Thermochemistry – identify the endothermic reaction Which of the following reactions is endothermic under standard conditions?

Introduction / Context:
Knowing whether a reaction absorbs or releases heat is vital for furnace design, reactor heat balance, and process safety. Endothermic reactions require heat input; exothermic reactions release heat and may need cooling or careful temperature control.

Given Data / Assumptions:

• Reactions are considered at standard conditions for qualitative identification.
• Combustion reactions are typically exothermic.
• Thermal decomposition of carbonates is known to be endothermic.

Concept / Approach:
Decomposition of calcium carbonate into quicklime and carbon dioxide consumes heat (endothermic), forming the basis of lime kiln operation. By contrast, oxidation reactions such as forming CO or CO2 and methanation (CO + 3 H2 → CH4 + H2O) are exothermic, releasing heat.

Step-by-Step Solution:

Verification / Alternative check:
Enthalpy data show positive heat of reaction for calcination, while the combustion and methanation reactions have negative heats (exothermic), aligning with industrial observations (heat release in burners and methanators).

Why Other Options Are Wrong:

• C + ½ O2 → CO: Exothermic partial oxidation.
• CO + 3 H2 → CH4 + H2O: Methanation is exothermic.
• CO + ½ O2 → CO2: Exothermic full/partial oxidation step.

Common Pitfalls:
Assuming all gas-producing reactions are exothermic; calcination is a classic endothermic process despite gas evolution.

Final Answer:
CaCO3 → CaO + CO2