Oxidation Numbers and Rules
Oxidation is when any element loses electrons.
If the Compound is Ionic, you can use the periodic table to assign the oxidation number. We know that covalent compounds are made of two nonmetals, and from the periodic table they are expected to be negative. However, since all covalent bonds are neutral, it is not possible for each element to keep its negative oxidation number. Only the element that is more electronegative stays negative, while the other changes to positive.
Oxidation numbers are different from formal charges, and using oxidation numbers gives us another way to take into consideration the electrons in chemical changes.
Rules
1. The Oxidation number of any element that is uncombined is zero.
2. The Oxidation number of any monoatomic ion will equal its charge.
3. Oxygen is always -2, unless it is in a peroxide (H2O2). In peroxides it will be -1, and when its in bonds with fluorine it will be +2.
4. Hydrogen is always +1, unless it's bonding with metals to form metal hydrides, where it would be -1. It is also -1 if it is in NH4.
5. Sum of oxidation numbers for a compound has to equal zero.
6. The sum of oxidation numbers in the formula of polyatomic ions will be equal to its charge.
Oxidation is when any element loses electrons.
If the Compound is Ionic, you can use the periodic table to assign the oxidation number. We know that covalent compounds are made of two nonmetals, and from the periodic table they are expected to be negative. However, since all covalent bonds are neutral, it is not possible for each element to keep its negative oxidation number. Only the element that is more electronegative stays negative, while the other changes to positive.
Oxidation numbers are different from formal charges, and using oxidation numbers gives us another way to take into consideration the electrons in chemical changes.
Rules
1. The Oxidation number of any element that is uncombined is zero.
2. The Oxidation number of any monoatomic ion will equal its charge.
3. Oxygen is always -2, unless it is in a peroxide (H2O2). In peroxides it will be -1, and when its in bonds with fluorine it will be +2.
4. Hydrogen is always +1, unless it's bonding with metals to form metal hydrides, where it would be -1. It is also -1 if it is in NH4.
5. Sum of oxidation numbers for a compound has to equal zero.
6. The sum of oxidation numbers in the formula of polyatomic ions will be equal to its charge.
How to find Oxidation Numbers
Ex. SO4
SO4 has a negative charge, it is polyatomic, the sum is -2. SO4 = -2 The 4 subscript is now the coefficient, O is -2 because it's not with peroxide or F. S + 4(-2) = -2 Solve. S - 8 = -2 S = +6 O = -2 |
Ex. PO4
PO4 = -3 P + 4(-2) = -3 P - 8 = -3 P = +5 O = -2 |
Ex. Cr2O7
2Cr + 7(-2) = -2 2Cr - 14 = -2 2Cr = 12 Cr = +6 O = -2 |
Reduction
Reduction is when any element gains electrons.
Reduction is when any element gains electrons.
Redox
Reduction and oxidation will never happen by themselves. Whenever an element is being oxidized, there is another that's being reduced, and vise versa.
An easy way to remember which does what is "LEO the Lion says GER"
Lose Gain
Electron Electron
Oxidized Reduced
Reduction and oxidation will never happen by themselves. Whenever an element is being oxidized, there is another that's being reduced, and vise versa.
An easy way to remember which does what is "LEO the Lion says GER"
Lose Gain
Electron Electron
Oxidized Reduced
Two Types of Chemical Reactions:
Endothermic- releases energy, bond making
Exothermic- absorbs energy, bond breaking
Endothermic- releases energy, bond making
Exothermic- absorbs energy, bond breaking