Since electrolytes in solution are in the form of ions, the reactions between solutions of salts, bases and acids are reactions between ions, i.e. ionic reactions. Some of the ions, participating in the reaction, lead to the formation of new substances (low-dissociating substances, precipitation, gases, water), while other ions, being present in the solution, do not give new substances, but remain in the solution. In order to show the interaction of which ions leads to the formation of new substances, molecular, complete and brief ionic equations are composed.
IN molecular equations All substances are represented as molecules. Complete ionic equations show the entire list of ions present in solution during a given reaction. Brief ionic equations are composed only of those ions, the interaction between which leads to the formation of new substances (slightly dissociating substances, precipitation, gases, water).
When compiling ionic reactions, it should be remembered that substances are slightly dissociated (weak electrolytes), slightly - and sparingly soluble (precipitating - “ H”, “M”, see appendix‚ table 4) and gaseous are written in the form of molecules. Strong electrolytes, almost completely dissociated, are in the form of ions. The sign “↓” after the formula of a substance indicates that this substance is removed from the reaction sphere in the form of a precipitate, and the sign “”, indicates the removal of a substance in the form of a gas.
The procedure for compiling ionic equations from known molecular equations consider the example of the reaction between solutions of Na 2 CO 3 and HCl.
1. The reaction equation is written in molecular form:
Na 2 CO 3 + 2HCl → 2NaCl + H 2 CO 3
2. The equation is rewritten in ionic form, while well-dissociating substances are written in the form of ions, and low-dissociating substances (including water), gases or hardly soluble substances are written in the form of molecules. The coefficient before the formula of a substance in the molecular equation equally applies to each of the ions that make up the substance, and therefore it is taken out in the ionic equation before the ion:
2 Na + + CO 3 2- + 2H + + 2Cl -<=>2Na + + 2Cl - + CO 2 + H 2 O
3. From both parts of the equality, ions that occur in the left and right parts are excluded (reduced) (underlined by the corresponding dashes):
2 Na++ CO 3 2- + 2H + + 2Cl-<=> 2Na+ + 2Cl-+ CO 2 + H 2 O
4. The ionic equation is written in its final form (short ionic equation):
2H + + CO 3 2-<=>CO 2 + H 2 O
If in the course of the reaction and / or slightly dissociated, and / or hardly soluble, and / or gaseous substances, and / or water are formed, and such compounds are absent in the starting substances, then the reaction will be practically irreversible (→), and for it it is possible to compose a molecular, full and short ionic equation. If such substances exist both in the reactants‚ and in the products, then the reaction will be reversible (<=>):
molecular equation: CaCO 3 + 2HCl<=>CaCl 2 + H 2 O + CO 2
Full ionic equation: CaCO 3 + 2H + + 2Cl -<=>Ca 2+ + 2Cl - + H 2 O + CO 2
When any strong acid is neutralized with any strong base, about heat is released for each mole of water formed:
This suggests that such reactions are reduced to one process. We will obtain the equation of this process if we consider in more detail one of the above reactions, for example, the first one. We rewrite its equation, writing strong electrolytes in ionic form, since they exist in solution in the form of ions, and weak electrolytes in molecular form, since they are in solution mainly in the form of molecules (water is a very weak electrolyte, see § 90):
Considering the resulting equation, we see that during the reaction, the ions and did not change. Therefore, we rewrite the equation again, excluding these ions from both sides of the equation. We get:
Thus, the reactions of neutralization of any strong acid with any strong base are reduced to the same process - to the formation of water molecules from hydrogen ions and hydroxide ions. It is clear that the thermal effects of these reactions must also be the same.
Strictly speaking, the reaction of formation of water from ions is reversible, which can be expressed by the equation
However, as we shall see below, water is a very weak electrolyte and dissociates only to a negligible degree. In other words, the equilibrium between water molecules and ions is strongly shifted towards the formation of molecules. Therefore, in practice, the reaction of neutralization of a strong acid with a strong base proceeds to the end.
When mixing a solution of any silver salt with hydrochloric acid or with a solution of any of its salts, a characteristic white cheesy precipitate of silver chloride is always formed:
Similar reactions are also reduced to one process. In order to obtain its ionic-molecular equation, we rewrite, for example, the equation of the first reaction, writing strong electrolytes, as in the previous example, in ionic form, and the substance in the precipitate in molecular form:
As can be seen, the ions and do not undergo changes during the reaction. Therefore, we eliminate them and rewrite the equation again:
This is the ion-molecular equation of the process under consideration.
Here it must also be borne in mind that the silver chloride precipitate is in equilibrium with ions and in solution, so that the process expressed by the last equation is reversible:
However, due to the low solubility of silver chloride, this equilibrium is very strongly shifted to the right. Therefore, we can assume that the reaction of formation from ions practically comes to an end.
The formation of a precipitate will always be observed when ions and are in a significant concentration in one solution. Therefore, with the help of silver ions, it is possible to detect the presence of ions in a solution and, conversely, with the help of chloride ions, the presence of silver ions; an ion can serve as a reactant for an ion, and an ion as a reactant for an ion.
In the future, we will widely use the ion-molecular form of writing the equations of reactions involving electrolytes.
To draw up ion-molecular equations, you need to know which salts are soluble in water and which are practically insoluble. The general characteristics of the solubility in water of the most important salts are given in Table. 15.
Table 15. Solubility of the most important salts in water
Ionic-molecular equations help to understand the features of reactions between electrolytes. Consider, as an example, several reactions involving weak acids and bases.
As already mentioned, the neutralization of any strong acid with any strong base is accompanied by the same thermal effect, since it comes down to the same process - the formation of water molecules from hydrogen ions and hydroxide ions.
However, when a strong acid is neutralized with a weak base, a weak acid with a strong or weak base, the thermal effects are different. Let us write the ion-molecular equations for such reactions.
Neutralization of a weak acid (acetic acid) with a strong base (sodium hydroxide):
Here, the strong electrolytes are sodium hydroxide and the resulting salt, and the weak ones are acid and water:
As can be seen, only sodium ions do not undergo changes during the reaction. Therefore, the ion-molecular equation has the form:
Neutralization of a strong acid (nitric acid) with a weak base (ammonium hydroxide):
Here, in the form of ions, we must write the acid and the resulting salt, and in the form of molecules, ammonium hydroxide and water:
Ions do not undergo changes. Omitting them, we obtain the ion-molecular equation:
Neutralization of a weak acid (acetic acid) with a weak base (ammonium hydroxide):
In this reaction, all substances, except for the resulting weak electrolytes. Therefore, the ion-molecular form of the equation has the form:
Comparing the obtained ion-molecular equations, we see that they are all different. Therefore, it is clear that the heats of the considered reactions are not the same.
As already mentioned, the reactions of neutralization of strong acids with strong bases, during which hydrogen ions and hydroxide ions combine into a water molecule, proceed almost to the end. Neutralization reactions, in which at least one of the initial substances is a weak electrolyte and in which molecules of weakly associated substances are present not only on the right, but also on the left side of the ion-molecular equation, do not proceed to the end.
They reach a state of equilibrium in which the salt coexists with the acid and base from which it is derived. Therefore, it is more correct to write the equations of such reactions as reversible reactions.
