Aqueous Solutions & Solubility | Chemistry for EmSAT Achieve PDF Download

Learning Objectives

• Definition of electrolytes
• Examples of electrolytes

Electrolytes

When certain substances are dissolved in water, they undergo a process that results in the formation of ions in the solution. These substances belong to a category known as electrolytes. On the other hand, substances that do not produce ions upon dissolution are referred to as nonelectrolytes.

Strong Electrolytes

Strong electrolytes are substances where the process of generating ions upon dissolving is highly efficient, meaning that nearly all of the dissolved compound forms ions.

Weak Electrolytes

Weak electrolytes are substances in which only a small portion of the dissolved compound undergoes the ion-producing process, resulting in a lower concentration of ions in the solution.

To distinguish between strong, weak, and nonelectrolytes, one can measure the electrical conductance of an aqueous solution containing the substance. For a substance to conduct electricity, it must contain charged species that can move freely. For instance, electricity can flow through metallic wires due to the movement of electrons. Solutions can also conduct electricity if they contain dissolved ions, with conductivity increasing as the concentration of ions rises.

By applying a voltage to electrodes immersed in a solution, one can evaluate the relative concentration of dissolved ions. This assessment can be done quantitatively by measuring the electrical current flow or qualitatively by observing the brightness of a light bulb included in the circuit.

Ionic Electrolytes

• Attraction of Water and Ions: Water and other polar molecules are attracted to ions due to an electrostatic attraction known as anion-dipole attraction. This attraction is crucial in the process of dissolving ionic compounds in water.
• Ion-Dipole Attraction: When ionic compounds dissolve in water, the ions separate and disperse uniformly in the solution. This occurs because water molecules surround and solvate the ions, reducing the strong electrostatic forces between them. This phenomenon is termed dissociation, and ionic compounds generally dissociate almost entirely when dissolved, classifying them as strong electrolytes.
• Molecular Level Interaction: Considering the example of adding solid KCl to water, ion-dipole forces attract water molecules to the ions. The water molecules surround individual ions, weakening the interionic forces that hold the ions together. This weakening allows the ions to move into solution as solvated ions, leading to increased disorder in the system. This increased disorder facilitates the dissolution of ionic compounds like KCl, often accompanied by heat absorption.

Solubility and Precipitation

Insolubility of Certain Compounds

• In some cases, crystals are insoluble due to strong electrostatic attractions among ions or weak ion-dipole forces with water molecules.
• Examples include calcium carbonate (limestone), calcium phosphate (found in bones), and iron oxide (rust).

Solubility Rules

When certain aqueous reactants combine, they may or may not produce a solid precipitate. The following guidelines, known as solubility rules, help predict these outcomes:

Soluble Compounds

• All Group I A and NH4 salts are soluble.
• Nitrates, chlorates, perchlorates, and acetates are generally soluble.
• Exceptions exist for sulfates (e.g., CaSO4, BaSO4, SrSO4, PbSO4) and some chlorides, bromides, and iodides (e.g., AgX, Hg2X2, PbX2).

Sparingly Soluble Compounds

• Carbonates and phosphates are generally sparingly soluble.
• Exceptions include certain hydroxides (Ba2+, Sr2+, Ca2+), sulfides (e.g., MgS, CaS, BaS), and oxalates.
• Some electrolytes (e.g., CH3COOAg, Ag2SO4, KClO4) have moderate solubility and may precipitate in concentrated solutions.

Predicting Precipitation

For instance, when mixing cesium bromide and lead (II) nitrate, you can use solubility rules to predict if a precipitate will form.

Solubility Rules in Chemistry

• Double-Replacement Reaction
  • When cesium nitrate and lead (II) bromide react, a double-replacement reaction occurs.
  • Cesium nitrate and lead (II) bromide precipitate out as cesium nitrate is soluble, but lead (II) bromide is insoluble.
  • The balanced net ionic reaction is: Pb2 (aq) + 2Br- (aq) → PbBr2(s)
• Example of Solubility Classification
  • Zn(NO3)2 is soluble because all nitrates are soluble in water.
  • PbBr2 is insoluble due to the exception of lead (II) bromide among soluble bromides.
  • Sr3(PO4)2 is insoluble as all phosphates are generally insoluble.
• Exercise on Solubility Classification
  • Mg(OH)2, KBr, and Pb(NO3)2 need classification as soluble or insoluble compounds.

Summary

• Electrolytes are substances that dissolve in water to produce ions, while nonelectrolytes do not ionize when dissolved.
• Solubility rules can be used to predict the insolubility of certain products in water.

Contributions and Attributions

• Paul Flowers, Klaus Theopold, and Richard Langley, along with contributing authors, have collaborated on the textbook content.
• The textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0.

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