The Solution Process

 Important characteristics of solutions:

• They are homogenous mixtures
• Solutions may be gasses, liquids or solids
• Each substance in a solution is a component of the solution. Usually, the component with the highest concentration is termed the solvent (other components are termed solutes)

Most solutions we will deal with are those in a liquid state, where the solvent is H₂O (i.e. aqueous solutions)

• The liquid state, and the solid state, are known as condensed states
• In condensed states, the attractive forces between molecules are strong enough (in comparison to the temperature-induce kinetic energy) to hold neighboring molecules together.
• In solids, the neighbors are held rigid
• In liquids, the neighbor molecules can slide past each other

Homogenous mixtures (solutions) can form only when the following attractive forces are approximately equivalent:

• Attraction between solvent and solute molecules
• Attraction of solute molecules for other solute molecules
• Attraction of solvent molecules for other solvent molecules

If the attractive forces of solute molecules for other solute molecules are greater than the attractive forces of solute molecules for water, then the solute will not dissolve

• For ionic solids, the lattice energy describes the attractive forces between the solute molecules (i.e. ions)
• For an ionic solid to dissolve in water, the water-solute attractive forces has to be strong enough to overcome the lattice energy

The process known as solvation is where the solute-solvent interactions are strong enough separate, surround and disperse a solute

• If the solvent is H₂O, then solvation is referred to as hydration

We have previously studied enthalpy changes associated with chemical reactions (e.g. combustion reactions, DH_rxn) and with physical processes (e.g. changes of state, DH_fusion, and the heating of matter in a specific state, Molar Heat Capacity)

• The process of solvation involves energy changes also, known as the enthalpy of solvation (DH_solv)
• It is a physical process, not chemical

What are the different processes that contribute to the enthalpy of solvation DH_solv?

• There is the energy (DH₁) associated with dispersing the solutes. It is the lattice energy (here's an example for an ionic solid):

• In order to accommodate the dispersed solutes within the H₂O solution, the H₂O molecules have to separate from one another to provide the necessary space (in other words, we have to disrupt solvent-solvent interactions). There is an energy (DH₂) associated with this process:

• And, finally, there is the energy (DH₃) associated with the formation of solvent-solute interactions for the solvated solute molecules:

What is the overall energy associated with these three distinct energetic steps involved in solvation of a solute?

• DH₁, separating the solute molecules from each other, requires an input of energy to overcome the attractive forces holding the solute molecules together. Thus, DH₁ will be positive in sign (endothermic).
• DH₂, disrupting solvent-solvent interactions, to allow space for dispersed solute molecules, will also require the input of energy. Thus, DH₂ will be positive in sign (endothermic).
• DH₃, formation of solvent-solute interactions, will release energy. Thus, DH₃ will be negative in sign (exothermic).

If the energy released by the formation of solvent-solute interactions (DH₃) is greater than the sum of the energies required to disrupt solute-solute interactions and solvent-solvent interactions (DH₁ + DH₂), then the overall enthalpy of solvation (DH_solv) will be exothermic and energetically favorable and spontaneous:

Some solvation enthalpies are actually positive (i.e. endothermic) and absorb heat from their surroundings (this process is used in cold packs for muscle sprains):

Although some heats of solvation are positive (e.g. the hydration of ammonium nitrate), their reactions proceed spontaneously. Why is this?

• Up to this point, when trying to predict whether a reaction or process is spontaneous we have considered the overall change in heat energy (enthalpy)

• However, the hydration of Ammonium Nitrate is spontaneous, but has a slightly positive value for the overall enthalpy (it has absorbed heat energy). Clearly, something else is going on that forces the hydration of Ammonium Nitrate to occur

The answer to this riddle lies in the behavior of collections of objects. Consider a field with a herd of sheep, some are black and some are white.

• If we let the sheep roam around and then take an aerial picture of the field, the black and white sheep would be randomly distributed:

• Suppose the farmer wants to separate the sheep. A sheepdog will have to do work to separate them:

• Not only that, but the dog will have to keep working to keep them separated, because their natural tendency would be to randomly distribute throughout the field
• We can postulate that if energy must be expended to keep the sheep from being randomly distributed, then if we reverse the process (i.e. start with sheep in an ordered arrangement and let them become randomly distributed) then energy is somehow released

The same situation is true with collections of molecules. In the case of the solvation of ammonium nitrate we start with a crystal of the ionic solid being placed in a container with water:

• When the crystal initially dissolves all the ions, though hydrated, are not randomly distributed throughout the water in the container - they are concentrated in the vicinity of the original crystal
• The solvated ions are therefore initially not in a random distribution throughout the container.
• They are initially in a more organized or ordered state (i.e. concentrated in one location in the container) and will naturally want to become more disordered or randomly distributed

The term for the degree of disorder in a system is ENTROPY

• A system with high entropy is disordered
• A system with low entropy is ordered
• Systems tend to go from low to high entropy
• The entropy of the universe is increasing (just check out your kitchen or bathroom for evidence of this)

Why don't water and gasoline mix?

• Gasoline (octane; a hydrocarbon) can only participate in dispersion forces (a very weak type of van der Waals force)
• Water molecules can form much stronger hydrogen bonds between each other
• It takes a lot of energy to disrupt the water molecules to make room for the octane molecules (lot of energy expended to disrupt hydrogen bonds). However, the interaction between water and octane is via much weaker dispersion forces (the only interaction the octane can participate in with water), thus, you pay a big energetic cost to separate the water molecules and don't recoup much energy when the octane is dispersed
• The entropic gain in dispersing the octane molecules is not enough to overcome the energetic cost of disrupting the water-water interactions