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CHEMISTRY
- science of properties and transformations of matter
CHEMICAL REACTIONS - central to CHEMSITRY
CLASSIFICATION of REACTIONS (Kotz, Ch. 4)
Precipitation
Oxidation-reduction
Acid-base
Gas forming
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OXIDATION-REDUCTION REACTIONS
Cu (s) + 2 AgNO3 (aq)  Cu(NO3)2 (aq) + 2 Ag(s)
(see Fig. 4.17 in Kotz)
Transfer of electrons:
Cu0 gives 2 electrons to 2 Ag+ to form Cu2+ and 2 Ag0
species that provides electrons …….. called the REDUCING AGENT
species that accepts electrons …….. called the OXIDIZING AGENT
species that loses electrons is said to have been OXIDIZED
species that gains electrons is said to have been REDUCED
Chemists use the concept of OXIDATION NUMBER
to recognize OXIDATION-REDUCTION reactions
Here
Cu in Cu (s)
O.N. = 0
Cu in Cu(NO3)2
O.N. = +2
Ag in AgNO3
O.N. = +1
Ag in Ag(s)
O.N. = 0
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RULES for OXIDATION NUMBERS
O.N.
(see Kotz, p. 191)
•
Atoms in pure ELEMENT (Cu, I2, S8)
•
Ions of single atom (Al#+)
•
Fluorine in compounds (NaF)
-1
•
other halogens in compounds (KCl, CsI)
-1
0
charge on ion
– EXCEPT when combined with F or O where O.N. = +1
•
hydrogen in most compounds (HCl, HI)
+1
– EXCEPT metal hydrides (CaH2) where O.N. = -1
•
oxygen in most compounds (MgO, Na2O)
-2
– EXCEPT in peroxides (H2O2) where O.N. = -1
•
the O.N. of all other elements in a compound are determined by
requiring that
SUM of O.N. of all elements = CHARGE on the compound
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EXAMPLES:
What is O.N. of S in H2SO4 ?
• H has an O.N. of
• O has an O.N. of
• 2 * O.N. (H) + 4 * O.N. (O) =
- overall charge of molecule is 0
therefore O.N. of S in H2SO4 is :
What is O.N. of N in HNO3 ?
• H has an O.N. of
• O has an O.N. of
• O.N. (H) + 3 * O.N. (O) =
- overall charge of molecule is 0,
therefore O.N. of N in HNO3 is :
this shows the nitrate group: “ NO3 “ has a net O.N. of
(and thus a charge) of
THUS the O.N. of Ag in AgNO3 is
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Another example of an OXIDATION-REDUCTION REACTION
Fe2O3 (s) + 3 CO (g) 
Species Fe in Fe2O3
O.N.
+3
C in CO
+2
2 Fe (s) + 3 CO2 (g)
Fe
0
C in CO2
+4
Thus:
Fe in Fe2O3 GAINS 3 electrons - Fe2O3 is the OXIDIZING AGENT
C in CO
LOSES 2 electrons - CO
is the REDUCING AGENT
The iron in Fe2O3 was REDUCED to Fe
The carbon in CO was OXIDIZED to CO2
NOTES:
• oxidation-reduction reactions DO NOT require ions; can involve gases
• OXIDATION NUMBERS do not represent actual charge
- rather they are a convenient book-keeping device which help to
identify and classify reactions
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ACID-BASE REACTIONS
(see alsoKotz Ch 17 - sections 1 to 5 (pp. 794-804)
TYPES OF ACIDS/BASES
• Arrhenius:
– ACID - substance that donates H+ in water
HCl  H+(aq) + Cl- (aq)
– BASE - substance that donates OH- in water
NaOH  Na+ (aq) + OH- (aq)
– SALT - ionic product of an ACID - BASE reaction, composed of a
+ve CATION from the base
and a -ve ANION from the acid
HCl (aq) + NaOH (aq) 
ACID
+ BASE

NaCl (aq) + H2O
SALT +
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water
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Brnsted - Lowry: ( sect 17.3, p. 707)
ACID - substance that donates H+ to another species
HCO3- (aq)  H+(aq) + CO32- (aq)
BASE - substance that accepts H+ from another species
NH3 + H2O  NH4+ (aq) + OH- (aq)
TYPICAL PROPERTIES of ACIDS and BASES
ACIDS
• sour tase
•corrosive
•reacts with bases
•turns natural dyes red
•generates CO2 from limestone
•generates H2 with metals
BASES
•soapy feel
•restores natural dyes to blue
•reacts with acids
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CONJUGATE ACID-BASE PAIRS
A pair of compounds that differ in composition by one H+
- H+
+ H+
HBr (aq) + NH3 (aq)  NH4+ (aq)
+
ACID
BASE
conjugate ACID
of NH3
Br- (aq)
conjugate BASE
of HBr
ALL ACID-BASE REACTIONS involve
TWO CONJUGATE ACID-BASE PAIRS
Acid(1)
+
HCl
+
H2 O
+
CH3CO2H +
Base(2)

NH3
NH3
H2 O



Conj-Acid(2) +
NH4+
NH4+
H3 O+
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+
+
+
Conj-Base(1)
ClOHCH3CO28
RELATIVE STRENGTHS of ACIDS and BASES
STRONG ACIDS - react completely with water to form H3O+ (aq)
HCl (aq) + H2O  H3O+ (aq) + Cl- (aq)
STRONG BASES - react completely with water to form OH- (aq)
Li2O +
H2O  2 Li+ (aq) + OH- (aq)
Weak ACIDS/ weak BASES only react partially with water
- an EQUILIBRIUM is formed : the conjugate ACID and the
conjugate BASE are both present at the same time
WEAK ACID: (acetic acid a.k.a. vinegar)
CH3CO2H + H2O  CH3CO2- (aq) + H3O+ (aq)
WEAK BASE:
NH3 (g)
+ H2O  NH4+ (aq)
+ OH- (aq)
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Relative Strengths of acids/bases depend on the solvent :
H3O+ is the STRONGEST ACID
OH- is the STRONGEST BASE
Once an acid or base is fully reacted (dissociated) to form these species,
one can no longer distinguish relative strengths. This is called the
LEVELLING ACTION of water
(The relative strengths of two strong acids can only be determined in
non-aqueous solvents)
In water:
STRONG ACIDS:
WEAK ACIDS:
HCl, H2SO4, H3PO4, HNO3, HClO4
CH3CO2H, “H2CO3” (= CO2), HS, HCN, HF
WEAK BASES:
NH3, Na2CO3
STRONG BASES:
NaOH, Li2O, NaH, Ca(OH)2
Kotz, Table 17.3
In Br nsted acid-base reactions - H+ transfer occurs from STRONGER to
WEAKER congugate acid-base pair
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GAS FORMING REACTIONS
- some reactions generate gases. When carried out in an OPEN system,
the escape of the gas acts as a DRIVING FORCE for the chemical reaction
- loss of gas product pulls reaction to completion just as formation of
a precipitate
e.g.
CaCO3 (s) + 3 HCl (aq) 
Zn (s) + 2 HCl (aq)

CaCl2 (aq) + H2O (l) + CO2 (g) 
ZnCl2 (aq) + H2 (g) 
NB. Each of these reactions can also be classified as one of the other
3 types of reactions - WHICH ONES ?
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GAS FORMING REACTIONS - alternative classification
A.
CaCO3 (s) + 3 HCl (aq) 
CaCl2 (aq) + H2O (l) + CO2 (g) 
There is no precipitate and no change in O.N. (Ca stays as +2, Cl as -1)
It is an ACID-BASE REACTION involving an intermediate H2CO3
called CARBONIC ACID, which immediately dissociates to H2O and CO2
2 H+ + CO32-  “H2CO3 “  H2O + CO2 
B.
Zn (s) + 2 HCl (aq)

ZnCl2 (aq) + H2 (g) 
There is no precipitate. However there is an obvious change in O.N.
It is also an example of an OXIDATION-REDUCTION REACTION
Zn goes from O.N. = 0 in the metal to O.N. = +2 in the chloride salt
H goes from O.N. = +1 in HCl (aq) to O.N. = 0 in the elemental gas
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CHEMISTRY of HALOGENS
The Elements
• Group 7A - all non-metals
• diatomc molecules: F2 (gas), Cl2 (gas), Br2 (liquid), I2 (solid)
• most stable oxidation state: -1
• Undergo OXIDATION-REDUCTION reactions readily
with metals (all elements on right and middle of P.T.)
Trends:
SIZE:
Electron affinity:
Oxidizing strength
I > Br > Cl > F
F > Cl > Br > I
F > Cl > Br > I
Compounds
most stable form: HALIDES (A+X-)
hydrogen halides: ACIDS: HCl (strong), others (weak)
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CHLORINE
prepared by electrochemical reaction (chlor-alkali process)
ELECTROLYSIS
2 NaCl + H2O -------> Cl2 + 2 NaOH
•
8th most important industrial chemical by mass (1010 kg/yr)
IMPORTANT USES
•
disinfectant of water (as hpyochlorous acid (HOCl(aq))
Cl2 + 2 H2O  H3O+ + Cl- + HOCl (aq)
•
bleach for textiles, paper, etc (as hypochlorite (OCl-)
Cl2 + 2 OH-  H2O + Cl- + OCl- (aq)
•
organochlorine compounds: plastics, drugs
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CHLORINE: BANE or BOON ???
Kotz, pp 88,89
Consumer friend . . . .
. . . or environmental foe ??
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SUMMARY: Key Concepts from “Types of Reactions” (Kotz Ch. 4)
•Classes of reactions:
precipitation
oxidation-reduction
acid-base
gas-forming
•Ionic versus non-ionic compounds
•Solubility
guidelines: Kotz, p. 168
•Oxidation Number
rules:
Kotz, p. 191
•Acid/base types: Arrhenius, Br nsted- Lowry
• Typical chemistry of halogens
•Some reactions PROCEED TO COMPLETION
e.g. precipitation, gas forming, strong acid/strong base
•Some reactions establish an EQUILIBRIUM of reactants and products
e.g. oxidation-reduction, weak acids, weak bases
LEARN TO BALANCE CHEMICAL EQUATIONS
- both MASS and CHARGE balance are important
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CHEMISTRY of DEMONSTRATIONS
Oxidation-Reduction
iron and chlorine:
2 Fe (s) + 3 Cl2 (g)  2 FeCl3 (s) (black)
sugar and sulfuric acid:
2C6H12O6 (s) + 12 H2SO4 (aq) 
6C (s) + 6 CO2 (g)  + 24 H2O (l) + 12 SO2 (g)
Acid-Base
acid and limestone:
2 H+ (aq) + CaCO3 (s)  Ca2+ (aq) + CO2 (g)  + H2O(l)
acid and metal:
2 H+ (aq) + Zn (s)
 Zn2+ (aq) + H2 (g)
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