Volume 1
1.18. MAIN CLASSES OF INORGANIC COMPOUNDS: SALTS
General Information and Nomenclature • Preparation of Salts • Chemical Properties of Salts. Experiment: Reaction of CuSO4 with Fe (iron nail) • Exercises
General Information and Nomenclature • Preparation of Salts • Chemical Properties of Salts. Experiment: Reaction of CuSO4 with Fe (iron nail) • Exercises
1.18.1. General Information and Nomenclature. As you are fully expected to know by now, salts are compounds comprising one or more metal atoms and one or more acid remainders. You are also expected to know the nomenclature of salts. To name a salt, one needs to name the metal first and then the acid remainder (Table 2 in section 1.12). As is the case with oxides and bases, if a metal can be in more than one state of valence, the latter should be specified as a Roman numeral in parentheses after the name of the metal. According to this rule, CuBr is called copper (I) bromide and CuBr2 copper (II) bromide. However, if a metal that can exhibit more than one valence forms only one salt with a particular acid remainder, there is no need to specify the valence in the name of the salt. For example, the valence of copper can be 1 and 2. In principle, one would expect copper to form two fluoride salts, CuF and CuF2. Of these two, however, only CuF2 exists. Consequently, we name CuF2 copper fluoride, not copper (II) fluoride. Similarly, CuI does exist, whereas CuI2 does not. For this reason, the correct name for CuI is copper iodide, not copper (I) iodide.
Numerous salts occur in nature both as solid minerals and in solution, such as sea water and mineral water. Among the most common native salts are NaCl and carbonates, sulfates, and chlorides of potassium, calcium, and magnesium. Calcium carbonate is the main component of such highly important minerals as limestone, marble, and chalk. The mineral calcite is essentially pure CaCO3, and so is the chicken eggshell.
1.18.2. Preparation of Salts. Since many salts are commercially available, there is rarely a need to make simple salts in the laboratory. If necessary, salts can be prepared by a number of reactions, including between (i) a base and an acid (neutralization); (ii) a base and an acidic oxide; (iii) an acid and a basic oxide; and (iv) a metal and an acid. Insoluble or poorly soluble salts are also conveniently prepared by a variety of exchange reactions between a soluble salt and another soluble salt, an acid, or a base (Figure 1-87).
Numerous salts occur in nature both as solid minerals and in solution, such as sea water and mineral water. Among the most common native salts are NaCl and carbonates, sulfates, and chlorides of potassium, calcium, and magnesium. Calcium carbonate is the main component of such highly important minerals as limestone, marble, and chalk. The mineral calcite is essentially pure CaCO3, and so is the chicken eggshell.
1.18.2. Preparation of Salts. Since many salts are commercially available, there is rarely a need to make simple salts in the laboratory. If necessary, salts can be prepared by a number of reactions, including between (i) a base and an acid (neutralization); (ii) a base and an acidic oxide; (iii) an acid and a basic oxide; and (iv) a metal and an acid. Insoluble or poorly soluble salts are also conveniently prepared by a variety of exchange reactions between a soluble salt and another soluble salt, an acid, or a base (Figure 1-87).
Figure 1-87. Preparation of insoluble salts by exchange (metathesis) reactions.
1.18.3. Chemical Properties of Salts. Experiment: Reaction of CuSO4 with Fe (iron nail). The most conventional and general types of transformations of salts are exchange and displacement reactions, in which the metal atom of a salt is exchanged for a different metal atom.
(1). Exchange reactions of salts are particularly common. The exchange partner can be another salt, a base, or an acid (Figures 1-72, 1-81, 1-85, and 1-87).
(2). Displacement (substitution) reactions of a salt of a metal with another metal are also characteristic of many salts. The "Silver Tree" reaction (Experiment 7) is an example of this type of transformation.
2 AgNO3 + Cu = Cu(NO3)2 + 2 Ag
In this reaction, copper metal displaces the silver from its nitrate salt. The reverse reaction, in which silver metal would displace the copper from a copper (II) salt, does not occur.
Is there a way of knowing what metals would react with certain salts and what metals would not? There is. We just need to refer to the order of reactivity of metals that we have already considered above, see section 1.16.3 and Figure 1-77. A metal M can displace another metal M' from its salt if M is located to the left from M' in the reactivity series (Figure 1-88). A substitution reaction occurs only if a salt of a less reactive metal is treated with a more reactive metal. The silver tree experiment works because copper is more reactive than silver. Likewise, the transformation of a copper coin into a "silver" coin (subsection 1.8.4, Figure 1-25 and Video 1-19) takes place because copper is more reactive than mercury.
1.18.3. Chemical Properties of Salts. Experiment: Reaction of CuSO4 with Fe (iron nail). The most conventional and general types of transformations of salts are exchange and displacement reactions, in which the metal atom of a salt is exchanged for a different metal atom.
(1). Exchange reactions of salts are particularly common. The exchange partner can be another salt, a base, or an acid (Figures 1-72, 1-81, 1-85, and 1-87).
(2). Displacement (substitution) reactions of a salt of a metal with another metal are also characteristic of many salts. The "Silver Tree" reaction (Experiment 7) is an example of this type of transformation.
2 AgNO3 + Cu = Cu(NO3)2 + 2 Ag
In this reaction, copper metal displaces the silver from its nitrate salt. The reverse reaction, in which silver metal would displace the copper from a copper (II) salt, does not occur.
Is there a way of knowing what metals would react with certain salts and what metals would not? There is. We just need to refer to the order of reactivity of metals that we have already considered above, see section 1.16.3 and Figure 1-77. A metal M can displace another metal M' from its salt if M is located to the left from M' in the reactivity series (Figure 1-88). A substitution reaction occurs only if a salt of a less reactive metal is treated with a more reactive metal. The silver tree experiment works because copper is more reactive than silver. Likewise, the transformation of a copper coin into a "silver" coin (subsection 1.8.4, Figure 1-25 and Video 1-19) takes place because copper is more reactive than mercury.
Figure 1-88. Metal reactivity series.
To further illustrate how to use the metal reactivity series (Figure 1-88), let us consider, for example, copper (Cu) and zinc (Zn). Will Cu metal react with a Zn salt, or is it Zn metal that will react with a Cu salt? Zinc is to the left from copper in Figure 1-88, which means that Zn is more reactive than Cu. Therefore, Zn can displace Cu from its salt, but not the other way around (Figure 1-89).
To further illustrate how to use the metal reactivity series (Figure 1-88), let us consider, for example, copper (Cu) and zinc (Zn). Will Cu metal react with a Zn salt, or is it Zn metal that will react with a Cu salt? Zinc is to the left from copper in Figure 1-88, which means that Zn is more reactive than Cu. Therefore, Zn can displace Cu from its salt, but not the other way around (Figure 1-89).
Figure 1-89. More reactive Zn can displace less reactive Cu from its salt, not the other way around.
Although iron (Fe) is less reactive than zinc (Zn), it is still to the left from Cu and, therefore, should react with CuSO4, too. If you have some leftovers of CuSO4•5H2O from the recrystallizations (Experiment 3), you may use them for another interesting reaction.
Experiment 11. Reaction of CuSO4•5H2O with Fe (iron nail). 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) discussed, listed, described, or otherwise mentioned 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 experiment, still read this subsection.
Dissolve about one-quarter of a teaspoon of CuSO4•5H2O in about half a cup of water in a clear glass. Remember (Section 1-4, Experiment 3) to use a spoon made of plastic or wood, not metal, for scooping out the copper salt and agitation of its solutions.
Place an iron nail in the solution and observe the formation of red Cu metal on the surface of the nail. The reaction that occurs is as follows.
Fe + CuSO4 = Cu + FeSO4
Watch a demonstration of this reaction in Video 1-42 below. Note that it is green iron (II) sulfate, FeSO4, not brown iron (III) sulfate, Fe2(SO4)3, that is formed in this chemical transformation.
Although iron (Fe) is less reactive than zinc (Zn), it is still to the left from Cu and, therefore, should react with CuSO4, too. If you have some leftovers of CuSO4•5H2O from the recrystallizations (Experiment 3), you may use them for another interesting reaction.
Experiment 11. Reaction of CuSO4•5H2O with Fe (iron nail). 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) discussed, listed, described, or otherwise mentioned 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 experiment, still read this subsection.
Dissolve about one-quarter of a teaspoon of CuSO4•5H2O in about half a cup of water in a clear glass. Remember (Section 1-4, Experiment 3) to use a spoon made of plastic or wood, not metal, for scooping out the copper salt and agitation of its solutions.
Place an iron nail in the solution and observe the formation of red Cu metal on the surface of the nail. The reaction that occurs is as follows.
Fe + CuSO4 = Cu + FeSO4
Watch a demonstration of this reaction in Video 1-42 below. Note that it is green iron (II) sulfate, FeSO4, not brown iron (III) sulfate, Fe2(SO4)3, that is formed in this chemical transformation.
Video 1-42. More reactive Fe displace less reactive Cu from its salt (source).
Now that you have read the above, learned the order of reactivity of metals (Figure 1-88), and watched Video 1-42, it should be clear to you why it is perfectly fine to use iron spoons, spatulas, rods, etc. for handling and dissolving sodium and potassium salts such as NaCl and KNO3, but not salts of copper such as CuSO4.
(3). There are many interesting reactions that are less general but rather characteristic of salts formed by a particular acid. A good example is nitrates, salts of nitric acid (HNO3), which all decompose on heating. The decomposition onset temperature for a nitrate salt and the products formed in the decomposition strongly depend on the nature of the metal. We will learn a good deal about these reactions in Volume 3 of our course.
1.18.3. Exercises.
1. Use the metal reactivity series (Figure 1-88) to determine which one of the two reactions can occur and which cannot: (a) Hg(NO3)2 + 2 Ag = Hg + 2 AgNO3; (b) 2 AgNO3 + Hg = Hg(NO3)2 + 2 Ag. Answer
2. Finish and balance the equations below.
(a) K + HCl =
(b) Ag2SO4 + BaCl2 =
(c) Zn(OH)2 + HCl =
(d) Cu(NO3)2 + Ca(OH)2 =
(e) CuCl2 + Zn =
(f) Cu + HCl =
(g) Al(OH)3 + H2SO4 =
(h) Al(OH)3 + KOH =
(i) Al2O3 + KOH =
(j) CaBr2 + Na2CO3 =
(k) SiO2 + Na2O =
(l) K2CO3 + HCl =
(m) Zn + HCl =
(n) Fe + H2SO4 =
Answer
Now that you have read the above, learned the order of reactivity of metals (Figure 1-88), and watched Video 1-42, it should be clear to you why it is perfectly fine to use iron spoons, spatulas, rods, etc. for handling and dissolving sodium and potassium salts such as NaCl and KNO3, but not salts of copper such as CuSO4.
(3). There are many interesting reactions that are less general but rather characteristic of salts formed by a particular acid. A good example is nitrates, salts of nitric acid (HNO3), which all decompose on heating. The decomposition onset temperature for a nitrate salt and the products formed in the decomposition strongly depend on the nature of the metal. We will learn a good deal about these reactions in Volume 3 of our course.
1.18.3. Exercises.
1. Use the metal reactivity series (Figure 1-88) to determine which one of the two reactions can occur and which cannot: (a) Hg(NO3)2 + 2 Ag = Hg + 2 AgNO3; (b) 2 AgNO3 + Hg = Hg(NO3)2 + 2 Ag. Answer
2. Finish and balance the equations below.
(a) K + HCl =
(b) Ag2SO4 + BaCl2 =
(c) Zn(OH)2 + HCl =
(d) Cu(NO3)2 + Ca(OH)2 =
(e) CuCl2 + Zn =
(f) Cu + HCl =
(g) Al(OH)3 + H2SO4 =
(h) Al(OH)3 + KOH =
(i) Al2O3 + KOH =
(j) CaBr2 + Na2CO3 =
(k) SiO2 + Na2O =
(l) K2CO3 + HCl =
(m) Zn + HCl =
(n) Fe + H2SO4 =
Answer