Calcium carbonate is a metal carbonate. When an acid reacts with dilute hydrochloric acid and forms carbon dioxide gas & solution of calcium chloride. Reaction: CaCO3 + 2HCl(a) = CaCl2 + H2O(l) + CO2(g) Change in Free Energy: ΔG(20C) = -25.2kJ (negative, so the reaction runs) Change in Enthalpy: ΔH(20C) = +79.9kJ (positive, so the reaction is endothermic)