Consider the reaction: 2H_2(g) + O_2(g) → 2H_2O(l). Does this reaction absorb or release energy?

• A. It absorbs energy (endothermic)
• B. It releases energy (exothermic)
• C. It depends on the temperature
• D. It neither absorbs nor releases energy

Answer: (B)

Verified step by step guidance

1. Identify the type of reaction given: 2H_2(g) + O_2(g) → 2H_2O(l) is a combustion reaction where hydrogen gas reacts with oxygen gas to form liquid water.
2. Recall that combustion reactions typically release energy because they form strong bonds in the product molecules, which results in a net release of energy.
3. Understand the terms: an exothermic reaction releases energy to the surroundings, often as heat, while an endothermic reaction absorbs energy from the surroundings.
4. Consider the bond energies: breaking the H-H and O=O bonds requires energy input, but forming the O-H bonds in water releases more energy than was used to break the initial bonds.
5. Conclude that since more energy is released in forming water than is consumed breaking the reactants' bonds, the reaction releases energy and is therefore exothermic.