Which one of the following is an exothermic process?

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

Answer: (A)

Verified step by step guidance

1. Understand the definition of an exothermic process: it is a chemical or physical change that releases heat to the surroundings, resulting in a negative enthalpy change (ΔH < 0).
2. Analyze the combustion of methane reaction: CH_4 + 2 O_2 \rightarrow CO_2 + 2 H_2O. Combustion reactions typically release energy because bonds in the products are stronger and more stable than in the reactants, making this process exothermic.
3. Examine photosynthesis: 6 CO_2 + 6 H_2O \rightarrow C_6H_{12}O_6 + 6 O_2. This process requires energy input from sunlight to form glucose, so it is endothermic (absorbs energy).
4. Consider evaporation of water: H_2O(l) \rightarrow H_2O(g). Evaporation requires energy to overcome intermolecular forces, so it absorbs heat and is endothermic.
5. Look at melting of ice: H_2O(s) \rightarrow H_2O(l). Melting also requires heat input to break the solid structure, making it an endothermic process.