Types of Chemical Reactions

and Solution Stoichiometry

Chapter 4 Notes

West Valley High School

AP ChemistryMr. Mata

Classification of MatterSolutions are homogeneous mixtures

SoluteA solute is the dissolved substance in a solution.

A solvent is the dissolving medium in a solution.

Solvent

Salt in salt water Sugar in soda drinks

Carbon dioxide in soda drinks

Water in salt water Water in soda

Saturation of Solutions A solution that contains the maximum

amount of solute that may be dissolved under existing conditions is saturated.

A solution that contains less solute than a saturated solution under existing conditions is unsaturated.

A solution that contains more dissolved solute than a saturated solution under the same conditions is supersaturated.

The ammeter measures the flow of electrons (current) through the circuit.

If the ammeter measures a current, and the bulb glows, then the solution conducts. If the ammeter fails to measure a current, and the bulb does not glow, the solution is non-conducting.

Electrolytes vs. Nonelectrolytes

An electrolyte is:

A substance whose aqueous solution conducts an electric current.

A nonelectrolyte is:

A substance whose aqueous solution does not conduct an electric current.

Try to classify the following substances as electrolytes or nonelectrolytes…

Definition of Electrolytes and Nonelectrolytes

1.Pure water2.Tap water3.Sugar solution4.Sodium chloride solution5.Hydrochloric acid solution6.Lactic acid solution7.Ethyl alcohol solution8.Pure, solid sodium chloride

Electrolytes?

ELECTROLYTES: NONELECTROLYTES:

Tap water (weak)

NaCl solution

HCl solution

Lactate solution (weak)

Pure water

Sugar solution

Ethanol solution

Pure, solid NaCl

But why do some compounds conduct electricity insolution while others do not…?

Answers…

Ionic CompoundsDissociate

NaCl(s)

AgNO3(s)

MgCl2(s)

Na2SO4(s)

AlCl3(s)

Na+(aq) + Cl-(aq)

Ag+(aq) + NO3-(aq)

Mg2+(aq) + 2 Cl-(aq)

2 Na+(aq) + SO42-

(aq)Al3+(aq) + 3 Cl-(aq)

The reason for this is the polar nature of the water molecule…

Positive ions associate with the negative end of the water dipole (oxygen).Negative ions associate with the positive end of the water dipole (hydrogen).

Ions tend to stay in solution where they canconduct a current rather than re-forming a solid.

Covalent acids form ions in solution, with the help of the water molecules.

For instance, hydrogen chloride molecules,which are polar, give up their hydrogens towater, forming chloride ions (Cl-) and hydronium ions (H3O+).

Some covalent compounds IONIZE in solution

Other examples of strong acids include:

Sulfuric acid, H2SO4

Nitric acid, HNO3

Hydriodic acid, HI Perchloric acid, HClO4

Strong acids such as HCl are completelyionized in solution.

Many of these weaker acids are “organic” acidsthat contain a “carboxyl” group.

The carboxyl group does not easily give up itshydrogen.

Weak acids such as lactic

acid usually ionize less than 5% of the time.

Other organic acids and their sources include:

o Citric acid – citrus fruito Malic acid – appleso Butyric acid – rancid buttero Amino acids – proteino Nucleic acids – DNA and RNAo Ascorbic acid – Vitamin C

This is an enormous group of compounds; these are only a few examples.

Because of the carboxyl group, organic acids are

sometimes called “carboxylic acids”.

Sugar (sucrose – C12H22O11),

and ethanol (ethyl alcohol – C2H5OH) do notionize - That is why they are nonelectrolytes!

However, most covalent compounds do not ionize

at all in solution.

Molarity

The concentration of a solution measured in moles of solute per liter of solution.

mol = M L

Preparation of Molar Solutions

Problem: How many grams of sodium chloride are needed to prepare 1.50 liters of 0.500 M NaCl solution?

Step #1: Ask “How Much?” (What volume to prepare?)

1.500 L

Step #2: Ask “How Strong?” (What molarity?)

0.500 mol

1 L

Step #3: Ask “What does it weigh?” (Molar mass is?)

58.44 g

1 mol= 43.8 g

Serial DilutionIt is not practical to keep solutions of many different concentrations on hand, so chemists prepare more dilute solutions from a more concentrated “stock” solution.

Serial Dilution

Problem: What volume of stock (11.6 M) hydrochloric acid is needed to prepare 250. mL of 3.0 M HCl solution?

MstockVstock = MdiluteVdilute

(11.6 M)(x Liters) = (3.0 M)(0.250 Liters)

x Liters = (3.0 M)(0.250 Liters) 11.6 M

= 0.065 L

Single Replacement Reactions

Replacement of:

Metals by another metal Hydrogen in water by a metal Hydrogen in an acid by a metal Halogens by more active halogens

A + BX AX + B

BX + Y BY + X

The Activity Series of the Metals

Lithium Potassium Calcium Sodium Magnesium Aluminum Zinc Chromium Iron Nickel Lead Hydrogen Bismuth Copper Mercury Silver Platinum Gold

Metals can replace other metalsprovided that they are above themetal that they are trying to replace.

Metals above hydrogen can replace hydrogen in acids.

Metals from sodium upward canreplace hydrogen in water

The Activity Series of the Halogens

Fluorine Chlorine Bromine Iodine

Halogens can replace other halogens in compounds, providedthat they are above the halogenthat they are trying to replace.

2NaCl(s) + F2(g) 2NaF(s) + Cl2(g)

MgCl2(s) + Br2(g) ???No Reaction

???

Double Replacement Reactions

The ions of two compounds exchange places in anaqueous solution to form two new compounds.

AX + BY AY + BX

One of the compounds formed is usually a precipitate (an insoluble solid), an insoluble gas that bubbles out of solution, or a molecular compound, usually water.

Double replacement forming a precipitate…

Pb(NO3)2(aq) + 2KI(aq) PbI2(s) + 2KNO3(aq)

Pb2+(aq) + 2 NO3-(aq) + 2 K+(aq) +2 I-(aq) PbI2(s) + 2K+(aq) + 2

NO3-(aq)

Pb2+(aq) + 2 I-(aq) PbI2(s)

Double replacement (ionic) equation

Complete ionic equation shows compounds as aqueous ions

Net ionic equation eliminates the spectator ions

Solubility Rules – Mostly Soluble

Ion Solubility

Exceptions

NO3- Soluble None

ClO4- Soluble None

Na+ Soluble None

K+ Soluble None

NH4+ Soluble None

Cl-, I- Soluble Pb2+, Ag+, Hg22+

SO42- Soluble Ca2+, Ba2+, Sr2+, Pb2+, Ag+,

Hg2+

Solubility Rules – Mostly Insoluble

Ion Solubility

Exceptions

CO32- Insolubl

eGroup IA and NH4

+

PO43- Insolubl

eGroup IA and NH4

+

OH- Insoluble

Group IA and Ca2+, Ba2+, Sr2+

S2- Insoluble

Groups IA, IIA, and NH4+

Oxidation and Reduction (Redox)

Electrons are transferred

Spontaneous redox rxns can transfer energy

Electrons (electricity) Heat

Non-spontaneous redox rxns can be made to happen with electricity

Oxidation and Reduction

Gain Electrons = Reduction

An old memory device for oxidation and reduction goes like this… LEO says GER

Lose Electrons = Oxidation

Oxidation Reduction Reactions(Redox)

11

2

00

22

ClNaClNaEach sodium atom loses one electron:

Each chlorine atom gains one electron:

eNaNa10

10 CleCl

LEO says GER :

eNaNa10

Lose Electrons = Oxidation

Sodium is oxidized

Gain Electrons = Reduction

10 CleCl Chlorine is reduced

Rules for Assigning Oxidation Numbers

Rules 1 & 21. The oxidation number of any

uncombined element is zero

2. The oxidation number of a monatomic ion equals its charge

11

2

00

22

ClNaClNa

Rules for Assigning Oxidation Numbers

Rules 3 & 4

3. The oxidation number of oxygen in compounds is -24. The oxidation number of hydrogen in compounds is +1

2

2

1

OH

Rules for Assigning Oxidation Number Rule 5

5. The sum of the oxidation numbers in the formula of a compound is 02

2

1

OH2(+1) + (-2) = 0 H O

2

122

)(

HOCa(+2) + 2(-2) + 2(+1) = 0 Ca O H

Rules for Assigning Oxidation Numbers

Rule 66. The sum of the oxidation numbers in the formula of a polyatomic ion is equal to its charge

3

2?

ONX + 3(-2) = -1N O

24

2?

OS

X = +5 X = +6

X + 4(-2) = -2S O

Reducing Agents and Oxidizing Agents

The substance reduced is the oxidizing agent The substance oxidized is the reducing agent

eNaNa10

10 CleCl

Sodium is oxidized – it is the reducing agent

Chlorine is reduced – it is the oxidizing agent

Trends in Oxidation and Reduction

Active metals: Lose electrons easily Are easily oxidized Are strong reducing agents

Active nonmetals: Gain electrons easily Are easily reduced Are strong oxidizing agents

Redox Reaction Prediction #1

Important Oxidizers

Formed in reaction

MnO4- (acid solution)

MnO4- (basic solution)

MnO2 (acid solution)

Cr2O72- (acid)

CrO42-

HNO3, concentrated

HNO3, dilute

H2SO4, hot conc

Metallic IonsFree Halogens

HClO4

Na2O2

H2O2

Mn(II)MnO2

Mn(II)Cr(III)Cr(III)NO2

NOSO2

Metallous IonsHalide ionsCl-

OH-

O2

Redox Reaction Prediction #2

Important Reducers Formed in reaction

Halide IonsFree MetalsMetalous IonsNitrite IonsSulfite IonsFree Halogens (dil, basic

sol)Free Halogens (conc, basic

sol)C2O4

2-

HalogensMetal IonsMetallic ionsNitrate IonsSO42-Hypohalite ionsHalate ionsCO2

Not All Reactions are Redox Reactions

Reactions in which there has been no change in oxidation number are not redox rxns.

Examples:

)()()()( 3

2511111

3

251

aqONNasClAgaqClNaaqONAg

)()()()(22

2

1

4

26

2

1

4

26

2

1121

lOHaqOSNaaqOSHaqHONa