Download Week 7

Chem 115: Week 7
Dr. Babb
Normality
A concentration unit similar to molarity.
Normality =
Ex.
Equivalent: (eq):
Ex.
equivalents of solute = N
liter of solution
0.75 N HCl 0.75 eq. HCl/1 L soln.
Amount of acid that furnishes one mole of H+ (i.e. 1 eq=1 mol H+).
OR
Amount of base that furnishes one mole of OH- (i.e. 1 eq=1 mol OH-)
1 mol HCl = 1 mol H+ = 1 eq.
1 mol H2SO3 = 2 mol H+ = 2 eq.
1 mol NaOH = 1 mol OH- = 1 eq.
1 mol Al(OH)3 = 3 mol OH- = 3 eq.
Equivalent Weight/Mass: Mass (in g) of substance that furnishes one equivalent.
(Similar to molar mass but has units of g/eq instead of g/mol)
Principle of Equivalence: One equivalent of an acid reacts with one equivalent of base to produce
one equivalent of each product.
Why?
The net ionic eqn: H+ + OH- H2O shows that 1 mol H+ (1 eq acid)
reacts with one mol OH- (1 eq. base).
Examples:
A.
Convert 3 M H2SO4 to normality and convert 0.1 N Ca(OH)2 to molarity.
B.
In 800. mL of 0.12 N Al(OH)3, what mass of Al(OH)3 is present?
C.
Calculate the equivalent weights for H2SO4 and Al(OH)3.
D.
What volume (in mL) of 6.0 N H2SO4 is needed to neutralize 200. mL of 0.34 N
Ca(OH)2?
E.
A 1.006 g sample of a solid acid was dissolved in distilled water. Titration of this
solution required 23.81 mL of 0.500 N NaOH. Calculate the equivalent weight of
the solid acid.
1
Chem 115: Week 7
Dr. Babb
Redox Reactions
Redox Reaction: reaction accompanied by the transfer of electrons between
reactants. The processes of oxidation and reduction occur at the
same time during a redox reaction.
To identify if charge (or oxid. #) changes on going from reactants to products,
then is a redox rxn.
Ex.
Oxidation:
Reduction:
2 Na
+
Cl2
2 NaCl
involves the loss of electrons; charge (or oxid. #) increases in value
Ex. Fe+2 Fe+3; O-2 O2
involves the gain of electrons; charge (or oxid. #) decreases in value
Ex. Sn+4 Sn+2; Cl2 Cl-1
Oxidizing Agent: reagent that oxidizes something else and is itself reduced (i.e.
reagent that gains electrons).
Reducing Agent: reagent that reduces something else and is itself oxidized (i.e.
reagent that loses electrons).
Oxidation Number
Rxns. involving ionic compounds/monatomic ions ions present and easy to assign
charges and decide what is being oxidized and reduced.
Rxns. involving molecular compounds ions are not present and can’t assign charges.
Ex.
C
+
2 H2
CH4
Instead…..a theoretical charge called the oxidation number is assigned.
Oxidation Number:
a theoretical charge used to keep track of electrons during
redox reactions. Electrons in bond are thought to be
transferred to the element with the most nonmetal character
(i.e. element that lies farthest to right in periodic table).
H
Ex.
H
C
H
C lies further to right in periodic table and has
most nonmetal character. Think of electrons in
all four bonds as being totally transferred to C.
H
2
Chem 115: Week 7
Dr. Babb
Rules for Assigning Oxidation Numbers
Remember…..the oxidation numbers of all atoms must sum to the overall charge on the
species. Apply the rules in the order given.
•
Elements in molecules consisting of just that element (i.e. elements alone w/no
charge) are assigned an oxidation number of zero. Ex. Na, Fe, Cl2, N2, P4, S8
•
Monatomic ions have an oxidation number equal to the ion charge.
Ex. Cl-1, S-2, Al+3, PbBr2, TiO2, MnSO4
3.
In compounds………... Group IA metals are assigned an ON = +1
Group IIA metals are assigned an ON = +2
Group IIIA metals (Al) are assigned an ON = +3.
4.
In compounds………… fluorine is assigned an ON = -1.
5.
In compounds……..…. hydrogen is assigned an ON = +1.
6.
In compounds………… oxygen is assigned an ON = -2.
Example: Find the oxidation number for each element in the following species.
P4O10
N2O3
Cr2O7-2
OF2
H2PO4-
CO2
HAsO4-2
H2O2
C6H6
NH3
BaO2
O2
NaH
FeBr3
S8
C2H6O
HNO2
Zn(NO2)2
3
Chem 115: Week 7
Dr. Babb
Examples: For the following reactions…..identify the oxidizing and reducing agents.
Which reagent is undergoing oxidation and which reduction? How many
electrons (or moles of e-) are being transferred between reactants?
A.
8 HCl + KMnO4 + 5 FeCl2 MnCl2 + 4 H2O + 5 FeCl3 + KCl
B.
Cr2O7-2 + 3 V+3 + 2 H+ 3 VO2+ + 2 Cr+3 + H2O
C.
16 Ag + S8 8 Ag2S
Balancing Redox Reactions
In a balanced redox reaction, the # of electrons lost is equal to the # of
electrons gained.
Half-Reaction Method:
1. Split the redox reaction into two separate half-reactions (one for oxidation and
one for reduction).
2. Balance the half-reactions.
3. Add together the balanced half-reactions so that electrons cancel.
(i.e. # e- lost = # e- gained)
Examples: Balance the following simple redox reactions.
A.
Ag+ + Cu Ag + Cu+2
C.
Zn+2 + Cr+2 Cr+3 + Zn
B.
Al + Tl+3 Al+3 + Tl+
D.
Br2 + Ti Br- + Ti+4
4
Chem 115: Week 7
Dr. Babb
Balancing Acidic and Basic Half-Reactions
Acidic Half-Reactions:
Examples:
A.
P4 H3PO4
1.
2.
3.
4.
(acidic)
B.
Mo+3 MoO2+2
(acidic)
C.
S2O8-2 HSO4-
(acidic)
Basic Half-Reactions:
1.
2.
3.
4.
5.
6.
Examples:
A.
SO3-2 S2O4-2
B.
S SO3-2
Balance atoms oxidized or reduced.
Add H2O to balance “O”.
Add H+ to balance H.
Add e- to balance charge.
Balance atoms oxidized or reduced.
Add H2O to balance “O”.
Add H+ to balance H.
Add e- to balance charge.
Add OH- to neutralize H+ (add same # OH- to
both sides of the eqn.)
Cancel out extra H2O.
(basic)
C.
Br2 BrO3-
(basic)
(basic)
D.
Zn Zn(OH)4-2
(basic)
5
Chem 115: Week 7
Dr. Babb
Balancing Acidic and Basic Redox Reactions via the
Half-Reaction Method
1.
2.
3.
4.
Split the redox reaction into two separate half-reactions (one for oxidation and
one for reduction).
Balance the half-reactions for acidic or basic solution.
Add together the balanced half-reactions so that electrons cancel.
(i.e. # e- lost = # e- gained)
Cancel out any extra H2O, H+, or OH-. If necessary, reduce and write with
smallest set of whole # coefficients.
Examples: Use the half-reaction method to balance the following redox reactions.
A.
MnO4- + SO3-2 Mn+2 + SO4-2
(acidic)
B.
MnO4- + Br- Mn+2 + Br2
(acidic)
C.
OCl- + S2O3-2 Cl- + S4O6-2
(acidic)
D.
AsO4-3 + S2O3-2 As4 + SO4-2
(acidic)
E.
MnO4- + SO3-2 MnO4-2 + SO4-2
(basic)
F.
MnO4- + Br- MnO2 + BrO3-
(basic)
G.
ClO3- + N2H4 Cl- + NO
(basic)
6
Chem 115: Week 7
Dr. Babb
Balancing Redox Reactions via the
Oxidation Number Method
1.
2.
3.
4.
5.
Assign oxidation numbers and determine species oxidized and reduced.
Balance atoms oxidized and reduced.
Determine #e- lost during oxidation and #e- gained during reduction.
Adjust coefficients so that #e- lost = #e- gained.
Balance rest of atoms by inspection. If in the form of a net ionic eqn:
A. Acidic…..add H2O to balance “O”. Then, add H+ to balance H.
B. Basic…..add OH- to balance charge. Then, add H2O to balance H or “O”.
Examples: Use the oxidation number method to balance the following redox reactions.
A.
H2S + HNO3 NO2 + S + H2O
B.
V+3 + Cr2O7-2 VO2+ + Cr+3
C.
Fe+3 + NaOH Fe+2 + Na2O2
D.
KMnO4 + H2C2O4 + H2SO4 K2SO4 + MnSO4 + CO2 + H2O
(acidic)
(basic)
7
Chem 115: Week 7
Dr. Babb
Metal Activity Series
A list of metals in order of increasing reactivity to act as
reducing agents.
Element
Au
Ag
Cu
H2
Pb
Sn
Fe
Al
Zn
Na
Ca
K
Li
Oxidized Form
Au+3
Au is WORST red. ag.
Ag+
(Wants to be oxidized least!!)
Cu+2
H+
Pb+2
Sn+2
Fe+2
Al+3
Zn+2
Na+
Ca+2
K+
Li+
Li is BEST red. ag,
(Wants to be oxidized most!!)
An element at bottom of table will spontaneously reduce
the cation of any element that lies above it in the
table.
Examples: Will the following redox reactions
occur spontaneously in the forward direction? If
so, write a balanced redox reaction.
A.
Li + Al+3 B.
Al+3 + Ag C.
H2 + Au+3 D.
Ca + H+ E.
Ag + Cu+2 F.
Cu + Ag+ G.
Zn+2 + H2 Experimental Determination of Metal Activity Series
Example: The following redox reactions were observed to occur. List the four metals
(Pb, Cu, Fe, and Al) in order of their reactivity as reducing agents.
I.
II.
III.
IV.
V.
VI.
2 Al + 3 Cu+2 2 Al+3 + 3 Cu
2 Al + 3 Fe+2 2 Al+3 + 3 Fe
Fe + Pb+2 Fe+2 + Pb
Fe + Cu+2 Fe+2 + Cu
2 Al + 3 Pb+2 2 Al+3 + 3 Pb
Pb + Cu+2 Pb+2 + Cu
8
Chem 115: Week 7
Dr. Babb
Nonmetal Activity Series
A list of nonmetals in order of decreasing ability to act as oxidizing
agents.
F2 > O2 > Cl2 > Br2 > I2 > S
F2 is best oxidizing agent
(i.e. Wants to be reduced MOST!!)
S is worst oxidizing agent.
(i.e. Wants to be reduced LEAST!!)
Examples: Will the following reactions proceed spontaneously in the forward direction?
If so, write a balanced redox reaction.
A.
KCl + F2 D.
NaCl + I2 B.
Li2S + Br2 E.
CaBr2 + Cl2 C.
CaF2 + Cl2 F.
CaO + F2 9