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Reaction Predictions
Oxidation-Reduction (Redox) Reactions
Oxidation-Reaction???? It’s more common than you think……..
Oxidation Numbers
No physical meaning
Assigned to atoms to aid in redox reactions
Treats all atoms as if they were ions
ACTUAL charge of monoatomic ions, ASSIGNED charge to molecular atoms/atoms with a polyatomic ion
Oxidation Number Rules
Priority 1st, if there is ANY conflict—go with highest priority
1) Sum of oxidation numbers = 0 for ALL neutral compounds (atoms, molecules) Any single atoms are also assigned
oxidation # = 0
Rule # 1 Examples
Br2
MgCl2
C6H12O6
Cu
Ag
Fe
Oxidation Number Rules
2) Sum of oxidation numbers for an ion = ion’s charge
Examples:
SO4-2
NH4+
Al ion
Cl ion
Oxidation Number Rules
3) Group 1A metals = +1
Group 2A metals = +2
Examples: MgSO4
K3PO4
Oxidation Number Rules
4) For the majority of chemical compounds—
F = -1 H = +1 (sometimes will have -1) O = -2
Examples: NH3
H2O
CO
HF
KF
Oxidation Number Rules
5) Group 7A Elements = -1
Group 6A Elements = -2
Group 5A Elements = -3
**When combined with a metal in a binary compound !
Rule #5 Examples
CaF2
Li2S
Mg3N
K3P
Let’s Practice !
Strategy: assign oxidation numbers you know 1st
1) Cr2O7 -2
2) CaH2
3) KClO4
4) Fe3O4
5) Al2O3
6) K2Cr2O7
NOW TO ACTUAL REDOX
REACTIONS! ! !
Oxidation-Reduction Reactions
Electron transfer between ionic compounds, change in oxidation numbers
One compound wants to GIVE electrons, other compound wants to TAKE electrons.
2 chemical processes
happening at the same time !
1) Reduction— GAIN of electrons in element Compound becomes more NEGATIVE DECREASE in oxidation number
2) Oxidation LOSS of electrons Compound becomes more POSITIVE INCREASE in oxidation number
Oxidation CANNOT happen without Reduction.
Reduction CANNOT happen without Oxidation
Both have to happen in a redox reaction ! ! ! (electrons gained = electrons lost)
Oxidation Numbers/State
Allows us to identify redox reactions
A change in these numbers------ REDOX REACTION ! ! !
Acts like all chemical compounds are ionic----gives atom a charge it would have IF it was ionic
How do we identify Redox Reactions?
Assign oxidation numbers to all atoms in the chemical reaction
If there is a CHANGE in the oxidation numbers, the reaction is a REDOX ! ! !
Example 1:
Mg (s) + Cu (aq) Mg+2(aq) + Cu
(s)
Example 2:
2 Na + Cl2 2NaCl
Assign oxidation numbers
Identify where oxidation occurs, where reduction occurs.
Redox Reaction? If so, identify what is oxidized and what is reduced.
1) Mg + 2HCl MgCl2 + H2
2) I2 + 3Cl2 2 ICl3
3) NaOH + HCl NaCl + H2O
4) 2Na + 2H2O 2NaOH + H2
Homework
Read pp. 140-146
Problems p. 142 Exercises 4.7A and 4.7B
