Acids and bases

After reading the article, you will be able to separate substances into salts, acids and bases. The article describes what is The pH of the solution, what common properties acids and bases have.

In simple terms, acid is everything with H, and base is c OH. but! Not always. To distinguish acid from base necessary... remember them! I'm sorry.

At the moment , they use the definitions of the theory created by S. Arrhenius and V. Ostwald. Substances that have similar properties in reactions are divided into classes for convenience use in practice, for example,

Acid

Dissociation is the process of disintegration of a substance into ions in solutions or melts. For example, the dissociation of hydrochloric acid is the decay HCl on H⁺ and Cl^-. From the standpoint of the Arrhenius theory, acids are substances that dissociate in solutions with the formation of hydrogen cations. Acids consist of hydrogen ions and acid residue.

General properties of acids

• During dissociation, hydrogen ions form
• Reacts with some metals, such as zinc, releasing hydrogen
• When reacting with metallic carbonates (for example, calcium carbonate), carbon dioxide (CO₂) is released as a gas
• Change the color of pH indicators

Acid in Lewis theory

An acid is a compound or ion capable of reacting with a Lewis base to form a new substance by covalent bond formation (electron exchange).

Acid in the Bernstved-Lowry theory

An acid is a compound or ion capable of giving a proton to a base or a corresponding chemical.

For example
H₂O, H₃O⁺, CH₃CO₂H, H₂SO₄, HSO₄⁻, HCl, CH₃OH, NH₃

Base

General properties of bases

• Soapy to the touch
• When the base reacts with many cations, a precipitate is formed
• Change the color of the pH indicator

The basis in Lewis theory

A compound or ion capable of giving away a pair of electrons and thus forming a covalent bond with Lewis acid.

The basis in the Bernstved-Lowry theory

A compound or ion capable of receiving a proton from an acid or a corresponding chemical.

For example
OH⁻, H₂O, CH₃CO₂⁻, HSO₄⁻, SO₄²⁻, Cl⁻

Dissociation of water

Dissociation is the breakdown of a substance into its constituent molecules. The properties of the acid or base depend on the equilibrium that is present in the water:

H₂O +H₂O↔H₃O⁺_(r-r) + OH^-_(r-r)
K_c = [H₃O⁺][OH^-]/[H₂O]²
The water equilibrium constant at t=25: K_c = 1.83-6, the following also holds equality: [H₃O⁺][OH^-] = 10^-14, which is called a constant dissociation of water. For pure water [H₃O⁺] = [OH^-] = 10^-7, from where -lg[H₃O] = 7.0.

This value (-lg[h₃O]) is called the pH potential of hydrogen. If the pH is 7, then the substance it has acidic properties, if the pH is 7, then the substance has basic properties.

Methods for determining pH

Instrumental method

A special device pH meter is a device that transforms the concentration of protons in a solution into an electric one the signal.

Indicators

A substance that changes color in a certain range of pH values depending on the acidity of the solution, using several indicators, you can achieve a fairly accurate result.

Classification of acids and bases

Acids

Strong acids

Such acids that completely dissociate in water, producing hydrogen cations H⁺ and anions. An example of a strong acid is hydrochloric acid HCl:

HCl_(r-r) + H₂O_(w) →H₃O⁺_(p-p) + Cl^-_(p-r)

Examples of strong acids: HCl, HBr, HF, HNO₃, H₂SO₄, HClO₄

Weak acids

They are only partially soluble in water, for example, HF:

HF_(r-r) + H2O_(w) → H3O⁺_(r-r) +F^-_(r-r) - in such a reaction, more than 90% of the acid does not dissociate:
[H₃O⁺]=[F^-] < 0.01M for a substance of 0.1M

Strong and weak acid can be distinguished by measuring the conductivity of solutions: the conductivity depends on the number of ions, the stronger the acid, the more dissociated it is, so the stronger the acid, the higher the conductivity.

Bases

Bases are substances containing a hydroxyl group (OH^-) releasing ions OH^- in an aqueous solution.

Strong bases

Strong bases completely dissociate in water:

NaOH_(r-r) + H₂O↔NH₄

The strong bases include metal hydroxides of the first (alkalines, alkali metals) and the second (alkalinoterrenes, alkaline earth metals) groups.

Weak bases

In a reversible reaction in the presence of water, it forms OH^- ions:

NH_{3 (r-r)} + H₂O↔NH⁺_{4 (r-r)} + OH^-_(r-r)

Most weak bases are anions:

F^-_(r-r) + H₂O↔ HF_(r-r) + OH^-_(r-r)

Acid-base reactions

Strong acid and strong base

Such a reaction is called neutralization: when the amount of reagents is sufficient for complete dissociation of the acid and the base, the resulting solution will be neutral.

Example:
H₃O⁺ + OH^- ↔ 2H₂O

Weak base and weak acid

General type of reaction:
Weak base_(r-r) +H₂O↔ Weak acid_(r-r)+ OH^-_(r-r)

Strong base and weak acid

The base completely dissociates, the acid dissociates partially, the resulting solution has weak properties grounds:

HX_(r-r) + OH^-_(r-r) ↔ H₂O + X^-_(r-r)

Strong acid and weak base

The acid completely dissociates, the base does not dissociate completely:

NH_{3 (r-r)} + H⁺ ↔ NH₄

Salt

A salt is an ionic compound formed by a cation other than H⁺ and an anion other than O^2-. In a weak aqueous solution, the salts completely dissociate.

To determine the acid-base properties of the salt solution, it is necessary to determine which ions are present in solution and consider their properties:

Neutral ions formed from strong acids and bases do not affect the pH: they do not give off ions of either H⁺ neither OH^- in water. For example, Cl^-, NO^-₃, SO^2-₄, Li⁺, Na⁺, K⁺.

Anions formed from weak acids exhibit alkaline properties (F^-, CH₃COO^-, CO^2-₃), there are no cations with alkaline properties.

All cations except metals of the first and second groups have acidic properties.

Buffer solution

Solutions that maintain the pH level when adding a small amount of strong acid or strong the bases mainly consist of:

• A mixture of a weak acid, a corresponding salt and a weak base
• Weak base, corresponding salt and strong acid

To prepare a buffer solution of a certain acidity, it is necessary to mix a weak acid or base with the appropriate salt, it is necessary to take into account:

• The pH interval in which the buffer solution will be effective
• Solution capacity is the amount of strong acid or strong base that can be added without affecting on the pH of the solution
• There should be no undesirable reactions that can change the composition of the solution