HEAT OF SOLUTION Dissolving gets hot REMEMBER THESE TERMS? • Endothermic • Exothermic • What do they mean? HEAT OF SOLUTION • The term Heat of Solution refers to the change in temperature that occurs when substances dissolve. • The bonds holding the solute together take energy to break. Any process that needs energy put into it is Endothermic. • The formation of bonds between the solvent and the solute particles is always Exothermic. • So, we have an exothermic and an endothermic process competing with each other. • One is releasing heat and the other is sucking it up. • Whether the solvation process as a whole is endothermic or exothermic depends on which is stronger: • the attempt to release energy, or the attempt to absorb it. • If the solvation process is Exothermic overall, we write the energy as a Product. • Example: Hydrochloric Acid: • If the solvation process is Endothermic overall, we write the energy as a Reactant. • Example: Instant Cold Packs: • We can also determine exactly how much heat is released. • Every solvation process has a specific amount of heat it will absorb or release. • Check your Heat of Solution chart. • Negative numbers mean heat is released (exothermic) EXAMPLE • In the solvation of HCl: • HCl(l) -–H2O(l) H+(aq) + Cl-(aq) • It releases 74.8 kJ/mol • So for every 1mol HCl, we will release 74.8 kJ/mol • If we had 2 mol, we’d release 149.6 kJ EXAMPLE If we dissolved Ca(OH)2 in water, what is the Solvation Equation? Ca(OH)2(s) ---H2O(l)--- Ca2+(aq) + 2 OH-(aq) + 16.2kj/mol If we dissolved 5.86g of Ca(OH)2 in water, how much heat would be releases? EXAMPLE If we dissolved Ca(OH)2 in water, what is the Solvation Equation? Ca(OH)2(s) ---H2O(l)--- Ca2+(aq) + 2 OH-(aq) + 16.2kj/mol If we dissolved 5.86g of Ca(OH)2 in water, how much heat would be releases? • 5.86g / 74.1g/mol = 0.079mol • 0.079mol x 16.2kj/mol = 1.28kj