2.1.5 - Redox

Question 1

Define oxidation number

Answer 1

Based on a set of rules that apply to atoms. It can be thought of as number of electrons involved in bonding to a different element.

Question 2

How can you use oxidation numbers?

Answer 2

Helps when writing formulae and in balancing electrons as a check that all electrons have been accounted for.

Question 3

What is the rule for elements?

Answer 3

The oxidation is always zero for elements!

Question 4

What is the oxidation number of H2?

Answer 4

0

Question 5

What is the oxidation number of S8?

Answer 5

0

Question 6

What is the oxidation number of Na?

Answer 6

0

Question 7

What is the rule for compounds and ions?

Answer 7

Each atom in a compound has an oxidation number. An oxidation number has a sign which is placed before the number.

Question 8

Give two examples of compounds with the oxidation number -2

Answer 8

• H2O
• CaO

Question 9

Give two examples of compounds with the oxidation number +1

Answer 9

• NH3
• H2S

Question 10

Give an example of a compound with the oxidation number -1

Answer 10

• HF

Question 11

Give two examples of compounds with the oxidation number +1

Answer 11

• NaCl
• K2O

Question 12

Give two examples of compounds with the oxidation number +2

Answer 12

• MgCl2
• CaO

Question 13

Give three examples of compounds with the oxidation number -1

Answer 13

• HCl
• KBr
• CaI2

Question 14

Give two examples of ‘H in Hydrides’ with the oxidation number -1

Answer 14

• NaH
• CaH2

Question 15

Give an example of ‘O in Peroxides’ with the oxidation number +1

Answer 15

H2O2

Question 16

Give an example of ‘O bonded to F’ with the oxidation number +2

Answer 16

F2O

Question 17

Define the oxidation number in an ion.

Answer 17

The oxidation number of an ion is numerically the same as the ionic charge but the sign comes before the number.

The sum of the oxidation numbers = total charge

Question 18

How do you work out the oxidation number for Sulfur in H2SO4?

Answer 18

• Step 1:
  
  • Assign any oxidation numbers from the rules
  • H2SO4 (Total hydrogens +1x2=+2) and (Total oxygens -2x4=-8)
• Step 2:
  
  • What is the sum of the oxidation numbers?
  • Sum of oxidation numbers = total charge = 0
• Step 3:
  
  • Work out the unknown oxidation numbers
  • Sum of oxidation numbers = (+2) + (x) + (-8) = 0
  • (-6) + (x) = 0
  • x = 6

Question 19

Redox in terms of electrons

Answer 19

Oxidation
Is
Loss
Reduction
Is
Gain

Question 20

Redox Reactions of Acids

Answer 20

Acids produce salts in neutralisation reactions. Dilute acids also undergo redox reactions with some metals to produce salts and hydrogen gas.

Question 21

What are polyatomic ions?

Answer 21

Polyatomic ions contain oxygen for example: NO2(-) (-ite) and NO3(-) (-ate). Although still in common usage using -ite and -ate in naming this is old fashioned, today we use oxidation numbers shown as Roman Numerals.

Question 22

Polyatomic ion NO2(-)

Answer 22

Ion - NO2(-)
Common Name - Nitrite
Oxidation Number of Nitrogen - +3
Modern Name - Nitrate (III)

Question 23

Polyatomic ion NO3(-)

Answer 23

Ion - NO3(-)
Common Name - Nitrate
Oxidation Number of Nitrogen - +5
Modern Name - Nitrate (V)