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Chapter 5: Introduction to Reactions in Aqueous Solutions
5.1 The Nature of Aqueous Solutions
Aqueous solutions are mixtures where water acts as the solvent. Water's unique properties make it an excellent medium for dissolving a wide variety of substances, many of which dissociate into ions.
Water is inexpensive and can dissolve a vast number of substances.
Aqueous solutions are found everywhere, including in seawater and living systems.
Electrolytes and Non-Electrolytes
Strong electrolyte: Completely ionizes in water, resulting in good electrical conduction.
Weak electrolyte: Only partially ionizes, resulting in fair electrical conduction.
Non-electrolyte: Does not dissociate into ions, resulting in poor electrical conduction.
Conduction of Electricity in Solution
When electrodes are placed in an aqueous solution and connected to an electricity source, ions migrate to the electrodes, carrying electric charge through the solution:
Cations (positive ions) move toward the cathode (negative electrode).
Anions (negative ions) move toward the anode (positive electrode).
Classification of Solutes
Solutes are classified based on their ability to provide ions in water:
If a solute does not provide ions, it is a nonelectrolyte.
If a solute provides ions, it is an electrolyte:
Strong electrolyte: Completely ionized.
Weak electrolyte: Partially ionized.
Examples of Electrolytes
Non-electrolyte: (methanol in water)
Strong electrolyte:
Weak electrolyte:
Dissociation and Ionization
Dissociation refers to the separation of an ionic compound into its constituent ions when dissolved in water. Ionization refers to the formation of ions from a molecular compound as it reacts with water.
Notation for Representing Concentrations
For ionic compounds:
In a 0.0050 M solution:
(all dissolved)
For molecular compounds, ions are generated by reaction with water, e.g.:
Or simplified:
5.2 Precipitation Reactions
Precipitation reactions occur when soluble ions in aqueous solution combine to form an insoluble compound, called a precipitate.
Example: Test for chloride ion in water:
Net Ionic Equations
Whole formula equation:
Ionic equation:
Net ionic equation:
Spectator ions are ions that do not participate in the actual chemical change.
Predicting Precipitation Reactions
To predict whether a precipitate will form, use solubility guidelines for common ionic solids.
Rule | Description |
|---|---|
1 | Salts of group 1 cations (with some exceptions for Li+) and the NH4+ cation are soluble. |
2 | Nitrates, acetates, and perchlorates are soluble. |
3 | Salts of silver, lead, and mercury(I) are insoluble. |
4 | Chlorides, bromides, and iodides are soluble. |
5 | Carbonates, phosphates, sulfides, oxides, and hydroxides are insoluble (sulfides of group 2 cations and hydroxides of Ca2+, Sr2+, and Ba2+ are slightly soluble). |
6 | Sulfates are soluble except for those of calcium, strontium, and barium. |
Note: The lower-numbered guideline takes precedence in cases of conflict.
Example of Precipitation Reaction
Net ionic equation:
Practice Example
Indicate whether a precipitate forms by completing each equation as a net ionic equation. If no reaction occurs, so state.
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Additional info: Students should refer to the solubility guidelines to determine if a precipitate forms in each case.
