Which of the following is an example of an exothermic reaction?

• A. Electrolysis of water: 2 H_2O ightarrow 2 H_2 + O_2
• B. Photosynthesis: 6 CO_2 + 6 H_2O ightarrow C_6H_{12}O_6 + 6 O_2
• C. Combustion of methane: CH_4 + 2 O_2 ightarrow CO_2 + 2 H_2O
• D. Melting of ice: H_2O(s) ightarrow H_2O(l)

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the definition of an exothermic reaction. An exothermic reaction is one that releases energy, usually in the form of heat, to the surroundings.
2. Step 2: Analyze each given reaction to determine whether it releases or absorbs energy. For example, electrolysis of water requires energy input to break water into hydrogen and oxygen, so it is endothermic.
3. Step 3: Photosynthesis also requires energy input from sunlight to convert carbon dioxide and water into glucose and oxygen, making it an endothermic process.
4. Step 4: Combustion of methane involves burning methane in oxygen to produce carbon dioxide and water, releasing energy as heat and light, which characterizes it as an exothermic reaction.
5. Step 5: Melting of ice requires heat absorption to change solid water into liquid water, so it is an endothermic physical change, not exothermic.