Oxidation, Reduction and Redox Equations - Oxidation State Rules

Question 1

What do oxidation states/numbers tell you?

Answer 1

• The total number of electrons that have been donated or accepted

Question 2

How are oxidation states written?

Answer 2

• Charge comes before number

- E.g +2

Question 3

Rule 1: OS of atoms and elements

Answer 3

• Oxidation states of atoms in an element are 0

- Elements bonded to identical elements have oxidation states of 0, e.g O2

Question 4

Rule 2: Simple monatomic ion

Answer 4

• The oxidation state of a simple monatomic ion is the same as its charge
• Na+ OS = +1
• Mg2+ OS = +2

Question 5

Rule 3: Neutral compounds

Answer 5

• Sum of oxidation states equals 0

Question 6

Rule 4: Compounds/compound ions

Answer 6

• Sum of oxidation states equals overall oxidation state
• Overall oxidation state is equal to overall charge on ion
• When writing formula of a compound, the element with the positive oxidation state is written first. One rare exception is ammonia, (NH3)

Question 7

Rule 5: Fluorine

Answer 7

• Oxidation state of fluorine in its compounds is always –1

- Fluorine is the most electronegative element it always gains 1 electron on bonding

Question 8

Rule 6: Oxygen

Answer 8

• Oxidation state of oxygen is usually -2
• Except when bonded to fluorine, peroxides or superoxides
• Peroxides are compounds with structure R-O-O-R, e.g H2O2
• Superoxides are compounds containing superoxide ion O2-, e.g NaO2

Question 9

Rule 7: Hydrogen

Answer 9

• Oxidation state of hydrogen in its compounds is usually +1

- Except when bonded to a metal ion, in which case it is –1

Question 10

Which elements have a fixed oxidation state of +1 in its compounds?

Answer 10

• H (except rule 7, with metal ions is -1)
• Li
• Na
• K

Question 11

Which elements have a fixed oxidation state of +2?

Answer 11

• Mg
• Ca
• Ba

Question 12

Which element has a fixed oxidation state of +3?

Answer 12

• Al

Question 13

Which element has a fixed oxidation state of -2?

Answer 13

• O (except for rule 6, with peroxides, fluorine and superoxides)

Question 14

Which element has a fixed oxidation state of -1?

Answer 14

• F

Question 15

Name all the elements with fixed oxidation states alongside their oxidation states

Answer 15

• +1

• H
• Li
• Na
• K

• +2

• Mg
• Ca
• Ba

• +3
- Al

• -2
- O

• -1
- F

Question 16

What is the maximum oxidation state an element can have?

Answer 16

• Maximum oxidation state an element can have is equal to its group number in the Periodic Table
• E.g maximum oxidation state of sulfur is +6 (Group 6), the maximum oxidation state of iodine is +7 (Group 7)

Question 17

What do the roman numerals mean in formulas?

Answer 17

• Roman numerals show that that element can have varied oxidation states
• The roman numeral shows what the oxidation state that atom has in that compound