One of the processes for obtaining an acidic or basic solution occurs based on the reaction of ions from some salts with water. This process is called salt hydrolysis.
See too:What is a hydrolysis constant?
How does saline hydrolysis take place?
In saline hydrolysis, theions from the solubilization of a salt react with the Water and form only weak bases or weak acids, since cations and anions from strong bases and acids do not undergo hydrolysis. These salts can have different origins according to the ions that compose them.
- The reaction of a generic cation with water can be represented as follows:
X+ + H2O ⇌ XOH + H+
Note that cation hydrolysis produces acidic solutions, characterized by the formation of H+.
- The reaction of a generic anion with water can be represented as follows:
Y- + H2O ⇌ oh- + HY
Note that anion hydrolysis produces acidic solutions, characterized by the formation of OH-.
Next, we will analyze each of the types of hydrolysis according to the available ions of the chosen salt.
Salt hydrolysis of strong acid and weak base
Let's analyze the NH salt hydrolysis4br, which features the ions br- from HBr (strong acid) and the ions NH4+ derivatives of NH4OH (weak base).
In this case, the hydrolysis of the cation occurs, coming from the weak base, and nothing happens with the anion Br-, as it is an ion derived from a strong acid.
NH4+ + H2O ⇌ NH4OH+H+
The hydrolysis of strong acid salt and weak base results in an acidic solution (pH < 7), due to the presence of the H ion+ that did not interact with the Br ion-.
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Saline hydrolysis of weak acid and strong base
Let's analyze the KCN salt hydrolysis, which features the ions CN- from HCN (weak acid) and the ions K+ derived from KOH (strong base). In this case, the anion hydrolysis, from the weak acid and nothing happens to the K cation+, ion derived from a strong base.
CN- + H2O ⇌ OH- + HCN
The hydrolysis of weak acid salt and strong base results in a basic solution (pH > 7), due to the presence of OH ions- that did not interact with the K ions+.
Saline hydrolysis of weak acid and weak base
Let's analyze the NH salt hydrolysis4CN, from HCN (weak acid) it's from NH4OH (weak base). In this case, the hydrolysis of the two ions, as they come from weak acid and base.
NH4+ + CN- + H2O ⇌ NH4OH + HCN
The hydrolysis of weak acid salt and weak base results in a solution slightly basic or acidic, depending on acid and base dissociation constants (KThe and KB, respectively).
For the example of salt NH4CN, we have:
HCN → KThe= 4,9. 10-10
NH4OH →KB= 1,8. 10-5.
Like KB is bigger than KThe, the base is more ionized than the acid, so we will have a higher concentration of OH-, which will result in a solution with pH > 7.
That said, we conclude that:
KThe > KB |
Slightly acidic solution, pH < 7. |
KThe < KB |
Slightly basic solution, pH > 7. |
Salt hydrolysis of strong acid and strong base
In this case, we have a salt with ions derived from a strong acid and a weak base, such as NaCl, which has Cl ions.- and on+ from HCl and NaOH, respectively. As stated, these ions do not undergo hydrolysis and, therefore, there is no interaction between the ions from the salt and the ions derived from water, leaving the H ions free.+ and oh-.
Such as H concentration+ and oh- it's the same (product of the self-ionization of water), we will have a solution neutral (pH = 7).
Read too: Strength of acids — learnThe to determine whether an acid is strong or weak!
Bronsted-Lowry acid-base theory
To have a clearer understanding of salt hydrolysis, it is necessary to know the acid-base theory of Bronsted-Lowry, named after the two scientists responsible for developing this theory: the physical-chemical Danish Johannes Nicolaus Bronsted and the British physical chemist Thomas Martin Lowry.
For this theory, the acid is any compound that tends to donate the H protons+, and the base are the compounds that tend to receivethe H protons+. Some compounds, like water, act as both an acid and a Bronsted-Lowry base, depending on who it's reacting to. We give these compounds the name of amphoteric.
In saline hydrolysis, the phenomenon we call acid-base conjugate pair, in which the acid donates the proton and the base receives it, resulting in its conjugate acid-base pair, as shown in the following scheme:
ACID1 + BASE1⇌ ACID2 + BASE2
See the example in the dissociation of hydrofluoric acid:
HF+H2⇌H3O+ + F−
The HF is acting as an acid, donating its H proton+ to H2O, which, in turn, is acting as a base. As a product, we have the F-, which is the HF conjugate base, and the hydronium ion, H3O+, which is the water conjugate acid.
Know more:Arrhenius, Brönsted-Lowry and Lewis acid-base theories
solved exercises
Question 1 — (UEL) Among the following substances, the only one that provides a decrease in pH when added to water is:
a) NH4AT THE3
b) CH4
c) NH3
d) NaOH
e) NaCH3COO
Resolution:Letter a. Of the alternatives presented, the only substance that causes adecrease in pH is NH4AT THE3, because it is a salt from a strong acid (HNO3) and a weak base (NH4OH). See the following hydrolysis reaction:
NH4+ + H2O ⇌ NH4OH+H+
The cation undergoes hydrolysis that results in the formation of the H ion+, which lowers the pH of the solution. The anion NO3 it does not undergo hydrolysis as it is derived from a strong acid.
question 2 — (FEI) Sodium cyanide (NaCN), zinc chloride (ZnCl) compounds2), sodium sulfate (Na2ONLY4) and ammonium chloride (NH4Cl), when dissolved in water, make the medium, respectively:
a) basic, acidic, acidic, neutral.
b) acidic, basic, neutral, acidic.
c) basic, neutral, acidic, acidic.
d) basic, acidic, neutral, acidic.
e) acidic, neutral, basic, basic.
Resolution:Letter D. Let's analyze the composition of the salts described in the question.
- NaCN comes from a strong base and a weak acid, so the solution will be basic.
- ZnCl2 comes from a weak base and a strong acid, so the solution will be acidic.
- At2ONLY4 comes from a strong base and a strong acid, so the solution will be neutral.
- NH4Cl comes from a weak base and a strong acid, so the solution will be acidic.
Question 3 — (Fuvest) Sodium carbonate, when placed in water at 25°C, dissolves.
At2CO3 + H2O → HCO3- + 2Na+ (aq) + X
X and the pH of the resulting solution should be:
steel2 greater than 7.
b) OH-(aq) greater than 7.
c) H+(aq) equal to 7.
d) CO2 equal to 7.
e) OH-(aq) less than 7.
Resolution: Letter B. the salt in2CO3 comes from a weak acid (H2CO3) and a strong base (NaOH), by this only the anion undergoes hydrolysis, forming the OH ion-. See the reaction below:
CO32- + H2O ⇌ HCO3- + OH-
Therefore, the solution has a basic character, with a pH greater than 7, and the X in the reaction can be replaced by OH-.
By Victor Ricardo Ferreira
Chemistry teacher
