Hydrolysis : tests = Гидролиз : тесты


The Ministry of Education and Science of the Russian Federation Kazan National Research Technological University











            Hydrolysis


    Tests


















Kazan
KNRTU Press
2018

        UDC 546; 54-386



The study guide is published in accordance with the decision of the Faculty of Chemical Technologies

Contributors:
Assoc. Prof. М. М. Petrova Full Prof. Е. М. Zueva

Reviewers:
Ph. D. (in chemistry), Full Prof. N. B. Berezin
Ph. D. (in chemistry), Full Prof. M. B. Gazizov







        Hydrolysis : tests / compiled by M. M. Petrova, E. M. Zueva; The Ministry of Education and Science of the Russian Federation, Kazan National Research Technological University. - Kazan : KNRTU Press, 2018. - 62 p.



       The study guide contains individual test tasks, which can be used to control student achievements in the topic "Hydrolysis".
       The tasks are developed for the first-year students of engineering degree programs, who study the discipline "General and Inorganic Chemistry" in English.
       The study guide was prepared at the Department of Inorganic Chemistry.

UDC 546; 54-386


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THEORETICAL BACKGROUND


       Dissolution of chemical compounds in water is often accompanied by exchange reactions. Such processes are called hydrolysis. In general case, hydrolysis is the reaction of exchange decomposition between the water and a compound, which is well soluble in water. In inorganic chemistry, hydrolysis is most typical for salts. The salt hydrolysis can be considered as the result of a polarization interaction between the salt ions and their hydrate shell in water solution. The character and the degree of dissociation of the hydrate-shell molecules depend on the nature of cations and anions: the stronger the polarizing effect of the ions, the higher the degree of hydrolysis.
       In water solution, cations exist as cationic aqua complexes formed by the donor-acceptor interaction between the cation and water molecules. Aqua complexes are hydrated by the water molecules of the second hydrate shell by means of hydrogen bonds (see the scheme below). The higher the charge and smaller the size of the cation, the stronger its acceptor character and higher the degree of polarization of the O-H bond in the coordinated water molecule. As a result, the hydrogen bond between the coordinated water molecule and water molecules of the second hydrate shell becomes stronger. All these lead to cleavage of the O-H bond in the coordinated water molecule. The Н...ОН2 bond becomes a covalent one. Thus, the OH3⁺ ion and hydroxo aqua complex are formed. The hydrolysis scheme for the Al³⁺ cation, which in water solution exits as the cationic aqua complex [Al(OH2)6]³⁺, is shown below.

     OH2   /H       3+             OH2         2+    
H;O\ 1 /   O                  H;O\   1 / ,O -H       
     \ I z XH...... ......OH2      \ 1 /       + OH3+
     Al                               Al             
H2OZ z г   OH2                H2OZ   ' г OH2         
     OH2      у                      OH2   у         

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As a result, the aquahydroxoaluminium(III) complex, [Al(OH2)5OH]²⁺, is formed, and an excess of hydrogen ions appears. Thus, the medium becomes acidic.
[А1(ОН2)б]³⁺ + Нон <     > [Al(OH2)5OH]²⁺ + НзО⁺,
This reaction can be written in a short form:
Al³⁺ + HOH <-----> A1OH2+ + H⁺.
We will use this short notation hereinafter.
       Anions are linked with water molecules by hydrogen bonds. The higher the charge and smaller the size of the anion, the stronger the hydrogen bond between this anion and a proton in water molecule. This hydrogen bond can be strong enough to withdraw a proton from water molecule. As a result, the О...Н bond becomes a covalent one (see the scheme below). The hydrolysis scheme for the phosphate anion is shown below as an example.

O ч ^O /Р-f
O''  4

3-

H-O
H

O ч' /O
   /Р ''
O'  \) —H

2-

+ OH-

One can see that the hydrogen phosphate anion, HPO4²⁻ is formed, and an excess of hydroxide ions appears. Thus, the medium becomes basic.

       The salt hydrolysis degree is determined by the strength of the polarizing effect of the salt cation and anion on water molecules. One can distinguish four cases of interaction between the water and a salt:
       1.             If the salt cation and anion have small charges and small sizes, they produce a weak polarizing effect on water molecules. This is true for such cations as К⁺, Na⁺, Са²⁺, and some others, and for such anions as Cl⁻, SO4²⁻ NO3⁻ and some others, that is, for cations and anions corresponding to strong bases (KOH, NaOH, Ca(OH)2, etc.) and strong acids (HCl, H2SO4, HNO3, etc.).

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Thus, the salt formed by a strong base and a strong acid does not undergo hydrolysis, and its water solution has a neutral medium.
       2.           The salt formed by a strong base (such as KOH, NaOH, Ca(OH)2, etc.) and a weak acid (such as H2S, CH3COOH, H2CO3, H2SO3, etc.) undergoes an anion hydrolysis. For example, hydrolysis of the sodium sulfide proceeds as
S²’ + HOH <--> HS" + OH’,
Na2S + H2O <-----> NaHS + NaOH.
As a result, the weakly dissociating HS" anion is formed. An excess of hydroxide ions yields a basic medium. The weaker the acid, the higher the degree of hydrolysis.
       3.           The salt formed by a weak base (such as NHyH2O, Cu(OH)2, Fe(OH)2, Zn(OH)2, etc.) and a strong acid (such as HCl, H2SO4, HNO3, etc.) undergoes a cation hydrolysis. For example, hydrolysis of the manganese(II) chloride proceeds as
Mg²⁺ + HOH <-----> MgOH⁺ + H⁺,
MgCl₂ + H₂O <----> MgOHCl + HCl.
As a result, the weakly dissociating MnOH⁺ cation is formed. An excess of hydrogen ions yields an acidic medium. The weaker the base, the higher the degree of hydrolysis.
       4.           The salt formed by a weak base (such as NHyH2O, Cu(OH)2, Fe(OH)2, Zn(OH)2, etc.) and a weak acid (such as H2S, CH3COOH, H2CO3, H2SO3, etc.) undergoes both cation and anion hydrolysis. For example, hydrolysis of the ammonium acetate proceeds as
NH4⁺ + HOH <-----> N IH l₂O + H⁺,

CH3COO’ + HOH <------> CH3COOH + OH",

CH3COONH4 + H2O <-------> NlklhO + CH3COOH.
Since both an acid and a base are formed, water solutions of such salts have a nearly neutral medium.
       All hydrolysis reactions discussed above are reversible, that is, a dynamical equilibrium is reached in a salt solution. Hydrolysis

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of salts formed by a very weak base and a very weak acid is irreversible, e.g., hydrolysis of the aluminium(III), chromium(III), and iron(III) sulfides or carbonates. These salts cannot be obtained in water solution. An aluminium(III), chromium(III) or iron(III) salt when interacting with a sulfide or carbonate salt in water solution yields the insoluble hydroxide Al(OH)3, Cr(OH)3 or Fe(OH)3:
      2А1С1з + 3Na2S +6H2O ------> 2Al(OI IR + 3H2ST + 6NaCl
2СгС1з + 3Na₂CO +ЗН2О --------> 2Cr(OHM +ЗСО2Т +6NaCl
In each case, hydrolysis of both salts (AlCl3 and Na2S or CrCl3 and Na2CO3) is mutual reinforced.
       The second example of irreversible hydrolysis is hydrolysis of covalent compounds. Compounds formed from nonmetals do not ionize in water and undergo irreversible hydrolytic decomposition. Two acids are usually formed:
ВС1з + ЗН2О -----> H3BO3 + 3HCl
ClF + H2O -----> HClO + HF

Hydrolysis and the medium acidity
       As mentioned above, hydrolysis yields an excess of either hydrogen or hydroxide ions. It follows that the solution medium is changed. To characterize its acidity or basicity, either the concentration of hydrogen ions or the pH index is used. The latter is defined as the negative decimal logarithm of the molar concentration, measured in units of moles per liter, of hydrogen ions:
pH = - lg[H⁺].
Thus, to compute this index, one needs to know the concentration of hydrogen ions in solution.
       The ionic product of water
Kw = [H⁺ ]-[OH- ]
is a constant not only for pure water, but also for dilute water solutions of different compounds. At room temperature, Kw is equal to 10⁻¹⁴ With a change in the [H⁺] or [OH⁻] concentration, the concentration of the other ion is changing in such a way that their

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product remains a constant. It follows that if the concentration of hydrogen ions is known, one can calculate the concentration of hydroxide ions, and vice versa:
[H ]=K /[OH- ],    [OH- ] = Kw /[H ].
Thus, to characterize both acidic and basic solutions, it is sufficient to specify either the concentration of hydrogen ions or the pH value.
       Neutral medium. Pure water contains equal concentrations of H⁺ and OH⁻ ions. At room temperature, [H⁺] = [OH⁻] = 10⁻⁷ mol/l and therefore for pure water pH = 7.
       Acidic medium. In this case, [H⁺] > 10⁻⁷ mol/l, while [OH-] < 10⁻⁷ mol/l, and рН < 7.
       Basic medium. In this case, [OH-] > 10⁻⁷ mol/l, while [H⁺] < 10⁻⁷ mol/l, and рН > 7.

       On practice, the medium acidity is conveniently determined by means of acid-base indicators - the substances which change colour with pH:

   Indicator        Indicator’s colour        
                 Acidic    Neutral    Basic  
                 medium    medium    medium  
Methyl orange      red     orange    yellow  
 Purple litmus     red     purple     blue   
Phenolphthalein colorless colorless raspberry

Hydrolysis constant
       Hydrolysis constant, KH, is an equilibrium constant for a hydrolysis reaction, which is used for its quantitative characterization. For example, hydrolysis of KCN and its hydrolysis constant are shown below:
CN⁻ + HOH <-----> HCN + OH⁻           KH = [HCN] '[OH ].
H [CN⁻ ]
                       K
If we replace [OH ] by ^jw^, we get

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[HCN] • ww   ww
KTT ——, H [CN ] •[H⁺ ]     Ka
where Ка is the ionization constant of HCN:

HCN <-> H⁺ + CN"

K — IH И¹ N ] — ₇.₉.₁₍ᵣ„ a [HCN]

      Similarly, hydrolysis constant for a cation hydrolysis is
K K-
K^ — —, H     Kb
where Kb is a base ionization constant.
       Let us consider both cation and anion hydrolysis. For example, hydrolysis of CH3COONH4 (the hydrolysis scheme is shown above). Hydrolysis constant for this salt is


NH4+ + CH3COO" + HOH <-> NH₃<H2O + CH3COOH

              [N _ [NH3 • H₂O] •[CH3COOH]
H~   [NH4+] •[CH3COO ]   .

If we multiply both numerator and denominator by [H⁺] and [OH"], we get

_ [NH₃ • H₂O]• [CH3COOH]• [H⁺ ]• [OH ] _ H ~  [NH₄⁺ ]• [OH ]• [CH3COO ]• [H⁺ ] "
where Kb is the ammonia solution ionization constant: N IO HO <---------------------> NH4+ + OH",
K — [W] .[OH- ] —1.76.10.5, b    [NH₃ • H₂O]
and Ка is the acetic acid ionization constant: CH3COOH <------------------> CH3COO’ + H⁺,
K — [H⁺]-[CH3C'OO ] —1.74,10-5.
               a    [CH₃COOH]

Kw

b

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        Obviously, the stronger the acid or the base, the larger their ionization constants. The Ka and Kb values are often replaced by their negative decimal logarithms, pKa and pKb:
рК = - lgK.
For weak acids or bases, the pK values are positive. The weaker the acid or the base, the larger pK value.


Multistage hydrolysis
       Salts containing a singly charged cation (anion) and a multicharged anion (cation) undergo a multistage hydrolysis.
       Let us first consider salts of weak tribasic acids. The acidic salt formed at the first stage of anion hydrolysis undergoes further interaction with water. However, the subsequent hydrolysis stages occur to a lesser extent due to decrease in ionization constants of the corresponding polybasic acid (Ka,1 > Ka,₂ > ... > Ka,n). For example, HPO4²⁻ dissociates to a lesser extent than H₂PO4“, and therefore it is mainly formed in course of hydrolysis of Na3PO4:
     ЖРО4 + H2O <--------> Na2HPO4 + NaOH
     Na₂HPO₄ + H₂O < > NaH₂PO₄ + NaOH (to a lesser extent). For salts of weak tribasic acids, the third stage occurs to a negligible extent, that is, its products present in solution in trace amounts.
       Let us consider salts of weak triacid bases, such as AlCl3. At the first stage of cation hydrolysis, the most weakly dissociating hydroxoaluminium(III) ion is formed:
     Al³⁺ + HOH <-----> AlOH²⁺ + H⁺     KH ₓ = [AlOH з] ~[H ]
<                        H,1      [Al³⁺ ]
     AlCl₃ + H₂O <----> AlOHCl₂ + HCl.
                      K
If we replace [H⁺] by ^^w , we get

_[AlOH²⁺ ] • K  K_
H ,1 [Al³⁺ ] •[OH- ] Kb,3,

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where Kb,3 is the ionization constant of Al(OH)3 at the third stage:
     A1OH2+ <-----> Al3+ + OH’         K з = [Al ]'[OH ].
<                         b,3    [AlOH²⁺ ]
At room temperature, hydrolysis of multicharged cations is practically confined to the first stage. Upon heating, the second stage occurs:


AlOH²⁺ + HOH <----> Al(OH)2⁺ + H⁺


[Al(OH)₂⁺ ]' [H⁺ ]
H,2     [AlOH²⁺ ]

AlOHCl2 + H2O <-----> Al(OH)2Cl + HCl.
                      K
If we replace [H⁺] by     , we get

£      [Al(OH)2⁺ ] ' Kw   K
H,2 [AlOH²⁺ ]'[OH-] Kb2 ,
where Kb ,₂ is the ionization constant of Al(OH)3 at the second stage:
     Al(OH)2⁺ <-----> А10Н2+ + OH’     Kb ₂ = [AlOH ] '[OH ].
b2     [Al(OH)2⁺ ]
Hydrolysis of Al(OH)2⁺ occurs to a negligible extent.
       In case of reversible hydrolysis, a dynamic equilibrium is established in a salt solution. Thus, according to the Le Chatelier’s principle, the equilibrium can be shifting by adding the acid or the base. This fact is often used to enhance or to inhibit the processes of hydrolysis. Hydrolysis is enhanced with a decrease in the salt concentration.

       In some cases, hydrolysis can proceed in a very complex way. The content of hydrolysis products depends on the applied conditions (concentration of solution, temperature, presence of other compounds). For multicharged ions, the equilibrium is usually reached slowly, and therefore the duration of the process is very important.

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