Which of the following reactions is exothermic?

• A. Combustion of methane: CH_4 + 2 O_2 ightarrow CO_2 + 2 H_2O
• B. Decomposition of calcium carbonate: CaCO_3 ightarrow CaO + CO_2
• C. Photosynthesis: 6 CO_2 + 6 H_2O ightarrow C_6H_{12}O_6 + 6 O_2
• D. Electrolysis of water: 2 H_2O ightarrow 2 H_2 + O_2

Answer: (A)

Verified step by step guidance

1. Understand that an exothermic reaction releases energy, usually in the form of heat, to the surroundings. This means the products have lower energy than the reactants.
2. Look at the types of reactions given: combustion, decomposition, photosynthesis, and electrolysis. Combustion reactions typically release energy because they involve burning a fuel with oxygen to produce CO_2 and H_2O.
3. Decomposition reactions often require energy input to break down compounds, so they are usually endothermic (absorb energy).
4. Photosynthesis is a process where plants absorb energy from sunlight to convert CO_2 and H_2O into glucose and oxygen, so it is endothermic.
5. Electrolysis uses electrical energy to split water into hydrogen and oxygen gases, so it also requires energy input and is endothermic.