Volume 1
1.17. MAIN CLASSES OF INORGANIC COMPOUNDS: BASES (METAL HYDROXIDES)

General Remarks and Nomenclature • Preparation of Metal Hydroxide Bases • Chemical Properties of Bases. Experiment: Reaction of Aluminum with NaOH • Exercises
1.17.1. General Remarks and Nomenclature. We already know that a base is a compound composed of a metal atom and one or more OH groups attached to it. Pure metal hydroxides are not nearly as widespread in nature as other minerals. Examples of naturally occurring metal hydroxides include gibbsite (Al(OH)3), brucite (Mg(OH)2), portlandite (Ca(OH)2), spertiniite (Cu(OH)2), dzhalindite (In(OH)3), and sweetite (Zn(OH)2).

The nomenclature of hydroxides is simple and similar to that of oxides. To name a metal hydroxide base, we first name the metal and then say "hydroxide". If a metal can be in more than one valence state and therefore forms more than one hydroxide, the valence is specified as a Roman numeral in parentheses after the name of the metal. For example, Fe(OH)2 is "iron (II) hydroxide" and Fe(OH)3 is "iron (III) hydroxide".

1.17.2. Preparation of Metal Hydroxide Bases. The chemical industry produces NaOH, KOH, and Ca(OH)2 on a very large scale. Both alkalis, NaOH and KOH, are made by electrolysis of an aqueous solution of NaCl and KCl, respectively (Figure 1-79). Electrolysis is a chemical reaction induced by passing a direct electric current through a chemical compound in solution or in the molten state. There will be a section on electrolysis in Volume 2. Meanwhile, just be aware of the fact that NaOH and KOH are made from the corresponding chloride salts using electricity.
Figure 1-79. Electrolysis of NaCl in water.


Large quantities of Ca(OH)2 are manufactured by hydration of CaO (Figure 1-80).
Figure 1-80. Reaction of calcium oxide with water.

In the laboratory, commercially available NaOH, KOH, and Ca(OH)2 are used to prepare insoluble hydroxides of other metals. For a couple of examples, see Figure 1-81. The precipitated hydroxides are separated by filtration, washed with water, and dried if necessary. The drying should be performed carefully since at high temperatures such insoluble hydroxides decompose to the corresponding oxides and water.
Figure 1-81. Precipitation of insoluble Cu(OH)2 and Fe(OH)3 on addition of KOH or NaOH to CuSO4 and Fe(NO3)3, respectively.


1.17.3. Chemical Properties of Bases. Experiment: Reaction of Aluminum with NaOH. We already know that bases react with acids and acidic oxides. Let us briefly summarize these types of transformations and learn a bit more about metal hydroxides.

(1). The most important chemical property of bases is their reaction with acids to give a salt and water (neutralization). Although we have already familiarized ourselves with this reaction more than once, two more examples are presented in Figure 1-82.
Figure 1-82. Two more examples of neutralization reaction.


(2). Bases react with acidic oxides to give a salt and water (Figure 1-83).
Figure 1-83. Bases react with acidic oxides.


(3). Strong soluble bases such as NaOH and KOH react with amphoteric oxides and hydroxides (Figure 1-84).
Figure 1-84. Strong water-soluble bases react with amphoteric hydroxides and oxides.


(4). Soluble metal hydroxides react with salts of other metals to give an insoluble hydroxide that precipitates out (Figure 1-81). Figure 1-85 provides two more examples of this type of reaction.
Figure 1-85. Precipitation of insoluble Mg(OH)2 and Al(OH)3 on addition of KOH or NaOH to MgCl2 and Al(NO3)3, respectively.


(5). Some metals that form amphoteric oxides and hydroxides react with aqueous NaOH or KOH to give hydrogen gas (Figure 1-86). This is a remarkable transformation that we have not yet discussed in our course and that is fun to watch. If you wish to try this reaction at home, you may.
Figure 1-86. Reaction of aluminum metal with NaOH in water.


Experiment 10. Reaction of Aluminum with NaOH. Please carefully read and understand the following:

DISCLAIMER: Although most of the experiments described in this subsection and elsewhere in this website are regarded as low hazard, I expressly disclaim all liability for any occurrence, including, but not limited to, damage, injury or death which might arise as consequences of the use of any experiment(s) listed or described in the free online course Chemistry from Scratch. Therefore, you assume all the liability and use these experiments at your own risk (see Terms of Use).

If you decide not to do the experiments, still read this subsection.

For this experiment, you will need aluminum foil and sodium hydroxide, NaOH. Aluminum foil is available from food and other stores. Sodium hydroxide (lye) is the sole or main component of solid drain openers sold in hardware stores. Food grade NaOH is available from a number of online vendors. Buy the smallest possible quantity of NaOH.

Let us now refresh our memory on NaOH and the reaction (Figure 1-86) in order to come up with safety measures for carrying out the experiment.

1. You should always remember that NaOH is a very caustic material that is hazardous to the skin and especially to the eyes. Therefore, direct contact with solid NaOH and its solutions must be avoided. Wear gloves and safety glasses when doing the experiment. If NaOH makes contact with your skin, immediately rinse the area with tap water until the slippery feel has subsided or gone altogether. Immediately after that rinse the affected area with a small amount of white vinegar and then water again. Why vinegar? Because vinegar is a weak solution of acetic acid that will neutralize the residual NaOH.

2. As NaOH dissolves in water, much heat is released. The reaction (Figure 1-86) is also exothermic. Consequently, the solution will get quite warm or even hot during the experiment. Be careful touching the reaction vessel.

3. The reaction produces H2 (Figure 1-86). Since hydrogen is flammable, there should be no open flame sources in the vicinity of the reaction. It is best to perform the experiment outdoors.

Place 5-10 aluminum foil strips at the bottom of a glass. The strips can be roughly a square inch in size. Pour tap water onto the strips until the glass is approximately one-quarter full. Using a teaspoon, pincer or forceps, add a few pellets of NaOH and gently swirl the contents of the glass. Observe the formation of bubbles of pure hydrogen formed in the reaction (Figure 1-86). After the hydrogen evolution has ceased, add vinegar to fill up the glass. Vinegar will neutralize the remaining NaOH (if any left) within the time of mixing, after which the mixture can be safely disposed of.

Watch this video showing two guys having lots of fun with the reaction between aluminum and NaOH. I discourage you from running this reaction on a large scale. First, you do not want a spillover of a hot caustic solution that can ruin your countertop and household goods and, even worse, damage your skin. Second, you should avoid evolution of large quantities of flammable hydrogen. Third, if run on a large scale, the reaction generates a lot of heat, which poses the risk of thermal burns.

Now take another look at the chemical equation of the reaction of aluminum with sodium hydroxide (Figure 1-86). Like the previously presented formula of sodium zincate (Na2ZnO2), that of sodium aluminate (NaAlO2) is simplified. An overly picky chemist might even blame me for teaching you incorrect formulas. However, I do this on purpose, in order to avoid oversaturating you with information that I believe is not only unnecessary for the learning process at this point, but could have a distracting and confusing effect. If you want to know the correct formulas right now anyway, here they are: Na2[Zn(OH)4] and Na[Al(OH)4]. They might look strange to you. How can Zn and Al be tetravalent? What are these square brackets in the formulas and why do we use them? Well, these substances are so-called coordination compounds, also known as metal complexes or "ate" complexes. Chances are that at this point you do not have enough knowledge of basic chemistry to study coordination compounds. One step at a time, so let us first learn and understand simpler things in chemistry.

1.17.4. Exercises.

1. Both NaOH and KOH occur in nature in the form of crystalline minerals. True or false? Answer

2. Calcium hydroxide is called slacked lime, made by the reaction of quicklime (CaO) with water, and used in large quantities to make concrete. True or false? Answer

3. Finish and balance the following chemical equations.

(a) NaOH + HNO3 =

(b) KOH + CuCl2 =

(c) Ca(OH)2 + CO2 =

(d) Fe(OH)3 + HCl =

(e) AlCl3 + 3 NaOH =

(f) AlCl3 + NaOH (excess) =

(g) Cu(OH)2 (heating) =

(h) Zn(OH)2 + KOH =

(i) MgCl2 + NaOH =

(j) Na2O + H2O =

Answer