When calcium carbonate decomposes at 298 K,
CaCO₃(s) → CaO(s) + CO₂(g)
the process absorbs heat from the surroundings. In a calorimetric experiment, 15.0 g of CaCO₃(s) required 26.7 kJ of heat to decompose completely under standard-state conditions.
The following standard molar enthalpies of formation are provided:
• CaCO₃(s): –1207 kJ·mol⁻¹
• CO₂(g): –393.5 kJ·mol⁻¹
• CaO(s): ?
Respond to the prompts below.
(1) Determine the enthalpy change per mole of CaCO₃(s) for the decomposition reaction, using the experimental data. Include the correct sign and units.
(2) Using the result from part (a) and the tabulated data, calculate the standard molar enthalpy of formation, ΔH°ₓ, for CaO(s).
(3) State whether the decomposition of CaCO₃(s) is endothermic or exothermic under standard conditions and justify your answer using your calculated values.