. Water (H2O) is always a product

C. Energy is released by the reaction

D. Carbon monoxide (CO2) is always produced

Answer 1

Answer: Option (B) is the correct answer.

Step-by-step explanation:

A chemical reaction in which a hydrocarbon combines with oxygen atom that is present in air to yield carbon dioxide and water along with the products.

For example,
`CH_(4) + 2O_(2) \rightarrow CO_(2) + 2H_(2)O`

Here, methane undergoes combustion reaction to yield carbon dioxide and water as the products.

Hence, we can conclude that the statement water (
`H_(2)O`) is always a product, is true of combustion reactions.

Answer 2

Gasoline contains C and H atoms. During combustion, the carbon (C) from the fuel combines with oxygen (O2) from the air to produce carbon dioxide (CO2).
2 C8H18 + 25 O2 → 16 CO2 + 18 H2O.

Combustion reactions release large amounts of heat. They have negative enthalpy. A negative enthalpy represents an exothermic reaction, releasing heat. This reaction is spontaneous and exothermic, since we can obtain energy from the reaction; the ΔG (free energy) is negative (So 1 is true).

ΔG < 0, so the free energy of the system decreases with the reaction. Remember that when there is a negative ΔG the reaction goes from higher free energy to lower free energy, like in this case.