Chapter 4 Part 1 “Aqueous Solutions” Honors Chemistry 2

Chemists have solutions.  A solution is a homogeneous mixture. solution  A solution consists of a solvent, the substance in which something is dissolved, and one or more solutes the substance(s) dissolved in the solvent. solventsolutessolventsolutes  Aqueous solutions are those in which water is the solvent.

You -lyte up my world!  Solutes cans be electrolytes or nonelectrolytes electrolytes nonelectrolyteselectrolytes nonelectrolytes –Solutions of electrolytes such as NaCl and HNO 3 contain ions that conduct electricity. –Solutions of nonelectrolytes such as sucrose (C 12 H 22 O 11 ) and methanol (CH 3 OH) do not form ions in solution and do not conduct electricity.

Strong or weak  An electrolyte may be either a strong electrolyte or a weak electrolyte. strong electrolyteweak electrolytestrong electrolyteweak electrolyte – A strong electrolyte exists in solution completely (or almost completely) as ions, while a weak electrolyte produces only a small concentration of ions when it dissolves in solution.

Electrolytes

Ionic dissociation  Ionic compounds that are soluble in water dissociate completely and exist in solution entirely as ions. dissociate –Soluble ionic compounds are strong electrolytes.  Molecular compounds such as sugar and alcohol are nonelectrolytes. –They have no tendency to come apart, and they exist in solution entirely as aqueous molecules.  Some molecular compounds, most notably acids and weak bases, are electrolytes.

Molecular ionization  The acids HCl, HBr, HI, HNO 3, HClO 3, HClO 4, and H 2 SO 4 are molecular compounds that ionize in aqueous solution and exist completely as ions. ionize –For example, hydrogen chloride gas dissolves in water and ionizes in solution to give aqueous hydrogen ion and aqueous chloride ion.  The seven acids listed above are strong electrolytes. –They are called strong acids.  The word strong denotes complete ionization. (Strong bases are ionic and are also strong electrolytes.)

Weak, weak, weak  Weak acids and weak bases are molecular compounds that are weak electrolytes.  Acetic acid is a weak acid. –Although it ionizes in water, the reverse process occurs more readily. –At any given time most of the acetic acid exists as aqueous molecules in solution.  Weak acids are weak electrolytes.  To show the equilibrium between the molecular acid and its ionized form, use a double arrow in the equation.

Follow the arrows.  Strong acids/bases completely change into ions so a single arrow is used  Weak acids/bases tend to return to their original form rather than stay as ions. A double arrow is used.

Let’s see what falls out.  A precipitation reaction is one that occurs in solution and results in the formation of an insoluble product. precipitation reactionprecipitation reaction –For example, an aqueous solution of the soluble ionic compound, sodium sulfate, can be mixed with an aqueous solution of the soluble ionic compound, barium hydroxide. –The result is the formation of a precipitate, the insoluble ionic compound barium sulfate. precipitate

 Note that only one of the products of the reaction is a solid. –Sodium hydroxide is soluble.  Many combinations of such solutions will not result in a precipitation because they produce no insoluble product.  Whether or not an ionic product of a reaction in solution will precipitate from the solution can be predicted using a set of solubility guidelines.

Let’s get to it!  Having learned that substances such as sodium sulfate, barium hydroxide, and sodium hydroxide are strong electrolytes and exist entirely as ions in solution, we are equipped to write these chemical equations in a way that better represents what actually happens in solution

 The molecular equation above shows the complete chemical formulas for the reactants and products.  We know, however, that several of the species in the equation dissociate completely in solution.  We convert the equation to a complete ionic equation by identifying the strong electrolytes and representing them as separated ions. complete ionic equationcomplete ionic equation

 The ionic equation reveals that two of the species in solution (sodium ion and hydroxide ion) do not undergo any change in the course of the reaction.  Ions that are present but play no role in the reaction are called spectator ions.  Eliminating the spectator ions from the equation gives the net ionic reaction net ionic reactionnet ionic reaction

 The net ionic equation for the combination of aqueous sodium sulfate and aqueous barium hydroxide is the same as the net ionic equation for any combination of a soluble sulfate and a soluble barium compound.

Concentration  An important feature of a solution is its concentration. concentration  A solution containing a large amount of solute is said to be "concentrated," while a solution containing a relatively small amount of solute is said to be "dilute."  These are simply relative terms, though, and a more quantitative expression of a solution's concentration often is necessary.

Molarity  The most commonly used expression of concentration is molarity. Symbolized with a capital M, molarity is the ratio of the number of moles of solute to the number of liters of solution. molarity

Solution Stoichiometry  Because electrolytes ionize or dissociate in water, it is necessary to consider stoichiometry when determining the concentrations of individual ions in an aqueous solution. –Sodium carbonate, for example, dissociates in solution to give sodium ions and carbonate ion.

 Because each unit of sodium carbonate yields two units of sodium ion and one unit of carbonate ion, the concentration of sodium ions and carbonate ions will not be equal. –If we were to mix up a 1 L solution containing 0.1 mole of sodium carbonate, the concentration of sodium carbonate would be 0.1 M. –The concentration of carbonate ion would be 0.1 M also. –The concentration of sodium ion would be twice that much, or 0.2 M.