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. Melting of ice: H_2O(s) ightarrow H_2O(l)
• D. Combustion of methane: CH_4 + 2 O_2 ightarrow CO_2 + 2 H_2O

Answer: (D)

Verified step by step guidance

1. Understand the definition of an exothermic reaction: it is a chemical reaction that releases energy, usually in the form of heat, to the surroundings.
2. Analyze each given reaction to determine whether it releases or absorbs energy:
3. 1. Electrolysis of water (\$2 H_2O \rightarrow 2 H_2 + O_2$) requires electrical energy input to break water into hydrogen and oxygen, so it is endothermic (absorbs energy).
4. 2. Photosynthesis (\$6 CO_2 + 6 H_2O \rightarrow C_6H_{12}O_6 + 6 O_2$) uses sunlight energy to produce glucose and oxygen, so it is also endothermic (absorbs energy).
5. 3. Melting of ice (\(H_2O(s) \rightarrow H_2O(l)\)) requires heat to change phase from solid to liquid, so it is endothermic (absorbs energy).
6. 4. Combustion of methane (\(CH_4 + 2 O_2 \rightarrow CO_2 + 2 H_2O\)) releases heat as methane reacts with oxygen to form carbon dioxide and water, making it an exothermic reaction.