Question

1. Explain in words what happens when a salt (i.e., NaCl) dissolves in water in terms of interactions that are lost and interactions that are gained. Explain for methanol (CH₃OH) in water. Explain for a wax (hydrocarbon – all C’s and H’s) in water.

2. If DH₁ is the enthalpy change for separating particles of solute, DH₂ is the enthalpy change for separating particles of solvent, and DH₃ is the enthalpy change for the formation of interaction between solute and solvent, write out an equation to determine DH_rxn.

3. Explain in words and using DH values WHY it is true to say “like dissolves like.”

4. What is entropy and how does it play a part in solution chemistry? Is it possible to have an endothermic solution-making process that is nevertheless spontaneous? How is this possible?

Answer 1

• Entropy can be thought of as the randomness or spread-outedness of a group of molecules. Increasing randomness is favorable.
• There is an entropy change associated with the formation of a solution, an increase in entropy (randomness) that thermodynamically favors the solution over the two original states.
• If the other energetics of dissolution are favorable, this increase in entropy means that the conditions for solubility will always be met. Even if the energetics are slightly endothermic the entropy effect can still allow the solution to form.