Chemistry 1210:
Introduction to General Chemistry
Dr. Gina M. Florio
20 Sept. 2012Jespersen, Brady, Hyslop
Chapter 5A
Molecular View of Reactions in Aqueous Solution
A solution is a homogeneous mixture in which the two or more components mix freely.
The solvent is taken as the component present in the largest amount.
A solute is any substance dissolved in the solvent.
Definitions
CH. 5.1
Example of a SolutionFormation of a solution of iodine molecules in ethyl alcohol where ethyl alcohol is the solvent and iodine the solute:
CH. 5.1
Crystal of I2 placed in ethanol: A solution of I2 in ethanol:
Properties of Solutions: Qualitative Composition
The relative amounts of solute and solvent are often given without specifying the actual quantities.
The dilute solution (left) has less solute per unit volume than the (more) concentrated solution (right).
Example:
CH. 5.1
Solutions have variable composition and they may be characterized using a solute-to-solvent ratio called the concentration.
Properties of Solutions: Qualitative & Quantitative
There is usually a limit to the amount of solute that can dissolve in a given amount of solvent.
Example: 36.0 g of NaCl can dissolve in 100 g of H2O at 20°C
Saturated solution – no more solute can be dissolved at a given T.
The solubility of a solute is the number of grams of solute that can dissolve in 100 grams of solvent at a given temperature, T.
Unsaturated solution – able to dissolve more solute at a given T.
Supersaturated solution – contains more solute than required for saturation at a given T.
CH. 5.1
Properties of Solutions: Qualitative
Supersaturated solutions
– can be formed by careful cooling of saturated solutions.– are unstable and often result in the formation of a precipitate.
Precipitate – a solid substance that separates from solution.
Precipitates can also form from reactions (precipitation reactions).
CH. 5.1
Molecular Compounds in Solutions
Most solutions of molecular compounds do not conduct electricity and are called nonelectrolytes (no ions are formed).
CH. 5.2
Ionic Compounds in Solutions
Ionic compounds are electrolytes (hydrated ions conduct electricity).
Ionic compounds dissociate when they dissolve in water.
Ions separate from the solid and become hydrated.
)(SO )(Na 2 )(SONa -2442 aqaqs
The dissociation reaction of an ionic compound:
The hydrated ions, with the symbol (aq), have been written separately. Polyatomic ions remain intact.
CH. 5.2
Reactions of Ions in SolutionExample:When a solution of Pb(NO3)2 is mixed with a solution of KI the yellow precipitate PbI2 rapidly forms:
CH. 5.2
Reactions of Ions in Solution
)(2KNO)(PbI)2KI()()Pb(NO 3223 aqsaqaq
)(2NO)(2K)(PbI)(2I)(2K)(2NO)(Pb -32
--3
2 aqaqsaqaqaqaq
)(PbI )(2I)(Pb 22 saqaq
This reaction may be represented with a molecular, ionic, or net ionic equation:
The most compact notation is the net ionic equation which eliminates all the non-reacting spectator ions from the equation.
Molecular
Ionic
Net ionic
CH. 5.2
Criteria for balanced ionic and net ionic equations:
Balancing Ionic Reactions
Mass balance – the same number of each type of atom on each side of the arrow
Charge balance – the net electrical charge on the left side of the arrow must equal the net electrical charge on the right side of the arrow
CH. 5.2
Acids & Bases
CH. 5.3
Arrhenius Definitions:
Acid – a substance that reacts with water to produce the hydronium ion
Base – a substance that reacts with water to produce the hydroxide ion
OH3
OH
The characteristic reaction between acids and bases is neutralization:
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
In general, the reaction of an acid and a base produces a salt and water.
HA + H2O A– + H3O+
B + H2O B+ + OH−
Deprotonation Reaction: Ionization of acids in water
CH. 5.3
HA + H2O A– + H3O+
Ionization of an acid in water:
Acids
CH. 5.3
(aq) Cl (aq)O HO H HCl(g) - 32
Acids are molecular compounds that undergo ionization reactions with water:
(aq)Cl (aq) HHCl(g) -OH 2
(aq)Cl(aq)OHOHHCl(aq) - 32
(aq)CO(aq)OHOH(aq)HCO
(aq)HCO(aq)OHOH(aq)COH--
-
23323
33232
(aq)PO(aq)OHOH(aq)HPO
(aq)HPO(aq)OHOH(aq)POH
(aq)POH(aq)OHOH(aq)POH
--
--
-
3432
24
243242
423243
proton
Acids can be classified as monoprotic or polyprotic:
Monoprotic:
Diprotic:
Triprotic:
1 hydronium ion
2 hydronium ions
3 hydronium ions
Acid and Basic Anhydrides
CH. 5.3
Some nonmetal oxides react with water to produce acids.
They are called acidic anhydrides (anhydride = without water).
acidcarbonic acidnitric 2
acidsulfuric :Oxides Nonmetal
3222
3252
4223
(aq)COHOH(g)CO(aq)HNOOH(g)O N(aq)SOHOH(g)SO
Soluble metal oxides are basic anhydrides.
hydroxide sodium 2hydroxide calcium
:Oxides Metal
22
22
NaOH(aq) OHO(s) Na(aq)Ca(OH)OHCaO(s)
Molecular Bases
CH. 5.3
(aq)OH(aq)NHOH(aq)NH - 423
Ammonia gas ionizes in water producing hydroxide ions:
This is an example of a molecular base.
Classification of Acids and Bases
CH. 5.3
Acids and bases can be classified as strong or weak and so as strong or weak electrolytes.
Strong electrolytes undergo complete ionization in water.
acidsulfuric )(SOHacidnitric )(HNO
acid hydroiodic )HI(acidc hydrobromi )HBr(acidic hydrochlor )HCl(
acid perchloric )(HClO
42
3
4
aqaq
aqaqaq
aqCommon Strong Acids:
hydroxide barium OH) Ba(hydroxide cesiumCsOH hydroxide strontium Sr(OH) hydroxide rubidiumRbOH
hydroxide calcium Ca(OH) hydroxide potassiumKOH hydroxide sodiumNaOH hydroxide lithiumLiOH
IIA Group IA Group
2
2
2
Common Strong Bases:
Weak Acids and Bases
CH. 5.3
Most acids and molecular bases are not completely ionized in water. They are classified as weak electrolytes.
Weak acids and bases are weak electrolytes because less than 100% of the molecules ionize.
HCl (strong) HC2H3O2 (weak) NH3 (weak)
Reactions: Strong Acid and Strong Base
OH)(OH)(H :ionic Net
)(Cl)(KOH)(OH)(K)(Cl)(H :Ionic
OH)KCl()KOH()HCl( :Molecular
2-
-2
--
2
aqaq
aqaqaqaqaqaq
aqaqaq
The reaction of a strong acid with strong base gives a salt and water:
This net ionic equation applies only to strong acids and bases.
This is called a neutralization reaction. The driving force is the formation of water.
CH. 5.3
Reactions: Weak Acid and Strong Base
The neutralization of a weak acid with a strong base involves a strong and weak electrolyte.
OH)(OHC)(OH)(OHHC :ionic Net
OH)(OHC)(Na
)(OH)(Na)(OHHC :Ionic
OH)(OHNaC)NaOH()(OHHC :Molecular
2-232
-232
2-232
-232
2232232
aqaqaq
aqaq
aqaqaq
aqaqaq
Consider the reaction of acetic acid with sodium hydroxide:
Note that in ionic equations the formulas of weak electrolytes are written in “molecular” form.
The driving force is the formation of the weak electrolyte.
CH. 5.3
Reactions: Strong Acid and Weak Base
O H(aq) NH(aq)OH(aq) NH
(aq) NH(aq)H(aq) NH
2433
43
or
Note that water only appears as a product if the hydronium ion is used.
)(243 lOHCl(aq) NHHCl(aq)(aq) NH
For ammonia and HCl the net ionic equation is:
The situation is similar when a strong acid reacts with a strong base.
For example, the reaction between hydrochloric acid and ammonia:
CH. 5.3
Weak Acids and Bases
CH. 5.3
Weak acids and bases are in dynamic equilibrium in solution.
Two opposing reactions occur in solution:
1. Ionization of the acid (the forward reaction)
2. Recombination of ions into molecules (the reverse reaction).
Chemical or dynamic equilibrium results when the rate of the forward and reverse reaction are equal.
Example: Acetic Acid
)()(3)()( 2322232 aqOHCaqOHaqOHaqOHHC
Naming Binary Acids
CH. 5.4
Binary compounds of many nonmetals and hydrogen are acidic (binary acids).
ic acidhydroaqgic acidhydroaqg
id Binary Ac Compound Molecular
sulfur )S(H sulfidehydrogen )S(Hchlor )HCl( chloridehydrogen )HCl(
22
They are named by adding the prefix hydro- and the suffix –ic to the stem of the nonmetal name, followed by the word acid.
Example:
Naming Oxoacids
CH. 5.4
Acids that contain hydrogen, oxygen, plus another element are called oxoacids.
They are named according to the number of oxygen atoms in the molecule and do not take the prefix hydro-.
Example:
When there are two oxoacids, the one with the larger number of oxygens takes the suffix –ic and the one with the fewer oxygen atoms takes the suffix –ous.
ous acidous acidic acidic acid
nitr HNO sulfur SOHnitr HNO suflur SOH
232
342
Halogens can form up to four different oxoacids. The oxoacid with the most oxygens has the prefix per- the one with the least has the prefix hypo-.
Example:ic acidperous acid
ic acidous acidhypochlor HClO chlor HClO
chlor HClO chlor HClO
42
3
Ions of Oxoacids
CH. 5.4
Anions are produced when oxoacids are neutralized.
Relationship between the name of the polyatomic ion and the parent acid:
–ic acids give –ate ions-ous acids give –ite ions
In naming polyatomic anions, the prefixes per- and hypo- carry over from the parent acid:
perchlorate ion (ClO4-) makes perchloric acid (HClO4)
Polyprotic Acid Salts
CH. 5.4
phosphate dihydrogen sodium PONaHphosphate hydrogen sodium HPONasulfate hydrogen sodium NaHSO
42
42
4
An acidic salt contains an anion that is capable of furnishing additional hydrogen ions.
The number of hydrogens that can still be neutralized is also indicated.
Naming Bases
CH. 5.4
Ionic compounds containing metal ions are named like any other ionic compound.
Molecular bases are specified by giving the name of the molecule
Reactions: Insoluble Hydroxide and Oxide Bases
Magnesium hydroxide has a low solubility in water, but can react with a strong acid.
The net ionic equation is:
Magnesium hydroxide is written as a solid because it is insoluble.
OH (aq) Mg(aq) H (s)Mg(OH) 22
2 22
OH(aq)Fe(aq)H(s)OFe
OH(aq)FeClHCl(aq)(s)OFe
23
32
2332
326 :ionic Net
326 :Molecular
A number of metal oxides also dissolve in acids.
For example, iron(III) oxide reacts with hydrochloric acid:
The driving force is the formation of water.CH. 5
Reactions: Formation of Insoluble GasesSome reactions with acids or bases produce a gas.
OH)(NH-OHNH Salts Ammonium NH
OH)(SO-HSOH SulfitesHydrogen
OH)(SO-SO2H Sulfites SO
OH)(CO-HCOH CarbonatesHydrogen
OH)(CO-CO2H Carbonates CO
)( HCN-CNH Cyanides HCN
)( SH-S2H Sulfides SH
2343
2223
22232
223
22232
22
2
g
g
g
g
g
g
g
onnic Equati Net Io pounds Gas Com
The driving force is the formation of the gas.CH. 5
