Which of the following reactions is endothermic?

• A. Neutralization: HCl + NaOH ightarrow NaCl + H_2O
• B. Freezing of water: H_2O(l) ightarrow H_2O(s)
• C. Photosynthesis: 6 CO_2 + 6 H_2O ightarrow C_6H_{12}O_6 + 6 O_2
• D. Combustion of methane: CH_4 + 2 O_2 ightarrow CO_2 + 2 H_2O

Answer: (C)

Verified step by step guidance

1. Understand the definition of an endothermic reaction: it is a process that absorbs heat from the surroundings, meaning the system gains energy.
2. Analyze the neutralization reaction (HCl + NaOH \rightarrow NaCl + H_2O): this reaction typically releases heat, so it is exothermic, not endothermic.
3. Consider the freezing of water (H_2O(l) \rightarrow H_2O(s)): freezing releases heat to the surroundings, so it is also exothermic.
4. Look at 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, indicating it absorbs energy and is endothermic.
5. Examine the combustion of methane (CH_4 + 2 O_2 \rightarrow CO_2 + 2 H_2O): combustion reactions release energy, so this is exothermic.