Transcript Chapter4 - OrgSites.com

Types of Chemical Reactions
and Solution Stoichiometry
Solutions Classification
are
homogeneous
mixtures
of Matter
Solute
A solute is the dissolved substance in a
solution.
Salt in salt water
Sugar in soda drinks
Carbon dioxide in soda drinks
Solvent
A solvent is the dissolving medium in a
solution.
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.
Electrolytes vs. Nonelectrolytes
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.
Definition of Electrolytes and
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…
Electrolytes?
1.Pure water
2.Tap water
3.Sugar solution
4.Sodium chloride solution
5.Hydrochloric acid solution
6.Lactic acid solution
7.Ethyl alcohol solution
8.Pure, solid sodium chloride
Answers…
ELECTROLYTES:
NONELECTROLYTES:
Tap water (weak)
Pure water
NaCl solution
Sugar solution
HCl solution
Ethanol solution
Lactate solution (weak)
Pure, solid NaCl
But why do some compounds conduct electricity in
solution while others do not…?
Ionic CompoundsDissociate
NaCl(s)  Na+(aq) + Cl-(aq)
AgNO3(s)  Ag+(aq) + NO3-(aq)
MgCl2(s)  Mg2+(aq) + 2 Cl-(aq)
Na2SO4(s)  2 Na+(aq) + SO42-(aq)
AlCl3(s)  Al3+(aq) + 3 Cl-(aq)
Ions tend to stay in solution where they can
conduct a current rather than re-forming a
solid.
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).
Some covalent compounds IONIZE in solution
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 to
water, forming chloride ions (Cl-) and
hydronium ions (H3O+).
Strong acids such as HCl are completely
ionized in solution.
Other examples of strong acids include:




Sulfuric acid, H2SO4
Nitric acid, HNO3
Hydriodic acid, HI
Perchloric acid, HClO4
Weak acids such as lactic
acid usually ionize less than
5% of the time.
Many of these weaker acids
are “organic” acids
that contain a “carboxyl”
group.
The carboxyl group does not easily give up its
hydrogen.
Because of the carboxyl group, organic acids are
sometimes called “carboxylic acids”.
Other organic acids and their sources include:
o
o
o
o
o
o
Citric acid – citrus fruit
Malic acid – apples
Butyric acid – rancid butter
Amino acids – protein
Nucleic acids – DNA and RNA
Ascorbic acid – Vitamin C
This is an enormous group of compounds; these
are only a few examples.
However, most covalent compounds do not ionize
at all in solution.
Sugar (sucrose – C12H22O11),
and ethanol (ethyl alcohol – C2H5OH) do not
ionize - That is why they are nonelectrolytes!
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?)
 Step #2: Ask “How Strong?” (What molarity?)
 Step #3: Ask “What does it weigh?” (Molar mass is?)
1.500 L
0.500 mol
58.44 g
1 L
1 mol
= 43.8 g
Serial Dilution
It
Problem:
is not practical
What volume
to keep
of stock
solutions
(11.6ofM)
many
different
hydrochloric
concentrations
acid is needed
on to
hand,
prepare
so chemists
250. mL
prepare
of 3.0 Mmore
HCl solution?
dilute solutions from a more
concentrated “stock” 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
A + BX  AX + B
BX + Y  BY + X
Replacement of:




Metals by another metal
Hydrogen in water by a metal
Hydrogen in an acid by a metal
Halogens by more active halogens
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 metals
provided that they are above the
metal that they are trying to
replace.
Metals above hydrogen can
replace hydrogen in acids.
Metals from sodium upward can
replace hydrogen in water
The Activity Series of the Halogens
Fluorine
Chlorine
Bromine
Iodine
Halogens can replace other
halogens in compounds, provided
that they are above the halogen
that they are trying to replace.
???
+ Cl2(g)
2NaCl(s) + F2(g)  2NaF(s)
???Reaction
MgCl2(s) + Br2(g)  No
Double Replacement Reactions
The ions of two compounds exchange places in an
aqueous 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…
Double replacement (ionic) equation
Pb(NO3)2(aq) + 2KI(aq)  PbI2(s) + 2KNO3(aq)
Complete ionic equation shows compounds as aqueous ions
Pb2+(aq) + 2 NO3-(aq) + 2 K+(aq) +2 I-(aq)  PbI2(s) + 2K+(aq) + 2 NO3-(aq)
Net ionic equation eliminates the spectator ions
Pb2+(aq) + 2 I-(aq)  PbI2(s)
Solubility Rules – Mostly Soluble
Ion
NO3-
Solubility
Soluble
Exceptions
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
CO32-
Solubility
Insoluble
Exceptions
Group IA and NH4+
PO43-
Insoluble
Group 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
An old memory device for oxidation
and reduction goes like this…
LEO says GER
Lose Electrons = Oxidation
Gain Electrons = Reduction
Oxidation Reduction Reactions
(Redox)
0
1
0
1
2 Na  Cl 2  2 Na Cl
Each sodium atom loses one electron:
1
0
Na  Na  e

Each chlorine atom gains one electron:
0

1
Cl  e  Cl
LEO says GER :
Lose Electrons = Oxidation
0
1
Na  Na  e

Sodium is oxidized
Gain Electrons = Reduction
0

1
Cl  e  Cl
Chlorine is reduced
Rules for Assigning Oxidation Numbers
Rules 1 & 2
1. The oxidation number of any uncombined
element is zero
2. The oxidation number of a monatomic ion
equals its charge
0
0
1
1
2 Na  Cl 2  2 Na Cl
Rules for Assigning Oxidation Numbers
Rules 3 & 4
3. The oxidation number of oxygen in
compounds is -2
4. The oxidation number of hydrogen in
compounds is +1
1
2
H2O
Rules for Assigning Oxidation Number
Rule 5
5. The sum of the oxidation numbers
in the formula of a compound is 0
1
2
H2O
2(+1) + (-2) = 0
H
O
2
2 1
Ca(O H ) 2
(+2) + 2(-2) + 2(+1) = 0
Ca
O
H
Rules for Assigning Oxidation Numbers
Rule 6
6. The sum of the oxidation numbers in the
formula of a polyatomic ion is equal to
its charge
? 2
N O3

? 2
S O4
2
X + 3(-2) = -1
N
O
X + 4(-2) = -2
S
O
 X = +5
 X = +6
Reducing Agents and Oxidizing Agents
The substance reduced is the oxidizing agent
The substance oxidized is the reducing agent
1
0
Na  Na  e

Sodium is oxidized – it is the reducing agent
0

1
Cl  e  Cl
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)
CrO42HNO3, concentrated
HNO3, dilute
H2SO4, hot conc
Metallic Ions
Free Halogens
HClO4
Na2O2
H2O2
Mn(II)
MnO2
Mn(II)
Cr(III)
Cr(III)
NO2
NO
SO2
Metallous Ions
Halide ions
ClOHO2
Redox Reaction Prediction
#2
Important Reducers
Formed in reaction
Halide Ions
Free Metals
Metalous Ions
Nitrite Ions
Sulfite Ions
Free Halogens (dil, basic sol)
Free Halogens (conc, basic sol)
C2O42-
Halogens
Metal Ions
Metallic ions
Nitrate Ions
SO42Hypohalite ions
Halate ions
CO2
Not All Reactions are Redox Reactions
Reactions in which there has been no change
in oxidation number are not redox rxns.
Examples:
1 5 2
1
1
1
1
1 5 2
Ag N O3 (aq)  Na Cl (aq)  Ag Cl ( s)  Na N O3 (aq)
1 2 1
1
6 2
1
6 2
1
2
2 Na O H (aq)  H 2 S O 4 (aq)   Na 2 S O 4 (aq)  H 2 O(l )