Transcript Chapter 1 – Matter and Measurement

Chemical Reactions
Goals:
1. Write balanced equations for simple chemical
reactions.
2. Understand the nature of ionic compounds dissolved
in water.
3. Recognize common acids and bases and understand
their behavior in aqueous solution.
4. Recognize and write equations for the common types
of reactions in aqueous solution and net-ionic
equations.
5. Recognize common oxidizing and reducing agents and
identify oxidation-reduction reactions.
Chemical Reactions
• A stream of chlorine gas is directed onto solid
phosphorus.
• The mixture bursts into flame, and a chemical
reactions produces liquid phosphorus trichloride.
What are Chemical Equations?
Depict the kind of reactants and
products and their relative amounts in a
reaction.
P4 (s) + 6 Cl2 (g) - - - > 4 PCl3 (l)
reactants
products
The numbers in the front are called
stoichiometric coefficients.
The letters (s), (g), and (l) are the physical
states of compounds.
Chemical Equations
P4 (s) + 6 Cl2 (g) - - - > 4 PCl3 (l)
This equation means
1 molecule of solid phosphorus (P4) reacts
with 6 molecules of chlorine gas producing 4
molecules of phosphorus trichloride.
or
___ mol of phosphorus reacts with ___
moles of chlorine producing ___ moles of
phosphorus trichloride.
What is the Law of Conservation
of Matter?
• Because the same atoms are
present in a reaction at the
beginning and at the end, the
amount of matter in a system does
not change.
• Because of the principle of the
conservation of matter, an
equation must be balanced.
It must have the same number of
atoms of the same kind on both
sides.
Lavoisier, 1788
Law of Conservation of Matter
We must lay it down as an incontestable
axiom, that in all the operations of art and
nature, nothing is created; an equal quantity
of matter exists both before and after the
experiment. …. Upon this principle, the whole
art of performing chemical experiments
depends.
– Lavoisier’s Traité Élémentaire de Chimie.
Law of Conservation of Matter
and Chemical Equations
6 x 2 = 12 Cl atoms
4 x 3 = 12 Cl atoms
P4 (s) + 6 Cl2 (g) - - - > 4 PCl3 (l)
4 P atoms
4 P atoms
• The numbers in front of each formula – stoichiometric
coefficients - in a balanced chemical equation are
required by the law of conservation of matter.
Students should become familiar with
writing and balancing chemical equations.
Write a reaction for Iron reacting with
Chlorine producing Iron (III) chloride
Notice the stoichiometric coefficients
and the physical states of the
Visualizing Chemical Reactions
•
The reaction between reactant A (red) and reactant
B (blue) is shown in the diagram:
Based on this diagram, which equation best
describes the reaction?
a) A2 + B  A2B
b) 2 A + B4  2 AB2
c) A2 + 4 B  2 AB2
d) A + B2  AB2
What is an Aqueous Solution?
• Aqueous solution: an homogenous mixture
where the solvent is water.
KMnO4 in water
K+(aq) + MnO4-(aq)
How can Water be a solvent for
Ionic Compounds?
d
d
Water is a polar molecule: it has
a ___________ (one side of
the molecule is +, the other is -).
This is due to the difference in
_________________ (the ability of
an atom in a molecule to attract
electrons to itself) between oxygen
and hydrogen.
O is more electronegative than H:
O appears partially ________
charged (d-), and H partially
__________ charged (d+).
How do we know ions are
present in aqueous solutions?
The solutions conduct
_______________!
The substances that provide ions
in an aqueous solution are called
__________________.
HCl, CuCl2, and NaCl are
_____ _________.
What is a Strong Electrolyte?
They ionize or dissociate completely (or
nearly so) into ions.
HCl, CuCl2, and NaCl are strong
electrolytes.
HCl (aq)
H+(aq)
+
Cl-(aq)
What is a Weak Electrolyte?
Acetic acid ionizes only to a
small extent, so it is a
electrolyte.
CH3CO2H(aq)
weak
CH3CO2-(aq) + H+(aq)
What is a Nonelectrolyte?
Some compounds
dissolve in water but
do not conduct
electricity. They are
called
nonelectrolytes.
Examples include:
sugar
ethanol
ethylene glycol
Summary of the Electrolytic Behavior
of Common Soluble Ionic and
Molecular Compounds
Strong
Electrolyte
Ionic
All
Weak
Electrolyte
None
Molecular Strong acids Weak acids
and
Weak bases
(NH3)
Nonelectrolyte
None
All others
Which Ionic Compounds are
Soluble in Water?
If one ion from the “Soluble Compd.”
list is present in a compound, the
compound is water soluble.
Book Ed 7:
Figure 3.10 (p. 126)
Book Ed 6:
Figure 5.3 (p. 179)
Learn this
table!
Solubility of Ionic Compounds
What is an Acid?
An acid - - ->
+
H
PROTON
(in water)
Because a hydrogen atom is one proton and one
electron, a hydrogen ion (H+) is simply a proton.
Some strong acids
(completely ionized) are:
HCl
hydrochloric
H2SO4
sulfuric
HClO4
perchloric
HNO3
nitric
HNO3
What is an Acid?
An acid ---> H+ in water
HCl(aq)
H+(aq) + Cl-(aq)
What is the Hydronium Ion?
HCl(aq)
H+(aq) + Cl-(aq)
HCl
H 2O
Cl-
hydronium
ion
H3O+
Bronsted – Lowry Theory:
An acid donates a proton
What is a Weak Acid?
CH3CO2H (aq)
H+(aq) + CH3CO2- (aq)
WEAK ACIDS (only partially ionized)
= weak electrolytes
CH3CO2H
H2CO3
H3PO4
HF
acetic acid
carbonic acid
phosphoric acid
hydrofluoric acid
Weak Acid
What is a Base?
Base ---> OH- in water
HYDROXIDE
NaOH(aq)
NaOH is
a strong
base
---> Na+(aq) + OH-(aq)
How is it that Ammonia, NH3
is a base?
Bronsted – Lowry Theory:
A base accepts a proton
Acids and Bases
Book Ed 7: p. 132
Book Ed 6: p. 187
Students should become familiar with
identifying acids and bases.
Representative Trends in Strength of Binary
Acids (containing only H atoms and a nonmetal)
Higher electronegativity
difference of atoms more polar bond
Smaller atom bonded to H
Shorter bond
Weaker acid
base
weak acid
neutral
strong acid
Larger atom bonded to H
Longer bond
Stronger acid
The Bronsted-Lowry Theory
• 1923 – J. N. Bronsted and T. M. Lowry:
•
Acids – a proton (H+) ________.
•
Bases – a proton (H+) _________.
HCl (g) + H2O (l)
•
H3O+ (aq) + Cl- (aq)
A reaction between an acid and a base consists
of a proton transfer from an acid to a base.
Conjugate Pairs
• The products of an acid-base reaction are also an acid
and a base. The overall reaction consists of two
combinations of acids and bases – conjugates pairs.
CH3COOH
acid (1)
+
H2O
base (2)
CH3COO- +
base (1)
H3O+
acid (2)
• The acid-base conjugate pairs differs in structure
only by a proton (H+):
– The conjugate acid of a specie is that specie plus _______.
– The conjugate base of a specie is that specie minus
__________.
Conjugate Pairs
NH3
+
base (1)
H2 O
acid (2)
NH4+ +
acid (1)
OH-
base (2)
Ammonia (NH3) acts as a base by accepting a
proton from H2O. In the reverse reaction,
ammonium ion (NH4+) acts as acid by losing a
proton to OH-.
Amphiprotic – a substance that can act ________
__________________________
Dual Role of Water
• Amphiprotic – a substance that can act
either as an acid or a base – like H2O.
pair of e- in bond
are kept by O atom
lone pair of eforms new bond
Water Equilibrium
• Self-ionization of water: about 1 water molecule in
500 million transfer a proton to another water
molecule:
H2O + H2O
H3O+ + OH-
• Water is in equilibrium with a hydronium ion and a
hydroxide ion (or using our convention with a
hydrogen ion and a hydroxide ion).
H2O
H+ + OH-
Water Equilibrium
• Equilibrium lies far to the left (it is mostly water
molecules). Experimentally determined
concentrations of ions (25oC) in molarity (moles per
liter) are:
[H+] = [OH-] = 1.0 x 10-7 M (= 0.0000001 M)
• Ion product of water (Kw) – product of the
concentrations of H+ and OH-, at 25oC is:
Kw = [H+] [OH-] = (1.0 x 10-7) (1.0 x 10-7) = 1.0 x 10-14
• The ion product relationship applies to all aqueous
solutions (acids, bases, salts).
What are Basic Oxides?
(Basic Anhydrides)
Metal oxides react with water to form
bases
CaO(s) + H2O(liq) -- > Ca(OH)2(aq)
CaO in water. Indicator
shows solution is basic.
What are Acidic Oxides?
(Acidic Anhydrides)
Nonmetal oxides react with
water to form acids
CO2(aq) + H2O(l) ---> H2CO3(aq)
SO3(aq) + H2O(l) ---> H2SO4(aq)
And can come from burning coal
and oil.
= ACID RAIN
What is a Net Ionic Equation?
Mg(s) + 2 HCl(aq) --> H2(g) + MgCl2(aq)
We really should write
Mg(s) + 2 H+(aq) + 2 Cl-(aq) --->
H2(g) + Mg2+(aq) + 2 Cl-(aq)
The two Cl- ions are ____________ IONS —
they do not participate (could have used NO3-).
We leave the spectator ions out — to write a
NET IONIC EQ:
Mg(s) + 2 H+(aq) ---> H2(g) + Mg2+(aq)
Review in Exp 8: Six bottles experiment
Which Types of Reactions occur in
Aqueous Solution?
• EXCHANGE REACTIONS
The anions exchange places
between cations:
Precipitation
Acid-base; neutralization
Gas-forming
Driving force
Formation of ________
compound
Formation of _____ and
______; proton transfer
Evolution of _________
____________
• OXIDATION-REDUCTION REACTIONS
Electron Transfer.
What are Precipitation Reactions?
The “driving force” is the
formation of an insoluble
compound — a precipitate.
Pb(NO3)2(aq) + 2 KI(aq)
-----> 2 KNO3(aq) + PbI2(s)
Net ionic equation:
Net Ionic Equation
Complete formula equation:
Pb(NO3)2 (aq) + 2 KI (aq)
-----> 2 KNO3 (aq) + PbI2 (s)
Ionic equation:
Net ionic equation:
What are Acid-Base Reactions?
• The “driving force” is the formation of water.
NaOH(aq) + HCl(aq) ---> NaCl(aq) + H2O(liq)
• Net ionic equation
OH-(aq) + H+(aq) ---> H2O(liq)
This applies to ALL reactions of
STRONG acids and bases.
Acid-Base Reactions
Complete formula equation:
HCl (aq) + NaOH (aq)
products do not have
characteristics of
acid or base.
Ionic equation:
Net ionic equation:
For strong acids and bases write the ionic form.
Write the equations for ammonia added to
phosphoric acid to make ammonium phosphate.
Complete formula equation:
3 NH3 (aq) + H3PO4 (aq)
Ionic equation:
Net ionic equation:
For weak acids and bases do not write the ionic form.
Acid-Base Reactions
• Acid-Base reactions are sometimes called
NEUTRALIZATIONS because the solution is
neither acidic nor basic at the end.
• The other product of Acid-Base reactions is a SALT,
with general formula: MX.
HX + MOH ---> MX + H2O
Salt
Water
Mn+ comes from base & Xn- comes from acid
This is one way to make compounds!
What are Gas-Forming Reactions?
This is primarily the chemistry of metal
carbonates.
CO2 and water ---> H2CO3
H2CO3(aq) + Ca2+ ---> 2 H+(aq) + CaCO3(s)
(limestone)
Adding acid reverses this reaction.
MCO3 + acid ---> CO2 + H2O + salt
carbonates
carbon dioxide
Gas-Forming Reactions
CaCO3(s) + H2SO4(aq) --->
2 CaSO4(s) + H2CO3(aq)
Carbonic acid is unstable and forms CO2 & H2O
H2CO3(aq) ---> CO2+ H2O
(Antacid tablet
has citric acid + NaHCO3)
Practice
• Write down the complete formula, ionic, and
net ionic equations for:
Sulfuric acid reacts with calcium carbonate.
Students should become familiar with
writing ionic and net equations.
Gas-Forming Reactions
Students should become familiar with identifying Book Ed 7. p. 140
the several types of reactions and predicting Book Ed 6: p. 194
products of the reactions.
Redox Reactions
Batteries
Corrosion
Manufacturing metals
Fuels
Photosynthesis
6 CO2 + 6 H2O
C6H12O6 + 6 O2
Combustion, respiration/metabolism
C6H12O6 + 6 O2
6 CO2 + 6 H2O
Oxidation and Reduction
• Oxidation is a ________________.
• Reduction is a __________________.
Mg + Cl2
Mg2+ + 2 Cl-
Mg atom _____________; it is __________.
Cl atoms _____________; Cl2 is ___________.
LEO says GER
What are Redox Reactions?
• An oxidation-reduction reaction - must always
have both an oxidation and a reduction.
Oxidation process – the oxidation number of
one or more elements ___________.
Reduction process – and the oxidation
number of one or more elements _________.
Mg (s) + Cu2+(aq)
Mg2+(aq) + Cu (s)
Half- Reactions
• A redox reaction is two half-reactions
that occur simultaneously.
reduction
0
Mg (s) +
+2
+2
Cu2+(aq)
0
Mg2+(aq) + Cu (s)
oxidation
Oxidation half-reaction:
Mg atoms lose 2 eReduction half-reaction:
Cu2+ ions gain 2-
Mg (s)
Mg2+(aq) + _____
Cu2+(aq) + _____
Cu (s)
Hydrogen-Oxygen Fuel Cell
Hydrogen-Oxygen Fuel Cell
2 H2 + O2
2 H2O
• Fuel cell - reactants are supplied
continuously from an external source.
• Cars can use electricity generated by
H2/O2 fuel cells.
• H2 carried in tanks or generated
from hydrocarbons– (if possible
solar)
2 H2
O2 + 4 e4 H+ + 2 O-2
2 H2 + O2
4 H+ + 4 e2 O-2
2 H2O
2 H2O
Identifying Redox Reactions
• Look for a change in the oxidation number
of an element in the course of the reaction.
What are oxidation numbers?
Is the charge an atom has, or appears to
have, as determined by the following
guidelines for assigning oxidation numbers.
Oxidation Numbers in HF: Animation
Students should become familiar with
assigning OXIDATION NUMBERS
– learn the rules. (Page 200)
Follow the rules for Oxidation Numbers
1.
2.
3.
4.
5.
6.
Each atom in a pure element has an oxidation number of
zero. Cu, I2, S8.
For monoatomic ions, the oxidation number is equal to the
charge on the ion. NaBr, MgBr2.
Fluorine always has an oxidation number of -1 in
compounds with all other elements.
Cl, Br, and I always have oxidation number of -1 in
compounds, except when combined with oxygen or
fluorine. NaCl, ClO-.
The oxidation number of H is +1 and of O is -2 in most
compounds. Exceptions: H in a binary compound with a
metal, CaH2. O in peroxides can have an oxidation number
of -1, H2O2.
The algebraic sum of the oxidation numbers for atoms in
a neutral compound must be zero; in a polyatomic ion, the
sum must be equal to the ion charge. HClO4
Give the Oxidation Number of each Element in
the following:
Br2
Na2SO4
CaH2
Fe2O3
P4
H2O2
KMnO4
What is Oxidizing Agent?
What is Reducing Agent?
• OXIDIZING AGENT:
The substance that accepts electrons in
an oxidation-reduction reaction and gets
reduced.
• REDUCING AGENT:
The substance that donates electrons in
an oxidation-reduction reaction and gets
oxidized.
Oxidizing and Reducing Agents
2 K (s)
+ 2 H2O (l)
2 KOH (aq) + H2 (g)
Common Oxidizing and Reducing
Agents (Table 5.4, p. 203)
Metals (Cu)
are reducing
agents
HNO3 is an
oxidizing agent
Cu + HNO3 --> Cu2+ + NO2
O2, F2, Cl2, Br2, I2, Cr2O2-, MnO4-
Metals (Na,
K, Mg, Fe)
are
reducing
agents
2 K + 2 H2O --> 2 KOH + H2
H2, Na, K, Fe, Al, C
Students should be familiar with common
oxidizing and reducing agents.
Oxidizing Agents
• Oxygen - Oxidizes coal in electric power plants, gasoline in
automobiles, “burns” the food we eat.
• Potassium dichromate - Lab: aldehydes to ketones; ethanol to
acetaldehyde.
Cr2O72- + 3 C2H5OH + 8 H+
2 Cr3+ + 3 C2H4O + 7 H2O
acetaldehyde
• Potassium permanganate - Lab: color test for many oxidizable
substances (purple color disappears).
MnO4- + 5 Fe2+ + 8 H+
Mn2+ + 5 Fe3+ + 4 H2O
• Halogens and Peroxides
– Antiseptics – substance applied to living tissue to kill
microorganisms or prevent their growth: Iodine, Hydrogen
peroxide (3% sol), Benzoyl peroxide.
– Disinfectants – substance that is applied to nonliving tissue to kill
microorganisms: Chlorine from [Ca(OCl)2].
Reducing Agents
• Carbon - To obtain free metals from their ores.
SnO2 (s) + 8 C (s)
Sn (s) + CO2 (g)
• Metals - To obtain free metals from their ores.
Cr2O3 (s) + 2 Al (s)
Al2O3 (s) + 2 Cr(s)
• Hydrogen - To obtain free metals from their ores and to reduce many
kinds chemical compounds.
WO3 (s) + 3 H2 (g)
W (s) + 3 H2O
N2 (g) + 3 H2 (g)
• Organic Compounds
Fe
2 NH3 (g)
– Hydroquinone: it reduces silver ions in photographic films
C6H4(OH)2 (aq) + 2 Ag+ (aq)
C6H4O2 (aq) + 2 Ag (s) + 2 H+
– Antioxidants – in food chemistry
Vitamin C (ascorbic acid) prevents the browning of fruit by inhibiting air oxidation.
Vitamin E (tocopherol) - Both vitamins retard oxidation reactions that damage vital
components of living cells.
Remember
• Go over all the contents of your
textbook.
• Practice with examples and with problems
at the end of the chapter.
• Practice with OWL tutor.
• Work on your assignment for OWL
Chapters 4, 5.
• Practice with the quiz on CD of
Chemistry Now.