In H₂O₂ (hydrogen peroxide), oxygen has an oxidation number of −1 rather than the usual −2. Which explanation is correct?
AHydrogen is more electronegative than oxygen in peroxides, pulling electron density away from oxygen
BEach oxygen is bonded to another oxygen of equal electronegativity, so neither can claim both electrons from that bond — leaving each oxygen with only one extra electron instead of two
CThe oxidation number rules do not apply to compounds with oxygen-oxygen bonds
DOxygen alternates between −1 and −2 depending on temperature and pressure
Oxidation numbers are assigned by giving all shared electrons to the more electronegative atom. In the O–O bond of H₂O₂, both oxygens are equally electronegative, so they split the bonding electrons equally — neither gets to claim both. This reduces each oxygen's 'extra' electrons from 2 (the usual −2) to 1 (hence −1). This is a principled application of the bookkeeping logic, not an exception to the rules.
Question 2 Multiple Choice
In the reaction 2Fe₂O₃ + 3C → 4Fe + 3CO₂, iron goes from +3 to 0 and carbon goes from 0 to +4. Which of the following correctly identifies what happened?
AIron was oxidized; carbon was the oxidizing agent
BIron was reduced; carbon was the oxidizing agent
CIron was reduced; carbon was the reducing agent
DIron was oxidized; carbon was the reducing agent
Iron goes from +3 to 0 — its oxidation number decreases, meaning it gained electrons — so iron was reduced. Carbon goes from 0 to +4 — its oxidation number increases, meaning it lost electrons — so carbon was oxidized. The reducing agent is the species that gets oxidized (loses electrons) and causes reduction in another species: that's carbon. The oxidizing agent is the species that gets reduced (gains electrons): that's the iron in Fe₂O₃. Confusing 'oxidizing agent' with 'the thing that gets oxidized' is the classic error here.
Question 3 True / False
Oxidation numbers represent the actual electrical charges on atoms in a compound.
TTrue
FFalse
Answer: False
Oxidation numbers are a formal bookkeeping device — they represent the hypothetical charge an atom would have if all bonds were fully ionic and all electrons were assigned to the more electronegative atom. In covalent compounds like CO₂, electrons are shared, not fully transferred; carbon does not carry an actual +4 charge. Oxidation numbers are a useful fiction that enables tracking electron flow in redox reactions without requiring full quantum mechanical analysis of each bond.
Question 4 True / False
In NaH (sodium hydride), hydrogen has an oxidation number of −1, even though hydrogen is typically +1 in compounds with nonmetals.
TTrue
FFalse
Answer: True
The assignment rule for hydrogen is context-dependent: hydrogen is +1 when bonded to nonmetals (which are more electronegative, so hydrogen 'loses' its electron), but −1 when bonded to metals in hydrides. Sodium is less electronegative than hydrogen, so in NaH the electrons are assigned to hydrogen, giving it −1. Sodium is +1. This is one of the key priority-rule exceptions students must apply correctly.
Question 5 Short Answer
An atom's oxidation number increases from +2 to +5 during a chemical reaction. Was this atom oxidized or reduced? What does this change tell you about electron movement, and what role does this atom play in the overall redox reaction?
Think about your answer, then reveal below.
Model answer: The atom was oxidized. An increase in oxidation number means the atom has (formally) lost electrons — in the bookkeeping system, it ends up with fewer electrons assigned to it. This atom is therefore the reducing agent: it donates electrons to another species, enabling that other species to be reduced. The magnitude of the change (+3 oxidation states) indicates that 3 electrons were transferred per atom of this species.
The mnemonic OIL RIG (Oxidation Is Loss, Reduction Is Gain) captures the directionality. Higher oxidation number = fewer electrons assigned = oxidized = reducing agent. Students often confuse the agent with the process: the reducing agent is the one that gets oxidized (it reduces others by donating electrons to them). The change in oxidation number also provides the stoichiometry of electron transfer, which is essential for balancing redox reactions using the half-reaction method.