Heat of Solution

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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
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