Oxidation Numbers Positive oxidation number Negative oxidation number - Loses partial or total control of electrons in a bond - Gains partial or total control of electrons in a bond Example Mg in the +2 state (Mg +2 ) Has lost partial or total control of 2 e- - Used to tell how many electrons an atom has lost or gained in a chemical reaction Redox - When one atom loses an electron another must receive it
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Oxidation Numbers Positive oxidation number Negative oxidation number - Loses partial or total control of electrons in a bond - Gains partial or total.
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Oxidation Numbers
Positive oxidation number
Negative oxidation number
- Loses partial or total control of electrons in a bond
- Gains partial or total control of electrons in a bond
Example
Mg in the +2 state (Mg+2)
Has lost partial or total control of 2 e-
- Used to tell how many electrons an atom has lost or gained in a chemical reaction
Redox - When one atom loses an electron another must receive it
Na Na+ + 1e-
Cl + 1e- Cl-
1) Oxidation - The lose of an electron by an atom
- Causes an increase in oxidation number
2) Reduction - The gain of an electron by an atom
- Causes a decrease in oxidation number
If Fe loses three electrons, we write this as
Fe Fe+3 +3e-
If the electrons appear on the products side, they are given off
oxidation
If Fe+3 gains three electrons, we write this as
Fe+3 +3e- Fe If the electrons appear on the reactants side, they are gained
reduction
Lose Electrons Oxidation
Gain Electrons ReductionLEO goes GER
L E O
G E R
Indicate if the atoms shown have lost or gained electrons
Then write down if the atom is oxidized or reduced
A. H H+ + e-
B. Cl2 + 2e- 2 Cl-
C. Fe+2 + 2e- Fe
D. Cl+5 + 6e- Cl-
E. S-2 S+4 + 6 e-
Lost or gained e- Oxidized or reduced
Lost
Gained
oxidized
reduced
Lost
Gained
Gained
oxidized
reduced
reduced
Lightening Underwater pg 207
Oxidizing Agent - Causes another substance to be oxidized- Is reduced
Ex. Sn+4 + 2 e- Sn+2
Sn+4 is the oxidizing agent
Reducing Agent- Causes another substance to be reduced
- Is oxidized
Ex. Na Na+1 + 1 e-
Na is reducing agent
Blue bottle demo
Blue is oxidized = lost electrons
Colorless is reduced = more electrons
Finding Oxidation States
The most common oxidation numbers are in the upper right corner of your periodic table
Pb
+2+4
Oxidation states
Using your periodic table, list the oxidation states of the following
Ca
Na
O
N
Examples
+2
+1 -3, -2, -1 and a bunch more!
-2
Rules for assigning oxidation numbers
1. Uncombined elements have an oxidation number of zero
Cu0 Mg0 S0
2.) The oxidation state of an ion is the same as its charge
Oxidation state of zero means the atom is not losing or gaining any electrons
Cu+ Mg+2 S-2
+1 oxid. state +2 oxid. state -2 oxid. state
Cl20
3) Group 1 metals in a compound always have a +1 oxidation state
4) Group 2 metals in a compound always have a +2 oxidation state
MgCl2 LiBr Na2O+2 +1+1
5) Hydrogen always has a +1 oxidation state in a compound
Exception - When H is attached to a group 1 or 2 metal, it has a -1 oxidation state
HCl H2O+1 +1
NaH CaH2
-1-1
6) Oxygen in a compound always has a -2 oxid. state
Exception - In a peroxide, oxygen becomes -1
Na2O2 H2O2
-1 -1
H2O HNO3
-2 -2
7) Halogens are usually -1 in compounds
SO4-2
+6 -2
-8+6 = -2
The sum of the one sulfur and the 4 oxygens should be -2
8) The sum of all oxidation numbers in a polyatomic ion must equal the charge of the ion
Examples - Assign the oxidation numbers to all elements in the following ions
NO3- CrO4
-2+6
-6+5
+5 -2-2
+6 -8= -1 = -2
Assigning Oxidation Numbers
- All of the oxidation numbers of all atoms in a neutral compound must add up to zero
For example, in H2O,
H2O+1 -2
The two hydrogens add up to +2,
+2 + -2 = 0
Since water is a neutral compound, all oxidation states must add up to zero
Neutral compounds
Practice - Assign oxidation numbers to the following
MgCl2 LiOH Na2S H2SO4 NaNO3
+2 -1 +1 -2 +1 +1 -2 +1 +6 -2 +1 +5 -2
+2 -2 +1 +1-2 +2 -2 +2 -8+6 +1 +5 -6
0 0 0 0 0
Cu(NO3)2 NaH NO2
+2 +5 -2 +1 -1 +4 -2
-12+10+2 +1 -1 +4 -4
Recognizing Redox Equations
- Not all reactions are redox
- Double replacement reactions are NOT
- Single replacement reactions are
- If a reaction is redox then the oxidation numbers of some of the elements must be different on either side of the equationIs the reaction redox?
LiOH + HCl H2O + LiCl
2H2 + O2 2 H2O
Mg + CuSO4 MgSO4 + Cu
In the reaction Mg + Cl2 --> MgCl2
There are 2 steps occurring
Mg loses 2 electrons
Each chlorine gains an electron
MgCl
Cl
Electrons are moving. If e- could move through a wire, this would be an electric current
Electrochemistry
Half reaction- Either the reduction or the oxidation portion of a redox reaction
Al+3 + 3e- --> Al
Ca --> Ca+2 + 2e-
reduction
oxidation
The reduction ½ reaction shows an atom or ion gaining e- and the oxidation number decreasing
The oxidation ½ reaction shows an atom or ion losing e- and the oxidation number increasing