뒤로Chapter 4: Reactions in Aqueous Solution – General Chemistry Study Notes
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Reactions in Aqueous Solution
Solutions and Aqueous Solutions
Solutions are homogeneous mixtures composed of two or more pure substances. The substance present in the greatest amount is called the solvent, while the other substances are solutes. When water acts as the solvent, the mixture is referred to as an aqueous solution.
Solvent: The component in greatest abundance.
Solute: The component(s) present in lesser amounts.
Aqueous solution: A solution where water is the solvent.
How Substances Dissolve in Water
Substances dissolve in water through different mechanisms:
Ionic compounds dissolve by dissociation, where water molecules surround and separate the ions.
Molecular compounds interact with water but typically do not dissociate into ions.
Some molecular substances react with water upon dissolving.
All dissolution involves solvation, the process of solvent molecules surrounding solute particles.
Electrolytes and Nonelectrolytes
Definitions and Properties
An electrolyte is a substance that dissociates into ions when dissolved in water, enabling the solution to conduct electricity. A nonelectrolyte dissolves in water but does not produce ions, so it does not conduct electricity.
Strong electrolyte: Dissociates completely in water.
Weak electrolyte: Dissociates partially in water.
Nonelectrolyte: Does not dissociate in water.
Electrolytic Behavior Table
The table below summarizes the behavior of common soluble ionic and molecular compounds:
Strong Electrolyte | Weak Electrolyte | Nonelectrolyte | |
|---|---|---|---|
Ionic | All | None | None |
Molecular | Strong acids (see Table 4.2) | Weak acids, weak bases | All other compounds |
Solubility of Ionic Compounds
Solubility Rules
Not all ionic compounds are soluble in water. Chemists use solubility rules to predict which combinations of ions will dissolve.
Soluble Ionic Compounds | Important Exceptions |
|---|---|
Compounds containing NO3−, ClO4−, ClO3−, SO42− | Compounds of Ag+, Hg22+, Pb2+ |
Compounds containing Cl−, Br−, I− | Compounds of Ag+, Hg22+, Pb2+ |
Insoluble Ionic Compounds | Important Exceptions |
Compounds containing S2−, CO32−, PO43−, OH− | Compounds of NH4+, alkali metal cations |
Precipitation Reactions
Formation of Precipitates
Precipitation reactions occur when two solutions containing soluble salts are mixed and an insoluble salt (precipitate) forms. The process can be predicted using solubility rules.
Identify ions present in reactants.
Consider possible cation-anion combinations.
Use solubility rules to determine if any combination is insoluble.
Metathesis (Exchange) Reactions
Metathesis reactions involve the exchange of ions between reactant compounds. The general steps for completing and balancing these equations are:
Determine ions present from reactant formulas.
Write product formulas by combining cations and anions.
Check solubility rules for precipitate formation.
Balance the equation.
Types of Equations for Metathesis Reactions
Molecular equation: Shows reactants and products without indicating ionic nature.
Complete ionic equation: Shows all strong electrolytes dissociated into ions.
Net ionic equation: Shows only the ions and molecules directly involved in the reaction.
How to Write a Net Ionic Equation
Write a balanced molecular equation.
Rewrite to show dissociated ions for strong electrolytes.
Identify and cancel spectator ions.
Acids and Bases
Acids
Acids are substances that ionize in aqueous solution to produce H+ ions. Since H+ is a proton, acids are often called proton donors.
Bases
Bases are substances that accept H+ ions or increase the concentration of OH− ions in water. Not all bases contain OH−.
Strong and Weak Acids/Bases
Strong acids: Completely dissociate in water.
Weak acids: Partially dissociate.
Strong bases: Dissociate to metal cations and hydroxide anions.
Weak bases: Partially react to produce hydroxide ions.
Strong Acids | Strong Bases |
|---|---|
Hydrochloric acid, HCl Hydrobromic acid, HBr Hydroiodic acid, HI Chloric acid, HClO3 Perchloric acid, HClO4 Nitric acid, HNO3 Sulfuric acid (first proton), H2SO4 | Group 1A metal hydroxides (LiOH, NaOH, KOH, RbOH, CsOH) Heavy group 2A metal hydroxides (Ca(OH)2, Sr(OH)2, Ba(OH)2) |
Neutralization Reactions
Acid-Base Neutralization
Neutralization reactions occur when an acid reacts with a base, producing water and a salt. These reactions can be written as molecular, complete ionic, or net ionic equations.
Molecular equation:
Complete ionic equation:
Net ionic equation:
Neutralization with Gas Formation
Some neutralization reactions produce gases, such as when carbonates or bicarbonates react with acids:
Antacids: Application of Neutralization
Antacids are commercial products used to neutralize stomach acid. The table below lists common antacids and their active ingredients:
Commercial Name | Acid-Neutralizing Agents |
|---|---|
Alka-Seltzer | NaHCO3 |
Amphojel | Al(OH)3 |
Di-Gel | Mg(OH)2 and CaCO3 |
Milk of Magnesia | Mg(OH)2 |
Maalox | Mg(OH)2 and Al(OH)3 |
Mylanta | Mg(OH)2 and Al(OH)3 |
Rolaids | Mg(OH)2 and CaCO3 |
Tums | CaCO3 |
Oxidation-Reduction (Redox) Reactions
Oxidation and Reduction
Redox reactions involve the transfer of electrons:
Oxidation: Loss of electrons.
Reduction: Gain of electrons.
One process cannot occur without the other.
Oxidation Numbers
Oxidation numbers are assigned to elements in compounds to track electron transfer. The rules are:
Atoms in elemental form: oxidation number = 0.
Monatomic ions: oxidation number = ion charge.
Nonmetals usually have negative oxidation numbers, but can be positive in certain compounds.
Sum of oxidation numbers in a neutral compound = 0; in a polyatomic ion = ion charge.
Displacement Reactions and Activity Series
Displacement reactions occur when ions oxidize an element, replacing it in solution. The activity series ranks metals by their tendency to be oxidized.
Metal | Displacement Reaction |
|---|---|
Lithium | Li(s) → Li+(aq) + e− |
Potassium | K(s) → K+(aq) + e− |
Hydrogen | H2(g) → 2H+(aq) + 2e− |
Gold | Au(s) → Au3+(aq) + 3e− |
... | ... |
Solution Concentration and Molarity
Molarity
Molarity (M) is a measure of the concentration of a solution, defined as the number of moles of solute per liter of solution:
Preparing Solutions
To prepare a solution of known molarity:
Weigh a known mass of solute.
Add solute to a volumetric flask.
Add solvent to the calibration mark.
Dilution
Dilution involves adding solvent to decrease the concentration of a solution without changing the number of moles of solute. The relationship is:
Where and are the molarity and volume of the concentrated solution, and and are those of the diluted solution.
Stoichiometry in Solution Chemistry
Stoichiometry Applied to Solutions
Stoichiometry allows chemists to relate quantities of reactants and products in solution reactions using molarity and volume.
Titration
Analytical Technique
Titration is a method used to determine the concentration of a solute in a solution by reacting it with a solution of known concentration (standard solution). The reaction is complete at the equivalence point, often indicated by a color change (end point).
Titration Calculations
The calculation involves using the volume and molarity of the standard solution to determine the moles of solute, then relating this to the unknown concentration using the balanced equation.
