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A substance that dissolves in water to give an electrically conducting solution is called an electrolyte. A substance that dissolves in water to give no conducting or very poorly conducting solutions is called a nonelectrolyte.
When electrolytes dissolve in water they produce ions, but they do so to varying extents. A strong electrolyte is an electrolyte that exists in solution almost entirely ions. A weak electrolyte is an electrolyte that dissolves in water to give equilibrium between a molecular substance and a small concentration of ions.
According to Svante Arrhenius concept:
Acid is any substance that, when dissolved in water, increase the concentration of hydrogen ion H+.
Base is any substance that, when dissolved in water, increase the concentration of hydroxide ion OH–.
NaOH → Na+ + OH–
HCl → H+ + Cl–
According to Johannes N. Brönsted and Thomas M. Lowry concept:
Acid is the species (molecule or ion) that donates a proton to another species in a proton-transfer reaction.
Base is the species (molecule or ion) that accepts a proton in a proton-transfer reaction.
HCl + NH3 → NH4Cl
acid base
NH3 + H2O → NH4+ + OH–
base acid acid base
A conjugate acid-base pair consists of two species in an acid-base equilibrium, one acid and one base, which differ by the gain or loss of a proton. The acid in such a pair is called the conjugate acid of the base, whereas the base is the conjugate base is the conjugate base of the acid.
According to G.N.Lewis concept:
Lewis acid is a species that can form a covalent bond by accepting an electron pair from another species.
Lewis base is a species that can form a covalent bond by donating an electron pair to another species.
H+ +:NH3 → NH4+
electron-pair electron-pair
acceptor donor
Lewis acid Lewis base
The Lewis and the Brönsted-Lowry concepts are simply different ways of looking at certain chemical reactions. The Lewis concept could be generalized to include many other reactions, as well as proton-transfer reactions.
Acids and bases are classified as strong or weak.
Strong acids are acids that ionize completely in water (that is, they react completely to give ions).
Weak acids are acids that are only partly ionized as the result of equilibrium reaction with water.
Strong bases are bases that are present in aqueous solution entirely as ions, one of which is OH–.
Weak bases are bases that are only partly ionized as the result of equilibrium reactions with water.
Acid and base with water produce hydrogen ion or hydroxide ion (relatively) and its conjugated ions. The process is called electrolyte ionization or electrolyte dissociation.
The weak electrolyte (acid and base) ionizes or dissociates to a small extent in water (about 1% or less, depending on concentration of electrolyte). For the weak electrolyte (acid or base) the concentration of ions in solution are determined from the acid ionization (or dissociation) constant (Ka) or the base ionization (or dissociation) constant (Kb), which is the equilibrium constant from the ionization of a weak electrolyte.
Value of ionization constants depends on:
1) nature of solvent,
2) nature of electrolyte,
3) temperature.
The degree of ionization (α) of a weak electrolyte is the fraction of molecules that react with water to give ions.
For very small concentration of electrolyte α have very small value and percent of ionization can be shown approximately on Ostwald’s dilution rule:
Sometimes electrolytes additionally classified on three types: weak (α<30%), medium (α=30-70%) and strong (α>70%). For strong electrolytes also distinguished seeming (100%) and true (real) (<100%) concentration.
ISWT 14. Properties of Ionic Compounds in Aqueous Solution
The properties of ionic compounds in solution are actually the properties of the individual ions themselves. These compounds are called strong electrolytes because their solutions conduct electricity well. For example, an aqueous solution of sodium chloride consists essentially of sodium ions and chloride ions in water. A similar solution of calcium chloride consists of calcium ions and chloride ions in water. If either solution is treated with a solution containing silver ions, the chloride ions will form silver chloride, which is insoluble. The chloride ions act independently of the cation that is also present, regardless of whether it is sodium ion, calcium ion, or any other ion. Because the properties of the compound are the properties of the component ions, we need to learn to write equations for only the ions that react, omitting the ions that remain unchanged throughout the reaction.
Strong acids, strong bases, and salts all provide ions in solution. They are all strong electrolytes, but the process by which these types of compounds form ions in solution differs. When they are pure, strong acids are covalent compounds, but they undergo a chemical reaction with water to form ions in solution. Salts and strong bases are ionic even when they are pure, and their interaction with water is more a physical process than a chemical reaction.
The solution process for them is called dissociation because the ions dissociate from each other; that is, they get out of each other’s sphere of influence and are able to move relatively independently of ions of the opposite charge.
Weak acids and bases ionize only slightly in aqueous solution. Because their solutions conduct electricity poorly, they are called weak electrolytes. Compounds whose solutions do not conduct electricity at all are called non-electrolytes.
A strong electrolyte in aqueous solution may be represented as separate ions because the ions of each type are free to move about independently of the ions of the other type. However, an ionic solid that is not dissolved in water is not written as separate ions; the oppositely charged ions in the solid lattice of an ionic compound are not independent of each other.
Thus, compounds must be both soluble and ionic to be written in the form of their separate ions. A listing of water-soluble compounds was given in Table 8.3. In addition to the compounds listed there, all strong acids are water soluble. In summary strong electrolytes—compounds that dissociate or ionize extensively in aqueous solution—include the following:
1. All soluble metal hydroxides
2. All salts (other compounds containing metal or ammonium ions) that are soluble
3. Strong acids (HCl, HBr, HI,)
All other compounds (for example, gases, other covalent compounds, and all solids) either contain no ions or have ions that are affected by the presence of the other ions. These weak electrolytes, nonelectrolytes, or solids (ionic or not) are written using their regular formulas.
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