SCH 3U - Solubility Predictions & Solubility Rules

Factors Affecting Solubility of Ionic Compounds

Ion Charge

The lower the charge on an ion, the more soluble the ionic compound.  The lower the charge, the weaker the attractive forces between the ions that make up the salt.

For instance, salts of alkali metal ions (Na⁺, K⁺, etc) tend to be soluble and the salts of sulfides (S^2-) tend to be insoluble.

ex. Which salt is more soluble in water: sodium chloride or copper(II) sulfide?

sodium chloride – Na⁺Cl^-

copper(II) sulfide – Cu²⁺S^2-

Thus, since sodium chloride has lower charges on its ions, it will be more water soluble.

Ion Size

The smaller the radii of the ions making up a salt, the less soluble the salt. The smaller the ionic radius, the stronger the forces of attraction between the ions.

For instance, salts of fluoride ions are less soluble than the salts of iodide ions.

ex. Which salt is more water soluble:  lithium chloride or cesium chloride?

lithium chloride – lithium is a small ion

cesium chloride – cesium is a large ion

Thus, since cesium chloride contains a larger ion, it is more water soluble.

Putting it All Together

To predict the solubility of a salt, you must look at all the factors (charge and size).  In the example below, I will use 😁 for water-soluble characteristics and 😒 for water-insoluble characteristics.  The salt with more 😁 will be more water-soluble.

ex.  Which salt is more water soluble: sodium sulfide or aluminum phosphide?

sodium sulfide           vs          aluminum phosphide

Na⁺                                          Al³⁺

single charge 😁                     multiple charge 😒

small ion 😒                            small ion 😒

S^2-                                           P^3-

multiple charge 😒                 multiple charge 😒

small ion 😒                           small ion 😒

Thus, sodium sulfide would be more water-soluble because it has more 😁.

Making Predictions

Since the oxide ion is small and has a multiple charge, we predict its salts are insoluble – which is true in most but not all cases.  Predicting solubility using the above guidelines is not an exact science.  Therefore chemists have, through experimentation, created a set of solubility rules.

Reactions in Aqueous Solutions

We have already discussed double replacement reactions, the use of solubility rules, total ionic equations and net ionic equations in the Chemical Reactions unit.  Go back and look at your notes if you need to brush up.

ex.  Predict the products when these two aqueous solutions are mixed.  Indicate soluble and insoluble products.  Write the total ionic and net ionic equations.

AgNO₃(aq)   +  H₂SO₄(aq)  →  

2AgNO₃(aq)  +  H₂SO₄(aq)  →  Ag₂SO₄(aq)  + 2HNO₃(aq)

2Ag⁺(aq) + 2NO₃^-(aq) + 2H⁺(aq) + SO₄^2-(aq) → 2Ag⁺(aq) + SO₄^2-(aq) + 2H⁺(aq) + 2NO₃^-(aq)

NR  

Note that, for the total ionic equation above, I reduced the font size considerably so it would fit on the line. There is no other reason.

Homework:

1. Which substance is more water-soluble?

(a) lithium fluoride or potassium bromide

(b) rubidium chloride or strontium chloride

2.  Practice Problems, p. 410 #1-3.  

 

 

Student Questions:
I was wondering if you could provide the solutions for lithium fluoride vs. potassium bromide, and rubidium chloride vs. strontium chloride?

Oops!  Meant to do that. 😎