Introduction
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:
compound
2 Zn(s) + O2(g) ® 2 ZnO(s)
and the combination of two compounds to form a third compound.
CaO(s) + H2O(l) ® Ca(OH)2(s)
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(s) ® CaO(g) + CO2(g)
and the use of an electric current to decompose water.
2 H2O(l) ® O2(g) + 2 H2(g)
Single Displacement Reactions: These can occur when a chemically active element (A) displaces (switches places with) a less active element (B) from a compound. These are Redox reactions.
A + BC ® AC + B
Two important examples would be the displacement of hydrogen from an acid by an active metal:
Zn(s) + 2 HCl(aq) ® ZnCl2(aq) + H2(g)
and the replacement of a less active metal in a compound by a more active metal.
Fe(s) + CuCl2(aq) ® FeCl2(aq) + Cu(s)
If you tried to perform the following reaction, nothing would happen. Why?
Cu(s) + FeCl2(aq) ®
Activity Series
K Using reactions such as the above, chemists have come with
Ca an activity series for metals. On the left is a 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(s) + Cu2+ ® Fe2+ + Cu(s)
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 reactivity 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:
(1) AgNO3(aq) + NaCl(aq) ® AgCl(s)+ NaNO3(aq)
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.
Solubility Rules
All NO3— , NH4+, K+, and Na+ salts are soluble in water
Cl— salts are soluble except for AgCl, Hg2Cl2, and PbCl2.
SO42— salts tend to be soluble. Important exceptions are SrSO4, BaSO4,and PbSO4.
CO32— salts are insoluble except those of the ammonium ion and the Group (I) metals.
S2- salts are insoluble except those of the NH4+ , Group I and Group II metals.
The second reaction is rather tricky. The double displacement reaction that occurs is
Na2CO3(aq) + 2 HCl(aq) ® H2O(l) + CO2(g) + 2 NaCl(aq), since the carbonic acid is unstable and breaks down to form carbon dioxide bubbles and water.
Other products that would break down to give a gas are:
H2SO3(aq) ® H2O(l) + SO2(g)
and
NH4OH(aq) ® H2O(l) + NH3(g)
The first two reactions would be easy to see taking place - a white precipitate forms in (1) and gas bubbles are produced in (3). 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 precipitate when two solutions is mixed.
b) A gas is produced when two solutions are mixed together.
c) A dramatic color change when two solutions are mixed.
d) Heat is produced.
PROCEDURE
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. Place about one gram of the compound into a test tube. 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?
Disposal: Put the cupric sulfate in the hazardous waste bottle.
KClO3(s) ®
THE LAB INSTRUCTOR WILL DO THIS AS A DEMONSTRATION.
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 1 ml of 1.0M CuSO4, in test tube #2 place 1ml of 1.0M FeSO4, in test tube #3 place 1 ml of 0.1M AgNO3, and in test tubes #4 and #5 place 1 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. Remove the nail from the test tube and examine it. 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.
Disposal: Pour the solutions into the hazardous waste bottle. Put the nail back in the box and the copper in the trash.
Double Displacement Reactions:
| 0.20 M Na2CO3(aq) | + | 1.0 M HCl(aq) | ® | |
| 6.0 M HCl(aq) | + | 6.0M NaOH (aq) | ® | |
| 0.20 M Na2CO3 (aq) | + | 0.10 MCaCl2(aq) | ® | |
| 6.0M NH4Cl(aq) | + | 6.0M NaOH | ® | |
| 0.20 M NaOH(aq) | + | 0.10 M AlCl3 | ® |
For each reaction above, mix one milliliter of each of the two solutions together. Use a 10 mL graduated cylinder to measure out one mL of water into a test tube. Use this as a rough guide for your other measurements. Record your observations. You might see 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.
Disposal: Pour the solutions down the sink when you are through.