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)

NaCl NaOH Na2CO3 Na2SO4 Na3PO4
AgNO3
Ca(NO3)2
Cu(NO3)2
Mg(NO3)2

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:

1

Dr. Alex Madonik College of Alameda Page

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