What do the oxidation numbers in a neutral compound add up to?

Zero. In an ion they add up to the charge on the ion.

What is the usual oxidation number of oxygen, and what is it in a peroxide?

−2 usually; −1 in peroxides such as H₂O₂.

What is the oxidation number of hydrogen in a metal hydride such as CaH₂?

−1, because hydrogen is more electronegative than the metal.

What is the oxidation number of oxygen in OF₂?

+2, because fluorine is always −1 and is more electronegative than oxygen.

Calculate the oxidation number of Cr in Cr₂O₇²⁻.

2Cr + 7(−2) = −2, so Cr = +6.

What does the Roman numeral in iron(III) oxide show?

The oxidation number of iron is +3.

Write the formula of iron(III) oxide.

Fe₂O₃ (two Fe³⁺ balance three O²⁻).

How can oxidation numbers show a reaction is redox?

If any element changes oxidation number, the reaction is redox.

Define disproportionation.

A redox reaction in which one element in a single species is simultaneously oxidised and reduced.

What are the oxidation numbers of N in NH₄⁺ and NO₃⁻?

−3 in NH₄⁺ and +5 in NO₃⁻.

How is an oxidation number written differently from an ionic charge?

Sign first: +2 for oxidation number, 2+ for ionic charge.

Oxidation numbersEdexcel A-Level Chemistry: Flashcards

Card 1 of 130 of 13 known

Question

What is the oxidation number of an uncombined element?

Tap or press Space to reveal

Tap card or press Space to flip

See all 13 cards

What is the oxidation number of an uncombined element?: Zero.
What do the oxidation numbers in a neutral compound add up to?: Zero. In an ion they add up to the charge on the ion.
What is the usual oxidation number of oxygen, and what is it in a peroxide?: −2 usually; −1 in peroxides such as H₂O₂.
What is the oxidation number of hydrogen in a metal hydride such as CaH₂?: −1, because hydrogen is more electronegative than the metal.
What is the oxidation number of oxygen in OF₂?: +2, because fluorine is always −1 and is more electronegative than oxygen.
Calculate the oxidation number of Cr in Cr₂O₇²⁻.: 2Cr + 7(−2) = −2, so Cr = +6.
What does the Roman numeral in iron(III) oxide show?: The oxidation number of iron is +3.
Write the formula of iron(III) oxide.: Fe₂O₃ (two Fe³⁺ balance three O²⁻).
How can oxidation numbers show a reaction is redox?: If any element changes oxidation number, the reaction is redox.
Define disproportionation.: A redox reaction in which one element in a single species is simultaneously oxidised and reduced.
What does an increase in oxidation number show?: The element has been oxidised.
What are the oxidation numbers of N in NH₄⁺ and NO₃⁻?: −3 in NH₄⁺ and +5 in NO₃⁻.
How is an oxidation number written differently from an ionic charge?: Sign first: +2 for oxidation number, 2+ for ionic charge.

Exam questions on Oxidation numbers

A technician is relabelling reagent bottles in a school store room. The bottles are marked only with the formulae KMnO₄, Fe₂O₃, H₂O₂ and CaH₂, and she must add the correct oxidation numbers to each label.
Explain why the oxidation numbers of oxygen in H₂O₂ and of hydrogen in CaH₂ differ from the values these elements usually have.2 marks
Sulfur forms many compounds and ions in which its oxidation number ranges from −2 to +6.
Calculate the oxidation number of sulfur in the thiosulfate ion, S₂O₃²⁻. Show your working.2 marks
Nitrogen forms compounds and ions in which its oxidation number ranges from −3 to +5. An environmental chemist is studying nitrogen species found in air and in rainwater.
Calculate the oxidation number of nitrogen in NH₄⁺, in N₂O and in NO₃⁻.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).