Which statement best describes the enthalpy change for methane combustion reaction CH4 + 2 O2 → CO2 + 2 H2O(l)?

• A. The reaction is endothermic
• B. The reaction is exothermic
• C. The reaction has no enthalpy change
• D. The reaction only occurs at high temperature

Answer: (D)

The main idea here is that the enthalpy change of a combustion reaction is negative, meaning heat is released. In methane burning with oxygen to form carbon dioxide and liquid water, the bonds formed in the products are stronger overall than the bonds broken in the reactants, so energy is released to the surroundings. Using standard enthalpies of formation, the overall ΔH for CH4 + 2 O2 → CO2 + 2 H2O(l) is about -890 kJ per mole of CH4, indicating a substantial exothermic release of heat.

The fact that the reaction can be initiated by a flame or spark does not change the sign of ΔH; it just shows there is an energy barrier (activation energy) to start the reaction. Once started, the reaction proceeds with a large release of energy, which is why combustion is such a vigorous exothermic process. The statement that it only occurs at high temperature is not correct for describing the enthalpy change.