Unit 5 – Aqueous Reactions & Solution Stoichiometry

Properties of Aqueous Solutions

Predicting Products of Chemical Reactions

Concentrations of Solutions

Solution Stoichiometry and Chemical Analysis

Aqueous Solutions

Many chemical reactions take place in an aqueous solution: a solution in which water is the

dissolving medium (aq)

Solution: a homogeneous mixture of two or more substances

Aqueous Solutions

Examples of Aqueous Reactions or Processes

Biological or physiological processes

photosynthesis metabolism of food transportation of oxygen or

nutrients in blood

Rust formation

Acid rain

Aqueous Solutions

Components of SolutionsSolvent:

the dissolver usually present in the greatest

quantity

Solute: the substance that dissolves

Aqueous Solutions

Sodium Chloride/WaterSolute: sodium chlorideSolvent: water

Sugar/waterSolute: sugarSolvent: water

Denatured alcoholSolutes: methyl alcohol, waterSolvent: ethyl alcohol

Aqueous Solutions

NaCl in H2O sucrose in H2O

NaCl sucrose

H2O

Aqueous Solutions

Compare and contrast salt (NaCl) water solution and sucrose (C12H11O22) dissolved in water

Similarities: Differences:

Aqueous Solutions

Salt water solutions are good electrical conductors.

Sucrose (C12H11O22)/water solutions are not good electrical conductors.

– Salt (NaCl) forms ions when dissolved in water• ions carry electrical charge

– Sugar is a molecular compound.– Molecules stay intact when the

compound dissolves so no ions are formed.

Aqueous Solutions

Electrolyte:a substance that forms ions when

dissolved in waterresulting solutions conduct

electricity all ionic compounds

MgCl2

NaHCO3

Ca(OH)2

some molecular compounds HCl Acetic acid

Aqueous Solutions

Nonelectrolyte:

a substance that does not form ions when dissolved in water

solutions do not conduct electricity

most molecular compounds sugar methyl alcohol acetone

When an ionic compound like NaCl dissolves in water, the ions are separated from their orderly crystalline structure in a process called dissociation.

The water molecules form a “shell” around the ions which helps stabilize the ions and keeps them from recombining.

Aqueous Solutions

Aqueous Solutions

Ions and their “shell” of water molecules are free to move around in solution.

The ions become uniformly dispersed throughout the solution. a homogeneous solution forms

Aqueous Solutions

How does water stabilize ions in solution? Water is an electrically neutral, molecular

compound but the electrons are not shared equally between the H and O atoms.

Oxygen attracts the electrons in the molecule more strongly than the hydrogen atoms. The O atom in water has a partial

negative charge (high electron density). The H atoms in water have a partial

positive charge (low electron density).

OH

H

Aqueous Solutions

- +

Solvation of anion Solvation of cation

Water solvates (surrounds and stabilizes) the ions.

• the H’s (+) interacts with the negatively charged anions

• the O (-) interacts with the positively charged cations

Aqueous Solutions

Example: What ions (and how many of each) are formed when KBr dissolves in water?

Example: What ions (and how many of each are formed when Na2SO4 dissolves in water?

Aqueous Solutions

Example: What ions (and how many of each form when Ba(NO3)2 dissolves in water?

Example: What ions (and how many of each form when Al2(SO4)3 dissolves in water?

Aqueous Solutions

Most molecular compounds dissolve in water without forming ions.Water molecules solvate the

molecular compound, but the structural integrity of the molecule is maintained.

The dissolution (dissolving) of methanol.

Aqueous Solutions

Some molecular compounds interact strongly with water and form ions.Acids

HCl (g) + H2O H+ (aq) + Cl- (aq)

Ammonia NH3 (g) + H2O NH4

+ (aq) + OH- (aq)

Molecular compounds that ionize are electrolytes.

Aqueous Solutions

There are two kinds of electrolytes:

Strong electrolyte: a substance that ionizes

completely when dissolved in water

ionic compounds strong acids strong bases

Aqueous Solutions

Weak Electrolyte:a substance that partially ionizes

when dissolved in water

part of the substance exists as ions and part exists as molecules

HC2H3O2 (aq) H+ (aq) + C2H3O2-

(aq)

Aqueous Solution

The double arrow indicates that the reaction goes in both directions.HC2H3O2 is ionizing to form H+ and

C2H3O2-

H+ and C2H3O2- are recombining to

form HC2H3O2

A single arrow indicates the reactiongoes only in one direction.

and (at the same time)

Predicting Chemical Reactions

Precipitation Reactions

Acid/Base Reactions

Oxidation/Reduction (Redox) Reactions

Precipitation Reactions

One visible sign of a chemical reaction is the formation of a solid:

AgNO3 (aq) + NaCl (aq) AgCl (s) + NaNO3 (aq)

One driving force for a chemical reaction (i.e. thing that pushes a reaction to occur) is the formation of a solid product (precipitate).

Precipitation Reactions

A reaction that forms an insoluble product is called a precipitation reaction.

AgNO3 (aq) + NaCl (aq) AgCl (s) + NaNO3 (aq)

Precipitate: the insoluble product formed by a reaction in solution

In the reaction above, AgCl is the precipitate.

Precipitation Reactions

AgCl forms a precipitate because it has very low solubility in water.

Solubility:the amount of a substance that can

be dissolved in a given quantity of solvent

AgCl is insoluble in water while NaNO3 is soluble.

Soluble: “dissolves”

Insoluble: “does not dissolve”

Precipitation Reactions

In order to predict whether a precipitation reaction will occur, you must know if an ionic compound is soluble in water or not.Experimental data have led to a series

of guidelines used to predict solubility.

On your exam, you must be able to use the guidelines in Table 4.1 to predict the solubility of a compound. I will give you a copy of these

guidelines on the exam.

Precipitation ReactionsSolubility Guidelines

Guidelines Based on Cation

All ionic compounds of the 1A metals are soluble.NaCl, NaHCO3, K2CO3, LiOH

All ammonium salts (ionic compounds) are soluble.NH4OH, NH4Cl, (NH4)3PO4

Precipitation ReactionsSolubility Guidelines

SOLUBLE Compounds

Anion Important ExceptionsNO3

- None

C2H3O2 - None

ClO4- Few common exceptions

SO42- Cpds. With Ca2+, Sr2+, Ba2+,

Pb2+, Ag+

Precipitation ReactionsSolubility Guidelines

SOLUBLE Compounds

Anion Important ExceptionsCl - Cpds. with Ag+, Pb2+, Cu+,

Hg22+

Br - Cpds. with Ag+, Pb2+, Cu+, Hg2

2+

I - Cpds. with Ag+, Pb2+, Cu+, Hg2

2+

F- Cpds. With Pb2+, Group 2A

Precipitation ReactionsSolubility Guidelines

INSOLUBLE Compounds

Anion Important ExceptionsS

2- Cpds. with NH4+, Group 1A, Ca2+,

Sr2+, or Ba2+

CO3 2- Cpds. with NH4

+ or alkali metalcations

PO43- Cpds. with NH4

+, alkali metal cations

OH - Cpds. with Group 1A, NH4+, Ca2+,

Sr2+, or Ba2+

Precipitation Reactions

Example: Use the solubility guidelines in Table 4.1 to predict the solubility of the following compounds.

Ba(NO3)2

CaCl2

Ca3(PO4)2

NH4ClPbSO4

Ba(OH)2

Precipitation ReactionsPredicting Products

Consider the reaction between AgNO3 and NaCl in aqueous solution:

AgNO3 (aq) + NaCl (aq) AgCl (s) + NaNO3 (aq)

Notice that the anions and cations appear to have exchanged partners.

Precipitation Reactions

Metathesis Reactionsreactions in which the positive ions

and negative ions present in the reactants appear to exchange partners

also called exchange reactions

AX + BY AY + BX

AgNO3 + NaCl AgCl + NaNO3

Precipitation Reactions To write a chemical equation for a

precipitation reaction:Write the formula including charge for

the ions formed by each reactant in aqueous solution.

Switch “partners” and write the correct formulas (electrically neutral) for the possible products.

Determine the solubility of each product.

Balance the equation. Include all physical states for reactants and products.

Precipitation Reactions

Example: Write a balanced equation for the reaction between aqueous solutions of NaF and Pb(NO3)2.

Precipitation Reactions

Example: Write the chemical equation for the reaction between aqueous solutions of Na2CO3 and Fe(C2H3O2)3.

Precipitation Reactions

Example: Write the chemical equation for the reaction between aqueous solutions of aluminum nitrate and sodium hydroxide.

Precipitation Reactions

Al(NO3)3 (aq) + 3 NaOH (aq) Al(OH)3 (s) + 3 NaNO3 (aq)

At the end of this reaction, the beaker contains two components:

The solid aluminum hydroxide that precipitates out and settles to the bottom of the beaker.

The aqueous solution that contains Na+ (aq) and NO3

- (aq)

Precipitation Reactions

Al(NO3)3 (aq) + 3 NaOH (aq) Al(OH)3 (s) + 3 NaNO3 (aq)

This is called a molecular equation.

It shows the chemical formulas of reactants and products but does not indicate the ionic nature of the reactants or products.

In reality, the chemicals with (aq) beside them exist as ions in solution

Precipitation Reactions

In order to indicate the ionic nature of the chemicals, we can write a complete ionic equation:

Explicitly indicates all of the ions in the reaction mixture

All soluble strong electrolytes are written as individual ions.

Precipitation Reactions

To write a complete ionic equation:

Break all soluble strong electrolytes (ionic compounds or strong acids with (aq) beside them) into their ions. indicate the correct charge of each

ion indicate the number of each ion write (aq) after each ion

Bring down all weak electrolytes unchanged.

Bring down all compounds with (s), (g), or (l) unchanged.

Precipitation Reactions

Example: Write the complete ionic equation for the following reaction:

Al(NO3)3 (aq) + 3 NaOH (aq) Al(OH)3 (s) + 3 NaNO3 (aq)

Precipitation Reactions

Notice that the Na+ and NO3- ions

appear unchanged on both sides of the reaction.

Spectator ions ions that are present but play no

direct role in a chemical reaction

Al3+ (aq) + 3 NO3- (aq) + 3 Na+ (aq) + 3 OH- (aq)

Al(OH)3 (s) + 3 Na+ (aq) + 3 NO3

-

(aq)

Precipitation Reactions

If we eliminate all spectator ions from the equation, we can write a net ionic equation

an equation that includes only the ions directly involved in the reaction and the insoluble products that are formed

Precipitation Reactions

To write a net ionic equation:Write the complete ionic equation.

Cross out the ions that appear in the same type, number, and physical state on both sides of the equation.

Write the “leftovers” as the net ionic equation. Don’t forget to include the physical

states of the reactants and products.

Precipitation Reactions

Net ionic equation:

Al3+ (aq) + 3 NO3 - (aq) + 3 Na+ (aq) + 3 OH- (aq)

Al(OH)3 (s) + 3 Na+ (aq) + 3 NO3- (aq)