Chemical Reactions

Chemical Reactions

In this experiment you will be observing some typical chemical reactions, identify some of the products, and summarize the chemical changes in terms of balanced chemical equations.

Chemical equations represent what transpires in a chemical reaction. For example, the equation

2KClO3(s) ∆ 2KCl(s) + 3O2(g)

Means that potassium chlorate, KClO3, decomposes on heating (∆ is the symbol used for heat) to yield potassium chloride, KCl, and oxygen, O2. Before an equation can be written for a reaction, someone must establish what the products are. How does one decide what the products are? Products are identified by their chemical and physical properties as well as by analysis. That oxygen and not chlorine gas is produced in the above reaction can be established by the fact that oxygen is colorless, odorless gas. Chlorine, on the other hand, is a pale, yellow-green gas with an irritating odor.

In this experiment you will observe that in some cases gases are produced, precipitates are formed, or color changes occur during the reactions. These are all indications that a chemical reaction has occurred. To identify some of the products of the reactions, you can consult Table 1, which lists some of the properties of the substances that could be formed in these reactions.

The chemical reactions you will be working with in this experiment are primarily oxidation-reduction or double-displacement reactions. In oxidation-reduction (redox) reactions electrons are transferred from one element to another element, most of the time these elements are in different reactants. The reactant that donates the electrons is the reducing agent. The electron accepting reactant is the oxidizing agent. Double-displacement reaction may be represented by equations of the following type:

AB + CD à AD + CB

As you can see this type of reaction involves the exchange of atoms or groups of atoms (ions) between interacting substances.

Table 1 Properties of Reaction Products

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Prelab: Before beginning this experiment in the laboratory, you should be able to answer the following questions:

1.  Before a chemical reaction is written, what must you know?

2.  What observations might you make that suggest that a chemical reaction has occurred?

3.  How could you distinguish between NO2 and NO, if you had a container of each in front of you?

4.  Define what occurs in an oxidation reduction reaction? Give an example.

5.  What is a precipitate?

6.  Balance these equations and then write net ionic equations:

KBrO3(s) à KBr(s) + O2(g)

ZnCl2(aq) + AgNO3(aq) à Zn(NO3)2(aq) + AgCl(s)

7.  Write the chemical equation for the decomposition of H2CO3(aq).

PROCEDURES

A.Oxidation-Reduction Reactions

Many metals react with acids to liberate a gas. Add a small piece of zinc to a test tube containing

2 mL of 6 M HCl. Repeat this by adding copper wire to hydrochloric acid. Next while holding a clean test tube in the fume hood, place a piece of copper wire in the test tube and add 1 mL of concentrated nitric acid, HNO3. After the reaction has proceeded for 5 minutes, carefully add 5 mL of water.

Potassium permanganate, KMnO4, is an excellent oxidizing agent in acidic media. To 1 mL of

0.1M sodium oxalate, in a clean test tube add 10 drops of 6 M sulfuric acid. Mix thoroughly. To the resulting solution add 1 to 2 drops of 0.1 M KMnO4 and stir. If there is no obvious indication that a reaction has occurred, warm the test tube gently in a hot water bath.

Place 3 mL of 0.1 M sodium hydrogen sulfite, NaHSO3, solution in a test tube. Add 1 mL of 10 M sodium hydroxide solution and stir. To the mixture in the test tube add 1 drop of 0.1 M KMnO4 solution. Record your observations. Add additional 0.1 M KMnO4 solution, one drop at a time, observing the effect of each drop until 10 drops have been added. While holding a test tube in the fume hood, add one or two crystals of potassium permanganate to 1 mL of 6 M HCl.

B.Double-Displacement Reactions

Place a small sample of sodium carbonate, Na2CO3, in a test tube and add several drops of 6 M HCl. In the fume hood, repeat the same test with sodium sulfite, Na2SO3, instead of sodium carbonate. Again in the fume hood, repeat this test with zinc sulfide, ZnS.

To 1 mL of 0.1 M lead (II) nitrate solution in a clean test tube add a few drops of 6 M HCl. To 1 mL of 0.1 M barium chloride solution add 2 drops of 1 M potassium chromate solution. Pour remains in waste disposal container. Next add several drops of ammonium carbonate solution to 1 mL of

0.1 M barium chloride solution in a test tube. After the precipitate from this reaction has settled, carefully decant (pour off) the excess liquid. Add 1 mL of water to the test tube, shake it, allow the precipitate to settle, and again decant the excess liquid.