Experiment-6: Double Displacement Reactions – Beyond Labz

The Chem 1A lab manual includes two “classic” experiments that look at reactions of ions in aqueous (water) solution. Experiment-6 looks at what happens when you mix solutions of different ionic compounds together, so that new combinations can form. There are two basic ways that a reaction can occur:

1) A cation and anion combine to form an insoluble product, which makes a solid precipitate

2) The H+ ion combines with an anion to form a soluble, molecular compound.

We’re looking at reactions in water, so all reactants start out as homogeneous, aqueous solutions (aq). If a solid product (a “precipitate”) forms, we indicate this by writing (s) after the formula. Note: (s) means “solid”, NOT “soluble.” Precipitates come in all colors; many are white. Most main group elements make colorless solutions and white solids. Transition metal ions are often colored.

CaCl2(aq) + Na2CO3(aq) CaCO3(s) + 2NaCl(aq)

In this example, the two colorless solutions (calcium chloride and sodium carbonate) react when mixed, and a solid white precipitate of calcium carbonate is formed. Sodium chloride is water-soluble and remains in solution. Remember that when ionic compounds dissolve in water, their ions separate:

CaCl2(aq) + Na2CO3(aq) Ca+(aq) + 2Cl-(aq) + 2Na+(aq) + CO3-2(aq)

When all of these ions mix in solution, the anions and cations can change partners. For some reason, textbooks call this process a “double displacement” reaction, but “exchange” might be a better term. Here’s the Complete Ionic Reaction:

Ca+(aq) + 2Cl-(aq) + 2Na+(aq) + CO3-2(aq) CaCO3 (s) + 2Cl-(aq) + 2Na+(aq)

Note that the sodium and chloride ions didn’t do anything in this reaction. We can write the Net Ionic Reaction, leaving out these so-called “spectator ions:”

Ca+(aq) + CO3-2(aq) CaCO3 (s)

Reactions between acids and bases follow a different pattern. In water, acids form the hydrogen (H+) ion, which can then replace other cations. If the product is a weaker acid than the one we started with, the reaction goes forward. Water is a very weak acid, so acid-base reactions that produce water are fast and release heat – a sure indication that a reaction took place.

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

Net Ionic: H+(aq) + OH-(aq) H2O(l)

When a strong acid transfers a hydrogen ion to another anion to form a weak acid, the ion concentration in the solution is reduced, as shown by conductivity measurements (Experiment-5). An important example is the reaction of an acid with the carbonate or bicarbonate anions, to form carbonic acid:

HCl(aq) + NaHCO3(aq) NaCl(aq) + H2CO3(aq)

H2CO3(aq) CO2(g) + H2O(l)

Since carbonic acid is unstable, the result is the formation of carbon dioxide.

Your Mission: Ionic reactions that form precipitates

Chapter 8.4 includes a simulation of these reactions, in which you can select a cation from the left column (shown in the left hand column of the table below) and an anion from the right-hand column (shown across the top of the table below).

For each reaction, record your observation: Yes (reaction makes a precipitate) or No (no reaction)

It’s also possible to simulate these reactions (and many more) in the Beyond Labz Inorganic lab. The procedure is to select “UNKNOWNS” and select a metal cation. Then, click on “Create Unknown.” A test tube appears in the UNKNOWN rack that contains that ion. Move the test tube to the clamp on the ring stand. That’s the only place you can do a reaction. Simply clicking on the dropper bottles on the right adds the selected reagent to the UNKNOWN tube, and you can observe the reaction in the image on the left side:

Click the red waste bin to remove the tube, then move a new tube to the ring stand for your next reaction.

For each combination that you mark “Yes” write the Net Ionic Reaction:

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Dr. Alex Madonik College of Alameda Page