Download Chap. 4 AQUEOUS RXNS O

Chap. 4 AQUEOUS RXNS
4.1 WATER AS A SOLVENT
• Describe solution composition in terms of
molarity
• Water is a polar
molecule
– “Bent” shape
– Covalent bonds
– Electrons not shared
equally
• Describe strong and weak electrolyte solutions,
including acids and bases
• Use ionic equations to describe neutralization
and other metathesis reactions
• Determine concentrations by applying solution
stoichiometry to titrations
IONIC SOLUTE: ELECTROLYTE
+ _+
+ _+
+ _+
+ _+
nB-(aq)
+ _+
+ _+
+ _+
AB(s) → AB(aq)
+ _+
+ _+
+ _+
+ _+
+ _+
+ _+
+ _+
+ _+
+ _+
+ _+
+ _+
+ _+
+ _+
+ _+
+
H
• Solution does not conduct electricity
• Weak electrolytes: low concentrations of ions
ABn(s) →
δ+
• Solute dispersed but not dissociated in solution
• Solution conducts electricity
A+(aq)
O
• Molecular solute in polar or nonpolar solvent
• Solute dissociated to ions in solution
STRONG:
H
NON-IONIC SOLUTE: NONELECTROLYTE
• Ionic solute in polar solvent
+ _+
δ+
– Electrostatic
– Dipole
• Understand the key event in the redox process
Solvated ion
δ–
• Water overcomes
solute particleparticle interactions
WEAK:
AB(s) ↔ A+(aq) + nB-(aq)
NON-IONIC ACID SOLUTE
4.2 AQUEOUS IONIC REACTIONS
• Covalent compounds of H
• Many reactions of ions involve ionic compounds
dissolved (dissociated) in H2O
• H—X bonds polar
• Solvation by water leads to dissociation
• Not all ions participate in reactions
• Solvation leads to formation of hydronium ion
HX(g) → H+(aq) + X-(aq)
H+(ag) + H2O(l) →
H3O+(aq)
• EXAMPLE: The reaction of sodium hydroxide with
hydrogen chloride (hydrochloric acid)
–
+
NaOH(aq) + HCl(g) → NaCl(aq) + H2O
MOLECULAR
MOLECULAR EQUATION:
EQUATION: Reactants
Reactants and
and products
products
shown
shown as
as intact,
intact, undissociated
undissociated compounds
compounds
+
Page 4-1
4.3 Metathesis (Double Displacement)
Reactions: Precipitation
Molecular equation does not show what happens:
NaOH(s) → Na+(aq) + OH-(aq)
HCl(g) → H+(aq) + Cl-(aq)
AX + BY → AY + BX
Na+(aq) + OH-(aq) + H+(aq) + Cl-(aq) →
2NaF + CaCl2 → 2NaCl + CaF2
Na+(aq) + Cl-(aq) + H2O(l)
TOTAL
TOTAL IONIC
IONIC EQUATION
EQUATION
All
All soluble
soluble ionic
ionic substances
substances shown
shown as
as dissociated
dissociated ions
ions
SPECTATOR
SPECTATOR IONS
IONS
Do
Do not
not participate
participate inin formation
formation of
of product
product
H+(aq) + OH-(aq) → H2O(l)
Two
Two compounds
compounds react
react and
and exchange
exchange
components
components to
to form
form two
two new
new compounds
compounds
NET
NET IONIC
IONIC EQUATION
EQUATION
Molecular
Molecular equation
equation less
less spectator
spectator ions
ions
Formation of insoluble product
Aqueous media:
AX + BY →
A+(aq)
Y-(aq)
B+(aq)
X-(aq)
AgNO3 soln
AgNO3 soln
NaHCO3 soln
→ A+(aq) + Y-(aq) + BX↑↓
NaCl soln
NaNO3 soln
• Reactants dissociate in solution
HCl soln
• For reaction to proceed, IONS MUST BE
REMOVED FROM SOLUTION:
– Formation of an insoluble product
(volatile gas or solid precipitate)
Na+ + Ag+ + NO3-
– Formation of a soluble weak/nonelectrolyte
(water)
NO REACTION
precipitate
Na+ + Cl- + H2O +
CO2(g)
volatile product
4.4 Metathesis Reactions:
Table 4.3, p 119
SOLUBLE
SOLUBLE
Na+ + NO3- +
AgCl(s)
ANION
NO3C2H3O2ClBr- , I2SO
SO442-
INSOLUBLE
INSOLUBLE
Acid-Base Reactions
COMMENTS
COMMENTS
All
All soluble
soluble
All
All soluble
soluble
2+
+
2+
EXCEPT:
Hg222+,, Pb
Pb2+
EXCEPT: Ag
Ag+,, Hg
+, Hg 2+
2+
2+
EXCEPT:
Ag
,
Hg
2+
+
2
EXCEPT: Ag , Hg2 , Hg2+,, Pb
Pb2+
EXCEPT: Ca2+2+, Sr2+2+, Ba2+2+, Ag++, Hg2+2+,
Arrhenius Theory :
S2PO43OH
OH-
Anything that produces hydrogen
ions when dissolved in water
HCl → H+ + Cl-
BASE
Anything that produces hydroxide
ions when dissolved in water
NaOH → Na+ + OH-
EXCEPT: Ca , Sr , Ba , Ag , Hg , Pb
Pb2+
2+
[X] < 0.01 M
CO32-
ACID
+
EXCEPT:
EXCEPT: Group
Group 1A,
1A, 2A,
2A, NH
NH44+
+
EXCEPT:
EXCEPT: Group
Group 1A,
1A, NH
NH4 +
4
+
EXCEPT:
EXCEPT: Group
Group 1A,
1A, NH
NH44+
2+
2+
2+
EXCEPT:
EXCEPT: Group
Group 1A,
1A, Ca
Ca2+,, Sr
Sr2+,, Ba
Ba2+
Page 4-2
Complete dissociation in water
STRONG
HX (X = Cl, Br, I), HNO3, HClO3,
ACIDS
HClO4, H2SO4
WEAK
ACIDS
Dissociation
STRONG ACIDS AND BASES
HCl(g) + H2O(l) → H3O+(aq) + Cl-(aq)
NaOH(s) + H2O(i) → Na+(aq) + OH-(aq)
Partial dissociation in water
HF and most other acids
Dissociation in water to yield OHSTRONG
Group 1A, 2A metal hydroxides:
BASES
NaOH, Ca(OH)2
WEAK
BASES
WEAK ACIDS AND BASES
HCN(l) + H2O(l) ↔ H3O+(aq) + CN-(aq)
NH3(g) + H2O(l) ↔ NH4+(aq) + OH-(aq)
Weak H+ acceptors (from H2O)
NH3, C6H5NH2
• Metal hydroxides react with (NEUTRALIZE)
acids to produce the salt of the acid and H2O:
Acid-Base Titrations
HBr + NaOH → NaBr + H2O
or
H+(aq) + OH–(aq) → H2O(l)
Weight
Weight FORMULA
A
A
WEIGHT
• Weak bases react with acids to produce the
salt of the acid:
Moles
Moles EQUATION Moles
Moles
A
A COEFFICIENT BB
SOLUTION
MOLARITY
NH3 + HCl → NH4Cl
Volume
Volume
Solution
Solution AA
Not really a net metathesis reaction, by the way:
FORMULA
WEIGHT
Weight
Weight
BB
SOLUTION
MOLARITY
Volume
Volume
Solution
Solution BB
NH3 + H+ → NH4+
Cl–
is a spectator ion
4.5 Oxidation-Reduction (Redox)
Reactions
Titrations
V0
Addition of a standard soln of
known concentration to a soln
of unknown concentration until
stoichiometrically equivalent
quantities have been reacted
(equivalence or end point)
Ca(s) + 2H+(aq) → Ca2+(aq) + H2(g)
Vf
OXIDATION — a reactant loses electron(s)
Half-reaction: Ca0 → Ca2+ + 2e-
STANDARD
SOLN of AX
REDUCTION — a reactant gains electron(s)
VAX = V0 - Vf
Half-reaction: H+ + e- → ½H2
AX + BY → AY + BX
MAX × VAX =
equivalents of AX added
Electrons
Electrons are
are transferred
transferred from
from one
one reactant
reactant
to
to aa second
second reactant
reactant
Oxidation
Oxidation is
is always
always accompanied
accompanied by
by reduction
reduction
VBY
Page 4-3
Examples
OXIDIZING AGENT
• Causes oxidation
• Is reduced
• Gains electrons
(becomes more negative)
OXIDATION
(LOSE ELECTRONS, become MORE POSITIVE):
REDUCING AGENT
• Causes reduction
• Is oxidized
• Loses electrons
(becomes more positive)
E0 → En+ + ne–
Zn → Zn2+ + 2e–
Em+ → E(m+n)+ + ne–
Fe2+ → Fe3+ + e–
En–
N3– → ½N2 + 3e–
→
E0
+
ne–
REDUCTION
(GAIN ELECTRONS, become MORE NEGATIVE):
Ca + 2H+ → Ca2+ + H2
E0 + ne– → En–
½O2 + 2e–→ O2–
Em+
→
E(m–n)+
Fe3+ + e– → Fe2+
→
E0
Ag+ + e–→ Ag
En+
2e–
+
ne–
+
ne–
Oxidation Number Rules
Oxidation Numbers (States):
Monitoring Electron Transfer
1. For any atom in its elemental form, O.N. = 0
2. For any monatomic ion, O.N. = ionic charge
(K+, O.N. = +1; S2–, O.N. = –2)
• The charge that an atom (X) in an XY bond
would have if its shared electrons were held
completely by the atom that attracts the
electrons more strongly
3. The O.N. for hydrogen is +1, with the sole
exception of metal hydrides (MHn), where
O.N. = –1
• The oxidation number (O.N.) of the oxidizing
agent DECREASES in a redox reaction
4. The O.N. for oxygen is –2, with the sole
exception of peroxides (O22–), where O.N. =
–1
• The O.N. of the reducing agent INCREASES in
a redox reaction
5. The O.N. for other nonmetals is <0, with the
exception of oxyanions, where O.N. > 0
6. The sum of all O.N. in a neutral compound is
0, otherwise ΣO.N. = ion charge
Decomposition
4.6 REDOX Reaction Types
A → B + C
• COMBINATION REACTIONS:
Two or more reactants form a single product
2H2O2 → 2H2O + O2
• DECOMPOSITION REACTIONS:
A single reactant forms two or more products
• DISPLACEMENT (SINGLE REPLACEMENT)
REACTIONS:
Reactant atoms or ions exchange places
O: -1 → 0
O: -1 → -2
Page 4-4
Displacement (Single)
Combination
A + BX → AX + B
A + B → C
H2 + CuO → Cu + H2O
2Mg + O2 → 2MgO
H: 0 → +1
Cu: +2 → 0
O: 0 → -2
Mg: 0 → +2
Can
Can we
we predict
predict which
which elements
elements will
will
displace
displace another
another element?
element?
• Nonmetals, particularly
halogens, have different
strengths as oxidizing
agents
• Redox reactants can be
organized by relative
strength in ACTIVITY
SERIES, allowing
prediction of a reaction’s
occurrence
Zinc
Bromine
½Br2 + e– → Br–
Iodine
½I2 + e– → I–
Zn → Zn2+ + 2e-
• • •
Lead
Hydrogen
Copper
Pb → Pb2+ + 2e½H2 → H+ + eCu → Cu2+ + 2e-
• • •
Gold
More easily reduced
½F2 + e- → F–
½Cl2 + e– → Cl–
Al → Al3+ + 3e-
• • •
REDUCTION
REACTION
Fluorine
Li → Li+ + eK → K+ + e-
• • •
• Redox activity is also
reflected in the O.N.s of
elements in covalent
compounds
Chlorine
OXIDATION REACTION
Lithium
Potassium
Aluminum
– A more active halogen
oxidizes a less active
halogen
More easily oxidized
HALOGEN
– A more active metal
reduces a less active
metal
METAL
Implication for O.N.:
Br—Cl
+1 –1
Page 4-5
Au → Au3+ + 3e-
More easily reduced
• In general:
• Metals have different
strengths as reducing
agents
More easily oxidized
Table 4.14, p 130
Activity Series