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Solutions, Solubility,and Reaction Types

Brown, LeMay Ch 4AP Chemistry

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4.1: Solutions & Expressing ConcentrationSolution: homogeneous mixture of 2 or more

substancessolute: component present in lesser quantitysolvent: component present in greater quantity

Molarity (M) = moles solute =mol

volume of solution L

Molality (m) = moles solute =mol

kg of solvent kg

*Normality (N): associated with acid & base strength = (moles solute)(# of acid/base equivalents) = mol

volume of solutionL

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Dilution: (Minitial)(Vinitial) = (Mfinal)(Vfinal)

Ex: How much water must be added to a solution that contains 100.0 g NaOH in 500. mL of solution to make the final concentration 2.00 M?

Minitial = (100.0 g NaOH)/(40.00 g/mol NaOH) = 5.00 M

0.500 L

(Mi)(Vi) = (Mf)(Vf) = (5.00 M)(0.500 L) = (2.00 M)(Vf)

Vf = 1.25 L, so 0.75 L water must be added.

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4.2: Properties of SolutesIonic compounds (salts): dissociate in water by solvation, in which polar H2O molecules surround

and separate ions from ionic crystal lattice

Molecular compounds: do not ionize because intramolecular bonds are stronger than bonds that form between H2O and molecule

Acids and bases, sometimes considered molecular (depending on strength), may also ionize.

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Electrolyte: substance which, in aqueous solution, ionizes and thus conducts electricity

Non-electrolyte: substance which, in aqueous solution, does not dissociate and thus does not conduct electricity

Strong & weak electrolytes: conductivity depends on degree of dissociation and equilibrium position: HA (aq) ↔ H+ (aq) + A- (aq)

Strong = nearly completely dissociatedWeak = partially dissociated

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4.3: Acids & BasesBronsted-Lowry definitions:

Acids: H+ donorBases: H+ acceptor

Figure 1: Strong & weak: depends on degree of dissociation (Ka value, Table 4.1, p. 117)

Strong Acids Strong Bases

HBr, HCl, HI

HNO3, H2SO4

HClO4, HClO3

Group 1A’s: LiOH, NaOH, KOH, RbOH,

CsOH

Heavy 2A’s: Ca(OH)2, Sr(OH)2,

Ba(OH)2

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4.4: Ionic Equations1. Molecular equation: shows complete chemical equation

with states of matter, undissociated

BaCl2 (aq) + Na2SO4 (aq) → 2 NaCl (aq) + BaSO4 (s)

2. Complete ionic equation: shows complete chemical equation with states of matter, dissociated if appropriateBa2+(aq) + 2 Cl-(aq) + 2 Na+(aq) + SO4

2-(aq) →2 Cl-(aq) + 2 Na+(aq) + BaSO4 (s)

Spectator ions: present in reaction but do not “participate”; depend on solubility rulesCl- (aq) and Na+ (aq)

3. Net ionic equation: shows chemical equation without spectator ionsBa2+ (aq) + SO4

2- (aq) → BaSO4 (s)

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Solubility Rules (memorize them!)Figure 2: Compounds that are always soluble.

Compounds containing…

Important Exceptions

NO3-

CH3COO-

Cl-, Br-, I-

SO42-

None

None

Ag+, Hg22+, Pb2+

Ag+, Hg22+, Pb2+,

Sr2+, Ba2+, Ca2+

• Also, compounds with Group 1A and NH4+ cations

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Scene from The G Block

Featuring 2003-04 AP Chemistry students Daniel Fong, Ashwin Nirmalkumar, Veselin Kolev, and Shintaro Funasaki. Cameo here by Rob Palmiotto.

Figure 3: Compounds that always insoluble.

Compounds containing…

Important Exceptions

• Other anions (e.g., BrO3- or Cr2O7

2-) are generally insoluble except with the soluble cations (Group 1A and NH4

+)

CO32-, CrO4

2-, C2O42-

PO43-

OH-

S2-

Group 1A, NH4+

Group 1A,Sr2+, Ba2+, Ca2+

Group 1A, NH4+,

Sr2+, Ba2+, Ca2+

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AP Exam: Predicting Reaction ProductsIn general, when writing the reactants:1. Write all soluble ionic compounds in

aqueous solution as their constituent ions. Never write NaCl, Ca(NO3)2, etc.

2. Write all strong acids in water as H+ or H3O+ and the anion. Never write HCl, HNO3, etc.

3. Write all strong bases as OH- and the cation. Never write NaOH, Ba(OH)2, etc.

4. Don’t omit any ions unless absolutely sure that they are be spectator ions. For example, although the Cl- in HCl is usually a spectator ion, it is sometimes a reactant.

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In general, after writing the products:1. Inspect all of the ions in the products to see

if there are any precipitates.2. Cross out any spectator ions.3. Balance the reaction.

Practice Predicting Precipitation ReactionsObservations from previous AP exams:

These types of reactions generally involve mixing two solutions, but may include the addition of hydrogen sulfide gas.Many involve formation of a single insoluble precipitate, but watch for the possibility of two insoluble products.

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ExamplesA. Dilute hydrochloric acid is added to a dilute

solution of mercury (I) nitrate.

B. Dilute sulfuric acid is added to a solution of barium chloride.

H+ + Cl- + Hg22+ + NO3

-→ Hg2Cl2 + NO3- + H+

H+ + SO42- + Ba2+ + Cl- → BaSO4

+ H+ + Cl-

2 Cl- + Hg22+ → Hg2Cl2

SO42- + Ba2+ → BaSO4

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C. Hydrogen sulfide gas is added to a solution of cadmium nitrate.

D. Solutions of silver nitrate and sodium chromate are mixed.

H2S + Cd2+ + NO3- → CdS + H+ + NO3

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Ag+ + NO3- + Na+ + CrO4

2-

→ Ag2CrO4 + Na+ + NO3-

2 Ag+ + CrO42- → Ag2CrO4

H2S + Cd2+ → CdS + 2 H+

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Other reactions to know:Acid + Base → Salt + WaterEx: A solution of cesium hydroxide is added to a solution of nitric acid.

Nonmetal oxide + Water → OxyacidEx: Sulfur trioxide gas is added to excess water.

Metal oxide + Water → BaseEx: Solid sodium oxide is added to water.

Cs+ + OH- + H+ + NO3- → Cs+ + NO3

- + H2O

H+ + OH- → H2O

SO3 + H2O → H2SO4

SO3 + H2O → 2 H+ + SO42-

Na2O + H2O → NaOH

Na2O + H2O → 2 Na+ + 2 OH-

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Nonmetal oxide + Metal oxide → SaltEx: Solid calcium oxide is heated in the presence of sulfur trioxide gas.

CaO + SO3 → CaSO4

If this were in water:CaO + H2O → Ca2+ + OH-

andSO3 + H2O → H2SO4 → H+ + SO4

2-

So, our overall reaction would be:Ca2+ + H2O + SO4

2- → CaSO4 + H2O

But this can happen without water:CaO + SO3 → CaSO4

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Metal oxide + Acid → Salt + WaterEx: Potassium oxide solid is added a solution of hydrochloric acid.

Nonmetal oxide + Base → Salt + WaterEx: Carbon dioxide is bubbled through a solution of barium hydroxide.

K2O + H+ + Cl- → K+ + Cl- + H2O

(K+ + O2-) + H+ + Cl- → K+ + Cl- + H2O

K2O + 2 H+ → 2 K+ + H2O

CO2 + Ba2+ + OH- → BaCO3 + H2O

CO2 + Ba2+ + 2 OH- → BaCO3 + H2O

Ion (aq)Co2+

[CoCl4]2-

RoseBlue

Cr3+

CrO42-

Cr2O72-

GreenYellowOrange

Cu2+

[Cu(NH3)4]2+

Light blueMedium blue

Fe2+

Fe3+

[Fe(SCN)]2+

GreenYellow-brown

Red

Mn2+ Pink

Ni2+ Green

Grp I-A+

Grp II-A2+

Al3+

Colorless

Zn2+ Colorless or “milky”

Compounds ColorPbI2

Grp I-A+/II-A2+ & CrO42-

Yellow

Grp I-A+/II-A2+ & Cr2O72- Orange

AgXGrp I-A+/II-A2+ & NO3

-, C2H3O2

-, X-, SO42-

White

AgNO3White, turns

black in sunlight

CuO Green

Fe2O3 Rust

KMnO4 Purple

HgO Dark red

NO2 (g) Brown

Any suggestions for others you’ve seen on old/practice exams, let me know.