Solutions

* Solutions

* Solutions are homogeneous mixtures of two or more pure substances.

* In a solution, the solute is dispersed uniformly throughout the solvent.

* Solutions

* The intermolecular forces between solute and solvent particles must be strong enough to compete with those between solute particles and those between solvent particles.

* How Does a Solution Form?

* As a solution forms, the solvent pulls solute particles apart and surrounds, or solvates, them.

* How Does a Solution Form

* If an ionic salt is soluble in water, it is because the ion-dipole interactions are strong enough to overcome the lattice energy of the salt crystal.

* Energy Changes in Solution

* Simply put, three processes affect the energetics of solution:

* separation of solute particles,

* separation of solvent particles,

* new interactions between solute and solvent.

* Energy Changes in Solution

* The enthalpy change of the overall process depends on H for each of these steps.

* Why Do Endothermic Processes Occur?

* Things do not tend to occur spontaneously (i.e., without outside intervention) unless the energy of the system is lowered.

* Why Do Endothermic Processes Occur?

* Yet we know that in some processes, like the dissolution of NH4NO3 in water, heat is absorbed, not released.

* Enthalpy Is Only Part of the Picture

* The reason is that increasing the disorder or randomness (known as entropy) of a system tends to lower the energy of the system.

* Enthalpy Is Only Part of the Picture

* So even though enthalpy may increase, the overall energy of the system can still decrease if the system becomes more disordered.

* Student, Beware!

* Just because a substance disappears when it comes in contact with a solvent, it doesn’t mean the substance dissolved.

* Student, Beware!

* Dissolution is a physical change — you can get back...