Chapter 4 Part 1 “Aqueous Chapter 4 Part 1 “Aqueous Solutions”Solutions”

Honors Chemistry 2Honors Chemistry 2

Chemists have solutions.Chemists have solutions.

A A solutionsolution is a homogeneous mixture. is a homogeneous mixture. A solution consists of a A solution consists of a solventsolvent, the , the

substance in which something is dissolved, substance in which something is dissolved, and one or more and one or more solutessolutes the substance(s) the substance(s) dissolved in the solvent. dissolved in the solvent.

Aqueous solutionsAqueous solutions are those in which are those in which water is the solvent.water is the solvent.

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You -lyte up my world!You -lyte up my world!

Solutes cans be Solutes cans be electrolyteselectrolytes or or nonelectrolytesnonelectrolytes – Solutions of electrolytes such as NaCl and Solutions of electrolytes such as NaCl and

HNOHNO33 contain ions that conduct electricity. contain ions that conduct electricity.– Solutions of nonelectrolytes such as sucrose Solutions of nonelectrolytes such as sucrose

(C(C1212HH2222OO1111) and methanol (CH) and methanol (CH33OH) do not form OH) do not form ions in solution and do not conduct electricity.ions in solution and do not conduct electricity.

Strong or weakStrong or weak

An electrolyte may be either a An electrolyte may be either a strong electrolytestrong electrolyte or a or a weak electrolyteweak electrolyte..– A strong electrolyte exists in solution A strong electrolyte exists in solution

completely (or almost completely) as ions, while completely (or almost completely) as ions, while a weak electrolyte produces only a small a weak electrolyte produces only a small concentration of ions when it dissolves in concentration of ions when it dissolves in solution.solution.

ElectrolytesElectrolytes

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Ionic dissociationIonic dissociation

Ionic compounds that are soluble in water Ionic compounds that are soluble in water dissociatedissociate completely and exist in solution entirely completely and exist in solution entirely as ions. as ions. – Soluble ionic compounds are strong Soluble ionic compounds are strong

electrolytes.electrolytes. Molecular compounds such as sugar and alcohol Molecular compounds such as sugar and alcohol

are nonelectrolytes. are nonelectrolytes. – They have no tendency to come apart, and they They have no tendency to come apart, and they

exist in solution entirely as aqueous molecules. exist in solution entirely as aqueous molecules. Some molecular compounds, most notably acids Some molecular compounds, most notably acids

and weak bases, are electrolytes.and weak bases, are electrolytes.

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Molecular ionizationMolecular ionization

The acids HCl, HBr, HI, HNOThe acids HCl, HBr, HI, HNO33, HClO, HClO33, HClO, HClO44, and , and HH22SOSO44 are molecular compounds that are molecular compounds that ionizeionize in in aqueous solution and exist completely as ions. aqueous solution and exist completely as ions. – For example, hydrogen chloride gas dissolves For example, hydrogen chloride gas dissolves

in water and ionizes in solution to give aqueous in water and ionizes in solution to give aqueous hydrogen ion and aqueous chloride ion.hydrogen ion and aqueous chloride ion.

The seven acids listed above are strong The seven acids listed above are strong electrolytes. electrolytes. – They are called strong acids. They are called strong acids.

The word The word strongstrong denotes complete ionization. denotes complete ionization. (Strong bases are ionic and are also strong (Strong bases are ionic and are also strong electrolytes.)electrolytes.)

Weak, weak, weakWeak, weak, weak

Weak acids and weak bases are molecular Weak acids and weak bases are molecular compounds that are weak electrolytes. compounds that are weak electrolytes.

Acetic acid is a weak acid. Acetic acid is a weak acid. – Although it ionizes in water, the reverse process Although it ionizes in water, the reverse process

occurs more readily. occurs more readily. – At any given time most of the acetic acid exists At any given time most of the acetic acid exists

as aqueous molecules in solution.as aqueous molecules in solution. Weak acids are weak electrolytes. Weak acids are weak electrolytes. To show the equilibrium between the molecular To show the equilibrium between the molecular

acid and its ionized form, use a double arrow in acid and its ionized form, use a double arrow in the equation.the equation.

Follow the arrows.Follow the arrows.

Strong acids/bases Strong acids/bases completely change into completely change into ions so a single arrow ions so a single arrow is usedis used

Weak acids/bases Weak acids/bases tend to return to their tend to return to their original form rather original form rather than stay as ions. A than stay as ions. A double arrow is used.double arrow is used.

Let’s see what falls out.Let’s see what falls out.

A A precipitation reactionprecipitation reaction is one that occurs in is one that occurs in solution and results in the formation of an solution and results in the formation of an insoluble product. insoluble product. – For example, an aqueous solution of the soluble For example, an aqueous solution of the soluble

ionic compound, sodium sulfate, can be mixed ionic compound, sodium sulfate, can be mixed with an aqueous solution of the soluble ionic with an aqueous solution of the soluble ionic compound, barium hydroxide.compound, barium hydroxide.

– The result is the formation of a The result is the formation of a precipitateprecipitate, the , the insoluble ionic compound barium sulfate. insoluble ionic compound barium sulfate.

Note that only one of the products of the Note that only one of the products of the reaction is a solid. reaction is a solid. – Sodium hydroxide is soluble. Sodium hydroxide is soluble.

Many combinations of such solutions will not Many combinations of such solutions will not result in a precipitation because they result in a precipitation because they produce no insoluble product. produce no insoluble product.

Whether or not an ionic product of a reaction Whether or not an ionic product of a reaction in solution will precipitate from the solution in solution will precipitate from the solution can be predicted using a set of solubility can be predicted using a set of solubility guidelines.guidelines.

Let’s get to it!Let’s get to it!

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

The molecular equation above shows the The molecular equation above shows the complete chemical formulas for the complete chemical formulas for the reactants and products. reactants and products.

We know, however, that several of the We know, however, that several of the species in the equation dissociate species in the equation dissociate completely in solution. completely in solution.

We convert the equation to a We convert the equation to a complete ionic equationcomplete ionic equation by identifying the by identifying the strong electrolytes and representing them as strong electrolytes and representing them as separated ions. separated ions.

The ionic equation reveals that two of the The ionic equation reveals that two of the species in solution (sodium ion and species in solution (sodium ion and hydroxide ion) do not undergo any change hydroxide ion) do not undergo any change in the course of the reaction. in the course of the reaction.

Ions that are present but play no role in the Ions that are present but play no role in the reaction are called spectator ions. reaction are called spectator ions.

Eliminating the spectator ions from the Eliminating the spectator ions from the equation gives the equation gives the net ionic reactionnet ionic reaction

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

ConcentrationConcentration

An important feature of a solution is its An important feature of a solution is its concentrationconcentration. .

A solution containing a large amount of A solution containing a large amount of solute is said to be "concentrated," while a solute is said to be "concentrated," while a solution containing a relatively small amount solution containing a relatively small amount of solute is said to be "dilute." of solute is said to be "dilute."

These are simply relative terms, though, and These are simply relative terms, though, and a more quantitative expression of a a more quantitative expression of a solution's concentration often is necessary.solution's concentration often is necessary.

MolarityMolarity

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

Solution StoichiometrySolution Stoichiometry

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

solution to give sodium ions and carbonate ion. solution to give sodium ions and carbonate ion.

Because each unit of sodium carbonate yields two Because each unit of sodium carbonate yields two units of sodium ion and one unit of carbonate ion, units of sodium ion and one unit of carbonate ion, the concentration of sodium ions and carbonate the concentration of sodium ions and carbonate ions will not be equal.ions will not be equal.– If we were to mix up a 1 L solution containing If we were to mix up a 1 L solution containing

0.1 mole of sodium carbonate, the 0.1 mole of sodium carbonate, the concentration of sodium carbonate would be 0.1 concentration of sodium carbonate would be 0.1 MM. .

– The concentration of carbonate ion would be The concentration of carbonate ion would be 0.1 0.1 MM also. also.

– The concentration of sodium ion would be twice The concentration of sodium ion would be twice that much, or 0.2 that much, or 0.2 MM..

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