Topic 3 Redox I

Question 1

Define Oxidation number:

Answer 1

The charge of an ion or theoretical charge of an atom in a covalently bonded compound assuming the bond becomes ionic.

Question 2

What is a reducing agent?

Answer 2

Reducing agents lose electrons and are oxidised themselves.

Question 3

What is an oxidising agent?

Answer 3

Oxidising agents gain electrons and are reduced themselves

Question 4

What are the oxidation states in compounds for these elements:
- Uncombined elements
- Ions
- Group 1
- Group 2
- Aluminium
- Hydrogen
- Chlorine
- Fluorine
- Oxygen

Answer 4

• Uncombined elements = Always 0
• Ions = Oxidation number is always same as charge on ion
• Group 1 = Always +1
• Group 2 = Always +2
• Aluminium = Always +3
• Hydrogen = +1 (except in hydrides where it’s -1 (NaH, LiH)
• Chlorine = -1 (except in a compound with F and O - it’s positive)
• Fluorine = Always -1
• Oxygen = -2 (except it’s -1 in peroxides and +2 in OF2)

Question 5

How does reduction and oxidation correlate to oxidation numbers?

Answer 5

• Reduction is a decrease in oxidation number
• Oxidation is an increase in oxidation number

Question 6

What is a disproportionation reaction?

Answer 6

When one species in a reaction is being simultaneously reduced and oxidised.

Question 7

What are the steps for balancing half equations in acidic solutions?

Answer 7

1. Write down the species before and after a reaction
2. Balance any atoms apart from oxygen and hydrogen
3. Balance any oxygens with H2O
4. Balance any hydrogens with H+ ions
5. Balance charges with electrons (e-)

Question 8

What is the 2 main rules when combining half equations?

Answer 8

• You must balance the electrons in both half equations (similarly to in simultaneous equations) and then you must cancel the electrons and combine the 2 equations.

Question 9

What are the steps for balancing half equations in alkaline solutions?

Answer 9

1. Write down the species before and after a reaction
2. Balance any atoms apart from oxygen and hydrogen
3. Balance any oxygens with H2O
4. Balance any hydrogens with H+ ions
5. Balance H+ ions with OH- ions - adding them to either side
6. Cancel out the H2O’s
7. Balance charges with electrons (e-)

Question 10

What does the roman numeral mean next to transition metal ions?

Answer 10

The roman numeral denotes the charge and the oxidation state of the transition metal ion.