To estimate the enthalpy change for the thermal decomposition of calcium carbonate,
CaCO₃(s) → CaO(s) + CO₂(g) ΔH_target = ?
a student performs two separate solution calorimetry experiments at constant pressure.
Experiment A
CaCO₃(s) + 2 HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l)
• Mass of CaCO₃: 2.50 g (molar mass = 100.1 g mol⁻¹)
• Mass of HCl solution: 150.0 g
• ΔT: +3.12 °C
Experiment B
CaO(s) + 2 HCl(aq) → CaCl₂(aq) + H₂O(l)
• Mass of CaO: 1.40 g (molar mass = 56.1 g mol⁻¹)
• Mass of HCl solution: 150.0 g
• ΔT: +7.84 °C
Assume the HCl solutions have the same specific heat capacity as water (4.18 J g⁻¹ °C⁻¹) and that the calorimeter cup absorbs negligible heat.
(a) Calculate the molar enthalpy change ΔH_A (kJ mol⁻¹) for Experiment A.
(b) Calculate the molar enthalpy change ΔH_B (kJ mol⁻¹) for Experiment B.
(c) Use appropriate algebraic manipulation of the two chemical equations to determine ΔH_target for the decomposition of CaCO₃(s). Present the combined equation before giving the numerical value.
(d) Identify two assumptions inherent in this calorimetric approach that could introduce systematic error into the value of ΔH_target, and explain the effect each assumption might have on the final result.