How to judge the ion concentration of the hydrolysis of weak acid and weak alkali salts 50

For example, for example, Na2CO3, the first step is to write the ionization equation (most salts are strong electrolytes and can be completely ionized), that is, sodium ions and carbonate ions are ionized, because the carbonate ions continue to be hydrolyzed, so the sodium ion concentration is the largest. In the second step, the carbonate ions are hydrolyzed in two steps, first hydrolyzed into bicarbonate and hydroxide, and then hydrolyzed into carbonic acid and hydroxide (the second step of hydrolysis is very weak, usually the ion concentration before hydrolysis is greater than the ion concentration produced by hydrolysis, except in the case of double hydrolysis), so the concentration of sodium ions Concentration of carbonate ions Concentration of hydroxide particles Concentration of bicarbonate ions Concentration of carbonic acid molecules Concentration of hydrogen ions. In fact, if you list both the ionization equation and the hydrolysis equation, you will come out after careful analysis.

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It depends on the relative strength of weak acids and weak bases, and the relatively strong ones can be regarded as strong in order.

Weak electrolytes have a strong hydrolytic capacity, and hydrolysis is greater than ionization in this case.

The law of hydrolysis of salts.

Rules: There is a weak hydrolysis, no weak is not hydrolyzed, all weak are hydrolyzed, the weaker the more hydrolyzed, who shows whose nature, the same strong is neutral.

Type of salt. Instance.

Hydrolysis of ions. The acidity and alkalinity of the solution.

Strong acids and alkaline salts.

NaCl, KNO3 none. Neuter.

Strong acid and weak alkali salt.

nh4cl、cuso4、fecl3

nh、cu2+、fe3+

Acidity. Strong base and weak acid [ ].

na2s、na2co3、nahco3

s2－、co、hco

Alkaline. Weak acid and weak alkali salt.

nh4)2co3、ch3coonh4

nh、co、ch3coo－

It is determined by the relative strength of the acid and base.

2 Factors influencing the hydrolysis of salts.

1) Internal causes. The nature of the salt itself is the most important factor in determining the degree of hydrolysis of salt, and the weaker the acid corresponding to the acid group that makes up the salt (or the weaker the base corresponding to the cation), the greater the degree of hydrolysis.

2) External factors. Temperature: The hydrolysis of salts is an endothermic reaction, so the higher the temperature, the greater the degree of hydrolysis.

Concentration: The smaller the concentration of salts, the less chance that electrolyte ions will collide with each other to form electrolyte molecules, and the greater the degree of hydrolysis.

Administered acid-base: promotes or inhibits the hydrolysis of salts. For example, the degree of hydrolysis of acid and salt is increased in CH3Coona solution, and the degree of hydrolysis of alkali and salt is reduced.

Add salt. a. Add salts with opposite acidity and alkalinity after hydrolysis, and the hydrolysis of salts promotes each other; When salts with the same acidity and alkalinity are added after hydrolysis, the hydrolysis of salts inhibits each other.

b. Solid salts that do not participate in hydrolysis are added, which has no effect on the hydrolytic equilibrium; Adding a salt solution that does not participate in hydrolysis is equivalent to diluting the original salt solution, and the degree of hydrolysis of salt increases.

The influence of external conditions on the reaction Fe3++3H2OFE(OH)3+3H+ (the positive reaction is an endothermic reaction) is as follows:

Condition. Direction of movement.

H+ number ]ph

Fe3+ degree of hydrolysis.

Phenomenon. Increase the temperature.

Rightward. Increase.

Lower. Enlarge.

The color becomes darker (yellow, reddish-brown).

Pass HCL to the left.

Increase. Lower.

Minish. The color becomes lighter.

Add h2o to the right.

Increase. Elevated.

Enlarge. The color becomes lighter.

Add magnesium powder. Rightward.

Minish. Elevated.

Enlarge. Reddish-brown precipitate, colorless gas.

Add NAHCo3

Rightward. Minish.

Elevated. Enlarge.

Reddish-brown precipitate, colorless gas.

The more dilute the more hydrolyzed the explanation is as follows:

In the hydrolysis of inorganic salts, the factors that affect the hydrolysis are the concentration of salts, temperature, and acidity. That is, the smaller the concentration of salt, the greater the degree of hydrolysis it has. The thinner, that is, the smaller the concentration, the greater the degree of hydrolysis.

Taking the hydrolysis of weak acid and strong alkali salt as an example, the hydrolysis equation is as follows:

In the reaction, when the amount of H2O increases, the concentration of CH3COOH on the right decreases more than the decrease in CH3COOH-, so the reaction proceeds to the right.

1. Definition of hydrolysis:Hydrolysis is a reaction in which ions ionized by salt combine hydrogen ions ionized from water and hydroxide ions to form weak electrolyte molecules.

2. Types of inorganic substances that occur in hydrolysis:Strong acid and weak alkali salt, strong alkali weak salt, weak acid and weak alkali salt.

3. Hydrolytic equilibrium constant:The hydrolysis constant is the amount that indicates the degree of hydrolysis.

The above content reference: Encyclopedia - Hydrolysis.

Encyclopedia - Hydrolytic equilibrium constant.

All of them are reversible, affected by hydrogen ions and hydroxide ions in water, and at the same time affected by temperature, because the degree of dissociation of water is related to temperature, the high temperature of the degree of dissociation is greater, and the hydrolysis of weak acid and weak alkali salts is greater.

Not all hydrolysis, but only partial hydrolysis.

The relationship between the concentration of a substance and the pH value mainly depends on the expression between the pH value and the concentration, including the following situations: 1. The concentration c of strong acid is proportional to the concentration of hydrogen ions [H+], that is: [H+] = C, pH = -LGC; 2. The relationship between the concentration c of a strong base and the concentration of hydroxide ions [OH-], i.e.:

oh-] = c，poh =-lgc; ph =14-poh =14-( lgc)

(ka*kw kb) make do with it, otherwise the exact formula is complicated) distribution coefficient.

Denominator: (hydrogen ion concentration to the power) times (the product of several terms ka), such as for six-membered acid, the sixth power of the first term h, the fifth power of the second term h multiplied by ka1, the fourth power of the third term h multiplied by ka1*ka2, and the seventh term is the zero power of h multiplied (the product of six ka).

Molecule: The number of h corresponding to the substance corresponds to the term in the denominator, such as the quaternary acid form of the hexaterniary acid in the previous example, which corresponds to the fourth power of the denominator of h multiplied (ka1*ka2) in the denominator, which is the numerator.

First of all, not all salts will be hydrolyzed. This weak base is hydrolyzed when the cation contained in the salt corresponds to a weak base, or the anion corresponds to a weak acid.

For example: NH4CL. In water, NH4Cl = NH4+ +Cl-

The ammonium hydroxide corresponding to the ammonium root is a weak base, and the hydrochloric acid corresponding to the chloride ion is a strong acid, so this is a strong acid and a weak alkali salt.

Then NH4+ is hydrolyzed: NH4+ + H2O = reversible = NH3·H2O + H+, so NH4Cl solution is weakly acidic.

Ion concentration magnitude comparison: Ions that do not hydrolyze have the largest concentration, and in this example, Cl- has the largest concentration.

Since hydrolysis occurs only for a portion of the ions, after the hydrolysis equilibrium is reached, the other NH4+ will not continue to hydrolyze.

Therefore, in NH4Cl solution, the ion concentrations are arranged in descending order: C(Cl-)>C(NH4+)>C(H+)>C(OH-).

When you want to judge whether the arrangement is correct or not, you can look at the order of oh- and h+.

For example, in some multiple-choice questions, the question gives a strong base and weak salt, then the OH- concentration must be greater than H+, and some options may be excluded according to this.

In addition, it can be judged by the charge balance. For example, if there is only one solute in a solution, there are 4 ions in the solution, if the largest concentration is anion, then the smallest concentration must be anion, and vice versa.

That's right, to add. If it is a strong acid and alkali salt, this kind of salt does not undergo hydrolysis, and the solution pH = 7 at room temperature, which is neutral; In the case of weak acid and weak alkali salts, double hydrolysis will occur. Double hydrolysis, i.e., cation and anion are both hydrolyzed.

Weak acids and weak alkali salts will promote hydrolysis with each other, and the degree of hydrolysis is often very high, or even completely decomposed.

As for the acidity and alkalinity of the weak acid and alkali salt solution, it depends on the relative strength of the acid and the alkali that form this salt.

For example, NH4F, because the acidity of HF is stronger than that of NH3·H2O, it will be weakly acidic;

However, (NH4)2CO3 is alkaline because the alkalinity of NH3·H2O is stronger than that of H2CO3.

In the case of NH4AC, the solution is almost neutral because NH3·H2O is almost as alkaline as HAC.

The relative strength of a weak acid or base is usually compared with the ionization constant, and the larger the ionization constant, the stronger the relative strength.

Of course, weak acid and weak alkali salts can be hydrolyzed, and the degree of hydrolysis is relatively large, and some are still completely double hydrolysis, that is, the hydrolysis is very thorough. As for the nature of the solution, it depends on the products of hydrolysis, such as bicarbonate and aluminum ions, and after the hydrolysis is completed, the solution may be neutral.

Hydrolysis occurs, also known as double hydrolysis, and can promote each other to increase the degree of hydrolysis.

Some weak acids and alkaline salts can be completely hydrolyzed, such as aluminum carbonate, aluminum metaaluminate, etc., hydrolyzed to form acidic gas and alkaline precipitation, and the hydrolysis reaction is no longer reversible.

There is a mantra "If there is weakness, it will be hydrolyzed, and all weak will be hydrolyzed, and the weaker it is, the more hydrolyzed, and whoever is strong will show its nature".

Therefore, both ions of weak acid and weak alkali salt are hydrolyzed. But it is not necessary to completely double hydrolyze, such as aluminum sulfide and aluminum bicarbonate, so they cannot form a solution in water, but like ammonium acetate, it is not so powerful.

Weak acids and weak alkali salts will undergo double hydrolysis, if the corresponding acid is easy to decompose into gas and detach from the system, and the corresponding alkali is easy to form precipitate from the system, then the hydrolysis is generally carried out thoroughly. As.

Carbonate, sulfur ions, etc. and aluminum ions, divalent iron ions will be completely hydrolyzed, if one of the corresponding acids and alkalis has a large solubility in water, then the hydrolysis is incomplete, and the degree of hydrolysis is not large when sulfurous acid and acetate encounter ammonia ions.

to acetic acid. HAC) and hydrocyanic acid.

HCN) as an example, they are weak acids, the ionization constant of acetic acid.

Order of magnitude 10 -5, hydrocyanic acid ionization constant order of magnitude 10 -10, hydrolysis constant = 10 -14 ionization constant, so acetate hydrolysis constant order of magnitude 10 -9, cyanide ion hydrolysis constant order of magnitude 10 -4

It can be seen that the degree of ionization of acetic acid is greater than the degree of hydrolysis, which is manifested as the ionization of acetic acid, and the ionization of H+, so the mixed solution of acetic acid (HAC) and acetic acid (NaAC) is acidic. However, the degree of hydrolysis of cyanide ions is greater than that of the electric mass blindness, and the hail collapse is the hydrolysis of cyanide ions, and the hydrolysis of OH-, so the mixed solution of hydrocyanic acid (HCN) and hydrocyanate (NaCN) is alkaline.

Acid salts with strong alkali and weak acids are not necessarily more hydrolyzed than ionized, and should be judged according to acidity and alkalinity.

Weakly acidic acid acid, because there are unionized hydrogen ions, so it will be both quietly ionized and hydrolyzed, and the ionization produces hydrogen ions to make the solution bright and acidic, and hydrolysis is just the opposite. Therefore, the acidity and alkalinity of the solution is the strength comparison of the two. Sodium bicarbonate is alkaline, so the hydrolysis is greater than ionization, and sodium dihydrogen phosphate is acidic, so the ionization is greater than the hydrolysis of the key Wang.

In fact, this example is inappropriate, phosphoric acid is a strong acid, sodium dihydrogen phosphate can be regarded as a positive salt, but it does have acidic acid salts.