Problem # 25 Salts & Electrolytes[Solved]

1. What’s a salt and how is it formed. 2. What is an Electrolyte, and what is a Non-electrolyte? Illustrate by examples. 3. What’s the difference between a Strong Electrolyte and a Weak Electrolyte? Illustrate by examples. 4. Are Salts strong or weak Electrolytes? Justify your answer.

1. Salt is a compound formed generally by a reaction of an acid with a base; it is an ionic compound with a metal cation from the base and an anion from the acid. A salt can also be formed by a metal that reacts with an acid.

HCl(aq)  +  NaOH(aq) →  NaCl(aq)  salt

Zn(s)  +  2HCl(aq) →  H₂(g)  +  ZnCl₂(aq) salt

a.  Neutral, acidic, basic properties of aqueous solutions of salts

According to the Arrhenius theory of acid and base, the acid and basic properties are due to the presence of H⁺ and OH^-respectively. From that concept, aqueous solutions of salts are expected to be neutral because the only H⁺ and OH^- ions present should be from the autodissociation of water molecules (see Problem 24). But this is not exactly what happens.

- Salts from strong acids and strong bases

Aqueous solutions of salts from strong acids and strong bases, e.g. NaCl, KNO₃, are neutral.

This is explained as follows: When such a salt as NaCl dissolves in water, they dissociate into ions and we have the following situation:

NaCl(aq)  +   H₂O(l)  →  Na⁺(aq)  +  Cl^-(aq)

H₂O(l)  =  H⁺(aq)  +  OH^-(aq), or [H⁺] = [OH^-]

Since Na⁺(aq) ion cannot combine with OH^-(aq) ion to form the strong base NaOH(aq), and Cl^-(aq) and H⁺(aq) cannot combine to form the strong acid HCl(aq); therefore the dissociation equilibrium of water is not changed and the pH of the solution is: pH=7, i.e. neutral solution

- Salts from strong acids and weak bases, and from weak acids and strong bases

Aqueous solutions of those salts, e.g, NH₄Cl, CH₃COONa, are not neutral and this is explained as follows:

NH₄Cl(aq)  +  H₂O(l)  →  NH₄⁺(aq)  +  Cl^-(aq)

In the presence of H⁺ and OH^-ions from the dissociation of water; H⁺ ion cannot combine with Cl^-(aq) to form HCl(aq), a strong acid; but NH₄⁺ ion can combine with OH^- to form NH₄OH, a weak base:

NH₄⁺(aq)  +  OH^-(aq) = NH₄OH(aq)

This reaction affects the dissociation equilibrium of water by taking OH^- ions; this results in inequality of the concentrations of H⁺ and OH^- ions: [H⁺] > [OH^-] or [H⁺] > 10^-7M and: pH < 7, i.e. acidic solution

This phenomenon is called salt hydrolysis, a reaction that changes the pH of water. Since the solution becomes acidic, this hydrolysis is called “acidic hydrolysis”.

But if we consider CH₃COONa, a salt from a weak acid and a strong base:

CH₃COONa(aq)  +  H₂O(l)  →  CH₃COO^-(aq)  +  Na⁺(aq)

In the presence of H⁺ and OH^-ions from the dissociation of water; H+ recombines with CH₃COO^-(aq) to form CH₃COOH(aq), a weak acid; but OH^- and Na⁺ cannot recombine to form NaOH(aq), a strong base. In this case the inequality of the concentrations of H+ and OH- ions becomes: [H⁺] < [OH^-] or [H⁺] < 10^-7M, and: pH > 7, i.e. basic solution. This is a “basic hydrolysis”

- Salts where the metal cation is highly charger (ex: M³⁺ or more)

The hydrolysis reaction happens also when metal cation of the salt is highly charged. When such a salt dissolves in water, it dissociates into its ions and those ions are surrounded by water molecules:

AlCl₃(s)  +  H₂O(l)  → [Al(H₂O)₆]³⁺  +  3Cl^-(aq)

Due to the strong attraction between the positive charge of Al³⁺ ion and the water molecules, some H-O-H bonds of water molecules break to give:

[Al(H₂O)₆]³⁺→  [AlOH(H₂O)₅]²⁺  +  H⁺(aq),  an acidic solution.

This is also an “acidic hydrolysis”

b.  Acid and Basic salts

Acidic and basic salts are salts that have still dissociable H⁺ and OH^- ions respectively.

An acid salt is formed during a partial neutralization of an acid with many acidic hydrogens, also called polybasic acids:

H₂CO3(aq)  +  NaOH(aq)  →   NaHCO₃(aq)  +  H₂O(l)

The salt NaHCO₃ is an acid salt:

NaHCO₃(s)  +  H₂O(l)  → Na⁺(aq)  +  HCO₃^-(aq)

HCO₃^-(aq)  +  H²O(l)  =  CO₃⁼(aq)  +  H⁺(aq),   this makes the solution acidic

A basic salt is formed during a partial neutralization of a base where some of the oxide or hydroxide ions are not neutralized:

Ba(OH)₂(s)  + HCl(aq)  →  BaOHCl(aq)  +  H₂O(l)

The salt BaOHCl is a basic salt:

BaOHCl(s)  +  H₂O(l)  →  Ba²⁺(aq)  +  Cl^-(aq)  +  OH^-(aq),  this makes the solution basic

2. Electrolyte is a liquid that conducts electricity due the presence of mobile ions. Any chemical substance that dissociates into its ions, when dissolved in water or any solvent, forms an electrolytic solution.

Arrhenius acids and bases are electrolytes because they dissociate into ions, H⁺ or OH^-, when dissolved in water.

Many salts form electrolytic solutions when dissolved in water due to their dissociation into their ions.

Molten salts are electrolytes because they conduct electricity due the presence of mobile ions.

Ex: NaCl(s) + Water → Na⁺(aq) + Cl^-(aq)

CH₃COOH(l) + Water  ↔  CH₃COO^-(aq) + H⁺(aq)

A Non-electrolyte doesn’t dissociate into ions when dissolved in a solvent. A non-electrolytic solution doesn’t conduct electricity.

Ex: CH₃CH₂OH(l) + Water  = CH₃CH₂OH + Water  (absence of mobile ions).

Metals are not electrolytes; they conduct electricity by electrons, not by ions.

Solid salts are generally non-electrolytes because their ions are not mobile.

3. A Strong electrolyte dissociates completely into its ions when dissolved in water.

Ex: NaOH(aq) + Water >>> 100% >>>> Na⁺(aq) + OH^-(aq)

HCl(g) + Water >>>>>100% >>>>H⁺(aq) + Cl^-(aq)

A Weak electrolyte dissociates partially into its ions when dissolved in water.

Ex: CH₃COOH(aq) + Water <<< partial dissociation >>> CH₃COO^-(aq) + H⁺(aq)

NH₄OH(aq) + Water <<<<< partial dissociation >>>>> NH₄⁺(aq) + OH^-(aq)

Due to its very weak auto-dissociation into its ions, [H⁺] = [OH^-] = 10^-7M, pure or distilled water is considered as non-electrolyte.

Mineral water or the water supplied in our homes by the city or other suppliers is not chemically pure, it generally contains mineral salts. Those electrolytes are needed by our bodies.

4. Many Salts are strong electrolytes because those that dissolve in water do it by dissociation into ions. But we may be careful with such a statement.

According to Chemistry/LibreTexts, many ionic compounds or salts of Transition Metals or Alkaline-Earth Metals are not strong electrolytes. When dissolved in water, some of the ions separate, but some stay together.

Ex: K₂SO₄(s) + water >>> K⁺(aq) (70%) + KSO₄^-(aq)(30%) + SO₄^2-(aq) (for a 0.36 M solution).

CdI₂(s) + water >>> Cd²⁺(aq) (2%) + CdI⁺(aq) (22%) + CdI₂(aq) (76%) + I^-(aq) (for a 0.5M solution)..

As we can see, these salts are electrolytes (they do produce ions) but if you do calculations assuming that they separate completely into their ions, you might get very wrong answers! They are not strong electrolytes. In general, the lower the concentration and the lower the charges on the ions, the “stronger” the electrolytes will be and vice-versa.

Alkali metal salts other than Lithium are usually strong electrolytes especially when the anion also has a small charge, such as Na⁺Cl^-, and dilute solutions (< 0.1M). Alkali-Earth metal compounds are weaker electrolytes, and other metals are even weaker still.