Investigation Part
Calcium oxide is also known as lime or quicklime and is used to make plaster, mortar, bricks, and many other construction materials. Calcium oxide is produced by heating limestone (calcium carbonate) in air. However, calcium oxide readily absorbs and reacts with carbon dioxide and water to form calcium carbonate (CaCO3) and calcium hydroxide [Ca(OH)2], respectively. When water is added to calcium oxide, an exothermic reaction occurs, producing calcium hydroxide and a large amount of heat. Calcium hydroxide is used to treat acidic soils, soften water, and prepare many building materials such as plaster, mortar, and bricks. The solubility of calcium hydroxide in water is very low, about 1.6 g/L. The product of the reaction of CaO and H2O is thus Ca(OH)2(s), not Ca(OH)2(aq).
CaO(s) + H2O(l) → Ca(OH)2(s) + heat ΔH = ΔHf (products) → ΔHf (reactants) ΔH = ΔHf [Ca(OH)2(s)] – {ΔHf [CaO(s)] + ΔHf [H2O(l)]} ΔH = –986.1 kJ/mole – [–635.1 kJ/mole + (–285.8 kJ/mole)] = –65.2 kJ/mole