Oxidation & Reduction (AQA A Level Chemistry)

What are the oxidation states of the elements in the following species?

a) C                b)  Fe³⁺                       c)  Fe²⁺   d) O^2-             e)  He                          f)  Al³⁺

Answers:

a) 0     b) +3    c) +2    d) -2    e) 0    f) +3

• So, in simple ions, the oxidation stateof the atom is the charge on the ion:

  • Na⁺, K⁺, H⁺ all have an oxidation state of +1

  • Mg²⁺, Ca²⁺, Pb²⁺ all have an oxidation state of +2

  • Cl^–, Br^–, I^– all have an oxidation state of -1

  • O^2-, S^2- all have an oxidation state of -2

Oxidation state and oxidation number are often used interchangeably, though IUPAC does not distinguish between the two terms.

Oxidation states are represented by Roman numerals according to IUPAC

Oxidising & reducing agents

Four reactions are shown. In which reaction is the species in bold acting as an oxidising agent?

1. Cr₂O₇^2-+ 8H⁺ + 3SO₃^2-  →  2Cr³⁺ + 4H₂O + 3SO₄^2-

2. Mg + Fe²⁺ →  Mg²⁺ + Fe

3. Cl₂ + 2Br^- →  2Cl^- + Br₂

4. Fe₂O₃+ 3CO →  2Fe + 3CO₂

Answer

The correct option is 2

• Oxidising agents are substances that oxidise other species, gain electrons and are themselves reduced.

• Write down the oxidation numbers of each species in the reaction

• In equation 2, Fe²⁺ oxidises Mg(0) to Mg²⁺(+2) and is itself reduced from Fe²⁺(+2) to Fe(0)

Systematic names of compounds

Give the full systematic names of the following compounds:

1. FeCl₂

2. HClO₄

3. NO₂

4. Mg(NO₃)₂

5. K₂SO₄

Answer

Answer 1: Iron(II) chloride: the oxidation state of 2 Cl atoms is -2 and FeCl₂ has no overall charge so the oxidation state of Fe is +2

Answer 2: Chloric(VII) acid: the oxidation state of H is +1, 4 O atoms is -8 and HClO₄ has no overall charge so the oxidation state of Cl is +7

Answer 3: Nitrogen(IV) oxide: the oxidation state of 2 O atoms is -4 and NO₂ has no overall charge so the oxidation state of N is +4

Answer 4: Magnesium nitrate: this is a salt of the common acid, so it is named without including the oxidation state of the non-metal

Answer 5: Potassium sulfate: this is a salt of the common acid, so it is named without including the oxidation state of the non-metal