Chapter 14: Solutions Consider the spontaneous mixing of gases: The mixing of gases. Two gases are initially in separate compartments (a). When the partition is removed (b) they mix spontaneously.

Spontaneous mixing is a strong driving force in nature –A system, left to itself, will tend towards the most probable state Same driving force goes for formation of liquid solutions Attractive forces are not important in the gas phase but are very important in liquid solutions

What does “immiscible” mean? Why are some compounds miscible and others are immiscible?

When the strengths of the intermolecular attractions are similar in solute and solvent, solutions form (miscible) What’s an example? Water and ________

What about water and benzene?

This can be summarized as the rule of thumb “like dissolves like” The basic principles remain the same when the solutes are solids When sodium chloride dissolves when it is added to water The sodium and chloride ions are hydrated or surrounded by water molecules The general term for surrounding a solute particle by solvent molecules is solvation

The hydration of a polar molecule. A polar molecule dissolves in water because its molecules are attracted to the very polar water molecules. The water molecules orientate themselves so their positive ends are near the negative ends of the solute and their negative ends are near the positive ends of the solute.

The formation of a solution from a solid and a liquid can be modeled as a two-step process Overall, the steps take us from the solid solute and liquid solvent to the final solution These steps are not the way the solution would actually be made in the lab This works because enthalpy is a state function

In the lab, the solutions is formed directly as indicated by “DIRECT” arrow. The energy changes can be analyzed using two steps. The enthalpy change for each path must be the same because enthalpy is a state function.

Consider the formation of aqueous potassium iodide

Consider the formation of aqueous sodium bromide

A similar treatment can be applied to a liquid solute: Enthalpy of solution. Step 1: Liquid solute molecules move apart to make room for solvent molecules (endothermic). Step 2: Solvent volume increased to make room for the solute molecules (endothermic). Step 3: Expanded solute and solvent come together to make a solution (exothermic).

If the sum of the energies in Step 1 and 2 is equal to the energy released in Step 3 the enthalpy change of solution is zero These are called ideal solutions In an ideal solution all the intermolecular attractive forces are equal Note that for an ideal gas there are no intermolecular forces This process can be summarized with an enthalpy diagram

Enthalpy is a state function so the direct and three-step paths must give the same enthalpy change. For an ideal solution, the overall enthalpy change is zero.

Enthalpy is a state function so enthalpy changes for nonideal solutions can be calculated using either a three-step or direct process. (a) The process is exothermic. (b) The process is endothermic.

(a) A gas dissolves in an organic solvent. Energy is absorbed to open “pockets” in the solvent that can hold the gas molecules. In the second step energy is released when molecules enter the pockets and are attracted to the solvent molecules. This is shown as an endothermic process. (b) Around room temperature, the network of hydrogen bonds in water already contains pockets, so little energy is needed to prepare the solvent to accept the gas. In the second step, energy is released as the gas enters the pockets and is attracted by solvent molecules. This is shown as an exothermic process.

Solubility is the mass of solute that forms a saturated solution with a given mass of solvent at a specified temperature The units are typically grams of solute per 100 g of the solvent If extra solute is added to a saturated solution, the extra solute will remain as a separate phase A dynamic equilibrium exists between the solute in the two phases

For a solute in contact with the saturated solution:

If a “stress” is applied to the equilibrium by increasing the temperature, the equilibrium will shift in a way to minimize the stress –If heat is absorbed when when solute dissolves, the solubility increases when the temperature is increases

–If energy is released when solute dissolves, the solubility decreases The solubility of most substances increases with temperature Most substances become more soluble as temperature increases. The amount of the solubility increase has considerable variation.