The number of possible reactions in inorganic chemistry is enormous. Fortunately, many of them fall into one of the following four categories: combination, decomposition, single displacement, and double displacement. During this experiment you will be performing reactions from each category.

Combination Reactions:

Here two substances are combined together to form a single, more complex substance. The general equation for this type of reaction is:

A + B ® C. Typical examples would be the combination of a metal with a nonmetal to form a compound

2 Zn + O₂ ® 2 ZnO
and the combination of two compounds to form a third compound.

CaO + H₂O ® Ca(OH)₂

Decomposition Reactions:

This type of reaction would be the reverse of a combination reaction. A compound is broken up (decomposed) into two or more simpler substances. The general equation would be: C ® A + B. Examples would include the decomposition of calcium carbonate using heat to drive the reaction

CaCO₃ ® CaO + CO₂
and the use of an electric current to decompose water.

2 H₂O  O₂ + 2 H₂

Single Displacement Reactions:

These can occur when a chemically active element (A) displaces (switches places with) a less active element (B) from a compound.

A + BC ® AC + B

Two important examples would be the displacement of hydrogen from an acid by an active metal

Zn + 2 HCl ® ZnCl₂ + H₂

and the replacement of a less active metal in a compound by a more active metal.

Fe + CuCl₂ ® FeCl₂ + Cu

If you tried to perform the following reaction, nothing would happen. Why?

Cu + FeCl₂ ®
 

K	Using reactions such as the above, chemists have come with
Ca	an activity series for metals. On the left is much abbreviated
Mg	activity series with the most active metals at the top and the least
Al	active metals at the bottom. Potassium, the most active metal on
Zn	this list, is never found as a free element in nature. It is too
Fe	reactive, that is, it loses electrons too easily. Another way of saying
Sn	this would be that potassium is too easily oxidized. At the other
Pb	extreme, gold is used in coins and jewelry because it is so
H	unreactive. Theoretically, any metal can displace the ion of another
Cu	metal in a compound if that metal is below it in the activity series.
Ag	For example,
Au	Fe° + Cu+2® Fe+2 + Cu°
	iron loses electrons more easily than copper and will displace
	copper in its compounds.

In addition, any metal should be able to displace hydrogen from an acid if hydrogen is below it in the activity series. Because of this hydrogen is included in the activity series.

But, as in life, what you see on paper doesn't always work out that way when you try it yourself. For example, the reaction of lead with an acid would be so slow that you would not be able to see any activity at all.. Also, metals tend to form a protective coating such as Al₂O₃, ZnO, and SnO. This allows us to use aluminum foil in the kitchen even though aluminum is a rather active metal.

Double Displacement Reactions:
 

AgNO3

+ NaCl

®

AgCl

+ NaNO3

Na2CO3

+ 2 HCl

®

H2O

+ CO2 + 2 NaCl

HCl

+ NaOH

®

NaCl

+ H2O

The above reactions take place in solution. They look quite different from each other but they are all double displacement reactions. They can all be represented by the same general equation:
 

AB

+ CD

®

CB

+ AD

It is easier to see what is happening if you think of the positively charged partners (A and C) in the reactants switching places as they form the products.

One can expect such a reaction to occur if an insoluble compound, a gas, or a weakly ionized compound is formed as a product.

In the first reaction the insoluble salt, AgCl, is one of the products so we would predict that the reaction would occur as written.

The following short list of solubility rules will help you in predicting whether an insoluble compound is formed in a given reaction.

All NO₃^-, NH₄⁺, K⁺, and Na⁺ salts are soluble in water.
Cl^- salts are soluble except for AgCl, HgCl, and PbCl.
SO₄^-2 salts tend to be soluble. Important exceptions are SrSO₄, BaSO₄,
and PbSO₄.
CO₃^-2 salts are insoluble except those of the ammonium ion and the Group I metals.
S-² salts are insoluble except those of the ammonium ion, Group I metals,
and Group II metals.

The second reaction is rather tricky. The double displacement reaction that occurs is
 

Na2O3

+ 2 HCl

®

H2CO3

+ 2 NaCl

but the carbonic acid, H₂CO₃, that is formed immediately breaks down to give H₂O and the gas CO₂. Other products that would break down to give a gas are
 

H2SO3	®	H2O	+ SO2
and
NH4OH	®	H2O	+ NH3

The third reaction involves the formation of water - a very weakly ionized compound.

The first two reactions would be easy to see taking place - white precipitate forms in one and gas bubbles are produced in the other. There is no visible evidence for the third reaction, but, if you held the test tube in your hand, you would definitely feel the heat produced by the reaction.

If none of the above conditions are present, then it is very unlikely that a reaction has taken place. For example, if we mixed solutions containing NaSO₄ and KCl, nothing would happen. NaCl and K₂SO₄ are soluble salts. Everything would stay in solution - no gas or precipitate would be formed.

To sum things up, the types of evidence that a chemist would use to determine if a reaction has taken place would include:

a) the formation of a precipitate, when two solutions are mixed
b) the formation of a gas, when two solutions are mixed or if a solution is
c) a dramatic color change, when two solutions are mixed or if a solution is added to a solid the liberation of heat.

Procedure
Spill/Disposal: C, for all procedures.

All chemical reagents & products (liquid and solid) of these reactions must be disposed of in the supplied Hazardous Waste container. Use the supplied funnel to make sure all waste makes it into the container. If the solid precipitate needs to be removed from the bottom of the test tube, please rinse it out into the Hazardous Waste container using a deionized water bottle. Make sure all precipitate is removed into the HW container. Metals should also be put into the HW container. When the HW container is not in use the cap must be put back on.

Wash your hands thoroughly when you leave the lab.

Combination Reaction: Example
In the laboratory "Empirical Formula of a Compound" we saw an example of a combination reaction.

Decomposition Reactions:

CuSO₄ · 5 H₂O (s) 

Compounds that contain water as part of their crystalline structure are called hydrates. Copper (II) sulfate pentahydrate is such a compound. Get a test tube that has the CuSO₄ crystal. Using a test tube holder, heat the test tube gently at first, then more strongly. Look at the formula of the compound being heated. What gas might be given off? Looking at the test tube, what evidence do you see for this? When the test tube is cool, add a few drops of water to the residue in the test tube. Why is there a change in color? Return the test tube (with the crystal).

KClO₃(s) ®

THIS WILL BE DONE, AS A DEMONSTRATION, BY THE LAB INSTRUCTOR. Approximately 0.5 gram of potassium chlorate (KClO3) is placed in a clean, dry test tube. The test tube is heated strongly. You should see the KClO3 liquefy and gas bubbles form. A wooden splint is lit, the flame is blown out and the glowing splint is quickly inserted into the mouth of the test tube as the reaction is still occurring. Then the splint should glow brightly and burst back into flame for oxygen is being produced by the reaction.

Single Displacement Reactions:

Place five clean test tubes in a test tube rack. Use a wax pencil to number the test tubes. In test tube #1 place 2 ml of 1.0M CuSO₄, in test tube #2 place 2 ml of 1.0M FeSO₄, in test tube #3 place 2 ml of 0.1M AgNO₃, and in test tubes #4 and #5 place 2 ml of 1.0M HCl.

Sandpaper an iron nail and place it in test tube #1. Sandpaper three small pieces of copper and place them in test tubes #2, #3, and #4. Finally, sandpaper a small piece of zinc and place it in test tube #5. If you don't see anything happening, set the test tubes aside for 10 minutes. At the end of that time observe the test tubes. If there is any evidence that a reaction has taken place, write a balanced equation for the reaction. If there is no such evidence, then write N.R. (no reaction) in place of products. It will be assumed that the equations involve reactions taking place in an aqueous solution.

Double Displacement Reactions:
 

#1	1.0M Na2CO3	+	1.0M HCl	®
#2	6.0M HCl	+	6.0M NaOH	®
#3	0.10M Ba(NO3)2	+	0.10M Na2SO4	®
#4	1.0M Pb(NO3)2	+	saturated NaCl	®
#5	6.0M NH4Cl	+	6.0M NaOH	®
#6	0.10M Pb(NO3)2	+	0.10M K2CrO4	®

Mix two milliliters of each of the two solutions, in each of the above sets of reactants. Use a 10 mL graduated cylinder to measure out two mLs of water into a test tube. Use this as a rough guide for your other measurements. Record your observations. You might see a colored precipitate or gas bubbles forming. If there is no visible evidence of a reaction, then feel the test tube to see if an exothermic or endothermic reaction has taken place. Also, keeping the test tube a foot from your face, you might gently wave any escaping vapors towards your nose. Use such findings in determining a balanced equation for the reaction that has taken place. Again, if there is no evidence for a reaction, just write N.R. in place of products.