Volume 1
1.16. MAIN CLASSES OF INORGANIC COMPOUNDS: ACIDS

General Remarks • Preparation of Acids • Chemical Properties of Acids • Exercises
1.16.1. General Remarks. Let us recall that acids are chemical compounds containing a hydrogen atom that can be displaced with a metal atom. Acids have common names (Table 2 in section 1.12) that should be memorized. Like oxides, acids are ubiquitous in nature. Naturally occurring acids, however, are mostly organic. These include citric (fruit), formic (ants), acetic (plants, fruit), and lactic (milk) acids. Unlike organic acids, inorganic acids, also known as mineral acids, are not widespread in nature and have to be manufactured for a broad variety of applications.
Digression. You may have heard of "acid trips" and LSD that is conventionally called just "acid". In fact, LSD is not an acid in a chemical sense, but a derivative of lysergic acid, its diethylamide.
1.16.2. Preparation of Acids. Water-soluble acids are formed upon treatment of an acidic oxide with water (Figure 1-71). The most important commodity chemical, sulfuric acid (H2SO4), is manufactured in this way.
Figure 1-71. Reactions of SO3 and P2O5 with water to give sulfuric and phosphoric acids, respectively.


A strong acid can react with a salt of a weaker acid to give the weaker acid and a salt of the stronger acid. Such processes are typical exchange (metathesis) reactions. Being one of the strongest mineral acids, sulfuric acid is often used to make other acids by this method (Figure 1-72).
Figure 1-72. Exchange (metathesis) reactions of sulfuric acid with salts of weaker acids.


To use this method for practical purposes, there should be a simple and efficient way to isolate the acid produced from the reaction mixture. This is easily done when one of the two products is a gas that escapes from the reaction medium or a precipitate that can be separated by filtration. The upper equation in Figure 1-72 is for the reaction that is most commonly used in the laboratory to make pure HCl. Heating a mixture of NaCl with concentrated H2SO4 produces HCl gas that can be collected and used.
Digression. While in the laboratory HCl is often made from NaCl and H2SO4 (Figure 1-72), the industry obtains HCl as a co-product of chlorination of organic compounds, mostly methane (CH4). We will learn about some of these reactions in Volume 4 of this course.
The middle equation in Figure 1-72 describes the reaction that is most closely related to the one used to make phosphoric acid on a large industrial scale (Volume 3). Agitating Ca3(PO4)2 with H2SO4 leads to the formation of H3PO4 and insoluble CaSO4 that can be separated by filtration.

The bottom equation in Figure 1-72 represents a special case. The silicic acid (H2SiO3) produced in this reaction is a solid that is insoluble in water, yet it does not precipitate out in the conventional sense and cannot be isolated by filtration. The particles of the silicic acid formed in this reaction are tiny, forming a sol, a colloidal solution that looks like an opaque (cloudy) liquid (Figure 1-73). Such very small particles, traditionally called colloids, have recently received a new fancy name "nanoparticles". Colloids are very interesting and important substances that are studied in a special branch of chemistry, called Colloidal Chemistry.
Figure 1-73. A colloidal solution of silicic acid (source).


1.16.3. Chemical Properties of Acids. As we already know, the most general and important chemical transformation of acids is their reaction with bases (metal hydroxides) to give water and salts (neutralization). We also already know that acids react with basic oxides to give salts and water, and with some metals to give salts and hydrogen. Let us summarize and expand our knowledge of chemical properties of acids.

(1). Neutralization of acids with bases and amphoteric hydroxides to give a salt and water (Figure 1-74).
Figure 1-74. Examples of neutralization reaction.


(2). Acids react with basic and amphoteric oxides to give a salt and water (Figure 1-75).
Figure 1-75. Examples of reactions of acids with a basic (top) and amphoteric (bottom) oxides.


(3). Acids react with many metals to produce hydrogen and a salt (Figure 1-76).
Figure 1-76. Examples of reactions of acids with metals.


Some metals react with acids readily, some considerably less so, and some do not react at all. The order of reactivity of selected metals toward acids is presented in Figure 1-77.
Figure 1-77. Order of reactivity of selected metals toward acids.


In Figure 1-77, the metals on the right from hydrogen (symbols in black) do not displace hydrogen from acids. The ones on the left (symbols in red) do, their reactivity increasing from right to left. We will learn why soon. Meanwhile, just remember that the ability of different metals to react with acids can be very different. For example, while tin (Sn) reacts with concentrated HCl only sluggishly, the reaction of zinc (Zn) is quite vigorous, sodium (Na) reacts violently, and potassium (K) even more so. The reaction of HCl with K is particularly spectacular (and dangerous)!

Another thing to keep in mind is that when it comes to reactions of acids with metals, nitric acid (HNO3) is exceptional. Nitric acid reacts even with some of those metals that do not react with other acids, the ones to the right from the hydrogen in Figure 1-77. These reactions, however, are of a different type. Only on a rare occasion do they produce H2. We will study reactions of nitric acid with various metals in considerable detail in Volume 3 of this course.

(4). Acids can react with some salts to give hydro salts (Figure 1-78).
Figure 1-78. Two examples of reactions of an acid with a salt to give a hydro salt.


1.16.4. Exercises.

1. Both organic and inorganic (mineral) acids are widespread in nature. True or false? Answer

2. The industrial method to make HCl is based on the reaction of H2SO4 with NaCl (Figure 1-72, top). True or false? Answer

3. Finish and balance the following equations.

(a) H2SO4 + Mg =

(b) SO3 + H2O =

(c) SO3 + KOH (excess) =

(d) CaCO3 + HCl (excess) =

(e) H3PO4 + NaOH (excess) =

(f) HNO3 + Cu(OH)2 =

(g) H2SO4 + BaCl2 =

(h) P2O5 + H2O =

Answer