When any strong acid is neutralized by any strong base, for each mole of water formed, about the heat is released:
This suggests that such reactions are reduced to one process. We will obtain the equation for this process if we consider in more detail one of the given reactions, for example, the first. Let's 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 the ions did not undergo changes during the reaction. Therefore, we will rewrite the equation again, eliminating these ions from both sides of the equation. We get:
Thus, the reactions of neutralization of any strong acid with any strong base come down to the same process - 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 the formation of water from ions is reversible, which can be expressed by the equation
However, as we will see below, water is a very weak electrolyte and dissociates only to a negligible extent. 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 completion.
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:
Such reactions also come down 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 sediment in molecular form:
As can be seen, the ions do not undergo changes during the reaction. Therefore, we exclude them and rewrite the equation again:
This is the ion-molecular equation of the process under consideration.
Here we must also keep in mind that the silver chloride precipitate is in equilibrium with the ions 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 is almost completed.
The formation of a precipitate will always be observed when there are significant concentrations of and ions 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 on an ion, and an ion can serve as a reactant on an ion.
In the future, we will widely use the ionic-molecular form of writing equations for reactions involving electrolytes.
To draw up ion-molecular equations, you need to know which salts are soluble in water and which are practically insoluble. general characteristics solubility in water essential salts is given in table. 15.
Table 15. Solubility of the most important salts in water
Ionic-molecular equations help to understand the peculiarities of reactions between electrolytes. Let us consider, as an example, several reactions that occur with the participation of weak acids and bases.
As already mentioned, the neutralization of any strong acid by 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 neutralizing a strong acid with a weak base, or a weak acid with a strong or weak base, the thermal effects are different. Let's write 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 electrolytes 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 (nitrogen) with a weak base (ammonium hydroxide):
Here we must write the acid and the resulting salt in the form of ions, and ammonium hydroxide and water in the form of molecules:
The ions do not undergo changes. Omitting them, we obtain the ionic-molecular equation:
Neutralization of a weak acid (acetic acid) with a weak base (ammonium hydroxide):
In this reaction, all substances except those formed are weak electrolytes. Therefore, the ion-molecular form of the equation looks like:
Comparing the obtained ion-molecular equations with each other, we see that they are all different. Therefore, it is clear that the heats of the reactions considered are also different.
As already indicated, the reactions of neutralization of strong acids with strong bases, during which hydrogen ions and hydroxide ions combine to form a water molecule, proceed almost to completion. Neutralization reactions, in which at least one of the starting 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 completion.
They reach a state of equilibrium in which the salt coexists with the acid and base from which it was formed. Therefore, it is more correct to write the equations of such reactions as reversible reactions.
When dissolved in water, not all substances have the ability to conduct electricity. Those compounds, water solutions which are capable of conducting electric current are called electrolytes. Electrolytes conduct current due to the so-called ionic conductivity, which many compounds with an ionic structure (salts, acids, bases) possess. There are substances that have highly polar bonds, but in solution they undergo incomplete ionization (for example, mercury chloride II) - these are weak electrolytes. Many organic compounds (carbohydrates, alcohols) dissolved in water do not disintegrate into ions, but retain their molecular structure. Such substances do not conduct electric current and are called non-electrolytes.
Here are some principles that can be used to determine whether a particular compound is a strong or weak electrolyte:
- Acids . The most common strong acids include HCl, HBr, HI, HNO 3, H 2 SO 4, HClO 4. Almost all other acids are weak electrolytes.
- Grounds. The most common strong bases are hydroxides of alkali and alkaline earth metals (excluding Be). Weak electrolyte – NH 3.
- Salt. Most common salts, ionic compounds, are strong electrolytes. Exceptions are mainly salts of heavy metals.
Electrolytic dissociation theory
Electrolytes, both strong and weak and even very diluted, do not obey Raoult's law And . Having the ability to conduct electrically, the vapor pressure of the solvent and the melting point of electrolyte solutions will be lower, and the boiling point will be higher compared to similar values pure solvent. In 1887, S. Arrhenius, studying these deviations, came to the creation of the theory of electrolytic dissociation.
Electrolytic dissociation suggests that electrolyte molecules in solution break down into positively and negatively charged ions, which are called cations and anions, respectively.
The theory puts forward the following postulates:
- In solutions, electrolytes break down into ions, i.e. dissociate. The more dilute the electrolyte solution, the greater its degree of dissociation.
- Dissociation is a reversible and equilibrium phenomenon.
- Solvent molecules interact infinitely weakly (i.e., solutions are close to ideal).
Different electrolytes have different degrees of dissociation, which depends not only on the nature of the electrolyte itself, but the nature of the solvent, as well as the concentration of the electrolyte and temperature.
Degree of dissociation α , shows how many molecules n disintegrated into ions, compared to total number dissolved molecules N:
α = n/N
In the absence of dissociation α = 0, with complete dissociation of the electrolyte α = 1.
From the point of view of the degree of dissociation, according to strength, electrolytes are divided into strong (α > 0.7), medium strength (0.3 > α > 0.7), weak (α< 0,3).
More precisely, the process of electrolyte dissociation is characterized by dissociation constant, independent of the concentration of the solution. If we imagine the process of electrolyte dissociation in general form:
A a B b ↔ aA — + bB +
K = a b /
For weak electrolytes the concentration of each ion is equal to the product of α by the total concentration of the electrolyte C, so the expression for the dissociation constant can be transformed:
K = α 2 C/(1-α)
For dilute solutions(1-α) =1, then
K = α2C
It's not hard to find from here degree of dissociation
Ionic-molecular equations
Consider an example of neutralization of a strong acid with a strong base, for example:
HCl + NaOH = NaCl + HOH
The process is presented as molecular equation. It is known that both the starting substances and the reaction products in solution are completely ionized. Therefore, let us represent the process in the form complete ionic equation:
H + + Cl - + Na + + OH - = Na + + Cl - + HOH
After “contraction” of identical ions in the left and right parts we get the equations abbreviated ionic equation:
H + + OH - = HOH
We see that the neutralization process comes down to the combination of H + and OH - and the formation of water.
When composing ionic equations, it should be remembered that only strong electrolytes are written in ionic form. Weak electrolytes solids and gases are written in their molecular form.
The deposition process is reduced to the interaction of only Ag + and I - and the formation of water-insoluble AgI.
To find out whether the substance we are interested in is able to dissolve in water, we need to use the insolubility table.
Let's consider the third type of reaction, which results in the formation of a volatile compound. These are reactions involving carbonates, sulfites or sulfides with acids. For example,
When mixing some solutions of ionic compounds, interactions between them may not occur, for example
So, to summarize, we note that chemical transformations observed when one of the following conditions is met:
- Non-electrolyte formation. Water can act as a non-electrolyte.
- Formation of sediment.
- Gas release.
- Formation of a weak electrolyte for example acetic acid.
- Transfer of one or more electrons. This is realized in redox reactions.
- Formation or rupture of one or more.
Since electrolytes in solution are in the form of ions, reactions between solutions of salts, bases and acids are reactions between ions, i.e. ion reactions. Some of the ions, participating in the reaction, lead to the formation of new substances (lowly dissociating substances, precipitation, gases, water), while other ions, present in the solution, do not produce new substances, but remain in the solution. In order to show which ions interaction leads to the formation of new substances, molecular, complete and brief ionic equations are drawn up.
IN molecular equations All substances are presented in the form of molecules. Complete ionic equations show the entire list of ions present in the 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 (lowly dissociating substances, sediments, gases, water).
When compiling ionic reactions It should be remembered that substances are slightly dissociated (weak electrolytes), slightly and sparingly soluble (precipitate - “ N”, “M”, see appendix, table 4) and gaseous ones are written in the form of molecules. Strong electrolytes, almost completely dissociated, 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 that the substance is removed in the form of a gas.
The procedure for composing ionic equations using known molecular equations Let's look at 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, with well-dissociating substances written in the form of ions, and poorly dissociating substances (including water), gases or sparingly soluble substances - in the form of molecules. The coefficient in front of the formula of a substance in a molecular equation applies equally to each of the ions that make up the substance, and therefore it is placed in front of the ion in the ionic equation:
2 Na + + CO 3 2- + 2H + + 2Cl -<=>2Na + + 2Cl - + CO 2 + H 2 O
3. From both sides of the equality, ions found in the left and right sides are excluded (reduced):
2Na++ 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 during the reaction, and/or slightly dissociated, and/or sparingly 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, complete and brief ionic equation. If such substances are present both in the reagents and in the products, then the reaction will be reversible (<=>):
Molecular equation: CaCO 3 + 2HCl<=>CaCl 2 + H 2 O + CO 2
Complete ionic equation: CaCO 3 + 2H + + 2Cl –<=>Ca 2+ + 2Cl – + H 2 O + CO 2