The chemistry of copper sulfate solutions and the "Law of Mass Action"

A deep blue copper compound is made white through the addition of heat, and then made blue again through the addition of water.

Ingredients: copper sulfate pentahydrate, watch glass, propane torch

Procedure: A complete recipe follows.

1. Place blue copper sulfate pentahydrate crystals in watch glass.

2. Heat crystals and observe transformation in color and character of crystals.

3. Allow crystals to cool.

4. Add drops of water to cooled powder crystals and observe transformation.

Understanding: There are five water molecules that hydrate the copper sulfate molecule, creating the deep blue copper sulfate pentahydrate species. The water molecules are easily driven off by heat, increasing the disorder of the reaction system by the creation of water vapor.

CuSO4·5H2O(s) → CuSO4(s) + 5H2O(g)

The resulting anhydrous copper sulfate is a nearly white crystalline powder. The beautiful blue copper sulfate pentahydrate can be recovered by the addition of water from a dropper.

CuSO4(s) + 5H2O(l) → CuSO4·5H2O(s)

The blue copper sulfate is found in the mineral chalcanthite. Also known as Roman vitriol, it has long fascinated chemists and alchemists alike, and was a central compound in the search for the Philosopher's Stone. While blue vitriol is copper sulfate, green vitriol is iron sulfate, and white vitriol is zinc sulfate. In ancient times, it was discovered that by heating a mineral sulfate, one could produce a smelly liquid that, following distillation, would yield a mostly odorless yellow liquid known as vitriol. The vitriol was found to dissolve human tissue and to be corrosive to most metals. We known vitriol to be sulfuric acid.

A clear solution of ammonium hydroxide is added to a pale blue solution of copper sulfate, producing a striking blue precipitate that remains suspended near the surface of the solution.

Ingredients: copper sulfate pentahydrate, ammonium hydroxide

Procedure: A partial recipe follows.

1. Prepare a solution of copper sulfate.

2. Prepare a solution of ammonium hydroxide.

3. Add drops of ammonium hydroxide to the copper sulfate and observe transformation.

Understanding: The water soluble copper sulfate produces Cu(II) ions in solution. The soluble ammonium hydroxide produces ammonium ions and hydroxide ions in solution in equilibrium with ammonia and water.

NH4OH(aq) ↔ NH3(aq) + H2O(l)

The ammonia molecules react with the cooper ion to produce a complex ion of Cu(II) coordinated with four ammonia molecules.

Cu2+(aq) + 4 NH3(aq) → [Cu(NH3)4]2+(aq)

That complex ion in turn associates with a water molecule and the sulfate ion resulting in the products of the overall reaction.

CuSO4(aq) + 4 NH4OH(aq) → Cu(NH3)4SO4H2O(s) + 3 H2O(l)

A clear solution of potassium dihydrogen phosphate is added to a pale blue solution of copper sulfate producing ... a somewhat more pale blue solution!

Ingredients: copper sulfate pentahydrate, potassium dihydrogen phosphate

Procedure: A complete recipe follows.

1. Prepare a solution of copper sulfate.

2. Prepare a solution of potassium dihydrogen phosphate.

3. Mix the two solutions and observe the transformation.

Understanding: Both copper sulfate and potassium dihydrogen phosphate are water soluble. For our purposes, we will describe the dissolution of potassium dihydrogen phosphate as the simple dissolution of the potassium and dihydrogen phosphate ions.

CuSO4(aq) + KH2PO4(aq) → Cu2+(aq) + SO42-(aq) + K+(aq) + H2PO4-(aq)

The resulting ions can then mingle in solution leading to nothing more than mingling ions!

Cu2+(aq) + SO42-(aq) + K+(aq) + H2PO4-(aq) → Cu2+(aq) + SO42-(aq) + K+(aq) + H2PO4-(aq)

The resulting net ionic equation can be written

→

There is no reaction between the copper sulfate and potassium dihydrogen phosphate.

A quantitative measure of reactivity

Question: Through these reactions involving copper sulfate, one can explore the fundamental principles underlying the "Law of Mass Action" in which the extent of the transformation of reactants into products is quantified as a ratio of chemical activities called the equilibrium constant.

Express the equilibrium constant for each of the three overall reactions.

You can check your answers here.