Lecture 9. Lecture 10 was just exam tips

In what type of solvents are ionic substances soluble in?

In most common solvents, ionic substances are slightly soluble. The exception are polar solvents (where there’s a permanent dipole) so water or ammonia e.g. Ionic substances tend to be more soluble in polar solvents.

What must happen for an ionic substance to dissolve in a solvent?

The attractive forces between the ions and the solvent molecules must be at least comparable to the lattice energy to overcome the strong attractive forces between the ions in the lattice.

What is the solvation energy equal to?

The sum of the attractive forces between the solvent molecules and the cation/anion. 

What type of energy is lost or gained when we dissolve salts?

Lattice energy is lost but solvation energy is gained (solvent molecules surrounding the salt and interacting with them).

How do we determine the polarity of a solvent

The dielectric strength gives a measure of the shielding effect of the solvent by separating the cation and anion. Water has a high dielectric constant as it is very polar.

What does the dielectric constant mean?

The dielectric constant of water is about 78.5. This means the force required to separate a cation and anion in water would be 1/78.5 of the force required to separate the cation and anion in a vacuum.

How to calculate the solvation energy per ion?

-Z² divided by 2r. This is to be multiplied by (1 - 1/e). Z is the charge of the ion, r is the radius and e is the dielectric constant.

What other factor that determines solubility of ionic materials?

The change in entropy for the process. The entropy of salts massively increases when it dissolves in the solvent. This is because in a solid lattice, cations/anions can hardly move around and have low entropy. In solution, the ions gain freedom to move around. The entropy of the solvent, upon dissolving a salt, decreases as the solvent molecules are now in part attached to the cations/anions and moves with the ions if in the coordination spheres. 

What physical property makes salts more soluble?

Salts are more likely to be soluble if the anion and cation differ substantially in size. This primarily comes from the lattice energy - salts with similar size pack well in the lattice and have high lattice energies hence more lattice energy is lost when dissolving in solution compared to ions that don’t pack well. We want to lose as little lattice energy as possible.

What are 3 trends/patterns of the lattice and solvation energies?

The solvation energies and solubilities of salts increase with the dielectric constant of the solvent. Both solvation and lattice energy are inversely proportional to the ionic radius due to a charge effect. When you increase the charge, the lattice energy increases faster than the solvation energy.

How does temperature affect solubility?

Solubility increase with temperature generally due to the fact that you get higher entropy changes.

How does hydrogen bonding/polarisation effects affect solubility?

Solvents with a lower dielectric constant than water e.g. ammonia are poor solvents for extreme ionic types but better at solubilizing intermediate salts.

How can we change the solubility of a substance? 

We can add a solubilisation agent such as a crown ether. Barium sulphate has very low solubility in water but with the addition of an agent, we can increase the solubility.

What are 2 general trends of solubility caused by entropy?

The solubilities of salts generally increase with the size of cations/anions because of more favourable entropy terms.The entropy changes accompanying the dissolution of a salt is usually less favourable for small ions or high charge ions.