Oxidation Numbers and Nomenclature

Redox Chemistry

What is nomenclature?

A system of rules used for naming chemical compounds and ions.

Why are oxidation numbers useful in nomenclature?

They distinguish between compounds or ions containing the same element in different oxidation states.

How is the oxidation number shown in a systematic chemical name?

Using a Roman numeral in brackets after the name of the element.

What does iron(II) mean?

Iron has an oxidation number of +2.

What does iron(III) mean?

Iron has an oxidation number of +3.

What do the Roman numerals in systematic names represent?

The oxidation number of the element immediately before the Roman numeral.

What does copper(I) mean?

Copper has an oxidation number of +1.

What does copper(II) mean?

Copper has an oxidation number of +2.

Why is FeO called iron(II) oxide?

Oxygen is −2, so iron must be +2 to make the compound neutral.

Why is Fe₂O₃ called iron(III) oxide?

Three O atoms contribute −6 overall, so two Fe atoms must contribute +6; each Fe is +3.

How do you determine the oxidation number from a chemical formula?

Use the known oxidation numbers and the rule that the total oxidation number equals the overall charge of the species.

How do you determine a formula from a systematic name?

Use the oxidation numbers or ion charges given by the name and combine the ions in a ratio that gives the correct overall charge.

What must the overall charge of an ionic compound be?

Zero.

What is the formula of iron(III) sulfate?

Fe₂(SO₄)₃.

Why is the formula of iron(III) sulfate Fe₂(SO₄)₃?

Fe³⁺ and SO₄²⁻ must combine in a 2:3 ratio so the charges cancel: +6 and −6.

What is the formula of copper(I) oxide?

Cu₂O.

Why is copper(I) oxide Cu₂O?

Cu⁺ and O²⁻ combine in a 2:1 ratio to produce a neutral compound.

What is the formula of copper(II) oxide?

CuO.

Why is copper(II) oxide CuO?

Cu²⁺ and O²⁻ combine in a 1:1 ratio.

What is the formula of chromium(III) sulfate(VI)?

Cr₂(SO₄)₃.

What does sulfate(VI) tell you?

Sulfur has an oxidation number of +6 in the sulfate ion.

What is the formula of lead(IV) iodide?

PbI₄.

Why is lead(IV) iodide PbI₄?

Pb⁴⁺ requires four I⁻ ions to give an overall charge of zero.

What is the formula of cobalt(III) nitrate(V)?

Co(NO₃)₃.

What does nitrate(V) mean?

Nitrogen has an oxidation number of +5 in the nitrate ion.

What is the systematic name of PCl₃?

Phosphorus(III) chloride.

Why is PCl₃ called phosphorus(III) chloride?

Each Cl is −1, giving −3 overall, so phosphorus must be +3.

What is the systematic name of PCl₅?

Phosphorus(V) chloride.

Why is PCl₅ called phosphorus(V) chloride?

Five Cl atoms contribute −5 overall, so phosphorus is +5.

What is the systematic name of V₂O₅?

Vanadium(V) oxide.

Why is V₂O₅ called vanadium(V) oxide?

Five O atoms contribute −10 overall, so two V atoms contribute +10; each V is +5.

What is the systematic name of NaClO?

Sodium chlorate(I).

What is the oxidation number of chlorine in ClO⁻?

+1.

What is the systematic name of NaClO₃?

Sodium chlorate(V).

What is the oxidation number of chlorine in ClO₃⁻?

+5.

What is the systematic name of NO₃⁻?

Nitrate(V).

What is the oxidation number of nitrogen in NO₃⁻?

+5.

What is the systematic name of NO₂⁻?

Nitrate(III).

What is the oxidation number of nitrogen in NO₂⁻?

+3.

What is the systematic name of ClO₄⁻?

Chlorate(VII).

What is the oxidation number of chlorine in ClO₄⁻?

+7.

What is the systematic name of VO₃⁻?

Vanadate(V).

What is the oxidation number of vanadium in VO₃⁻?

+5.

What is the systematic name of VO₄³⁻?

Vanadate(V).

Why are Roman numerals particularly useful for transition metals?

Transition metals can commonly have more than one oxidation number, so the Roman numeral identifies which oxidation state is present.

Why is it normally unnecessary to call sodium chloride sodium(I) chloride?

Sodium has only one common oxidation number, +1, so the Roman numeral is unnecessary.

Why is it normally unnecessary to call magnesium oxide magnesium(II) oxide?

Magnesium has only one common oxidation number, +2, so the Roman numeral is unnecessary.

When should a Roman numeral be included in a compound's name?

When it is necessary to specify which oxidation state of an element is present.

What is the key difference between iron(II) and iron(III)?

Iron(II) contains Fe²⁺ whereas iron(III) contains Fe³⁺.

What should you do first when working out a formula from a systematic name?

Identify the ions and their charges or oxidation numbers.

What should you check after constructing the formula of an ionic compound?

Check that the total positive and negative charges cancel to give zero overall charge.