Oxidation and reduction by oxygen
Core definitions: a substance is OXIDISED when it gains oxygen and REDUCED when it loses oxygen. In a redox reaction both happen together, so there is no oxidation without reduction. Example: — CuO loses oxygen (reduced), hydrogen gains oxygen (oxidised). An oxidising agent gives oxygen and is itself reduced; a reducing agent removes oxygen and is itself oxidised.
Redox as electron transfer (OIL RIG)
(Extended) In electron terms, OXIDATION is loss of electrons and REDUCTION is gain of electrons (OIL RIG) — but write the definition in full, as the mnemonic alone earns nothing. Half-equations show it: (oxidised) and (reduced). The oxidising agent accepts electrons (itself reduced); the reducing agent donates them (itself oxidised).
Oxidation numbers and how to assign them
(Extended) An oxidation number is the charge an atom would have if the compound were fully ionic. Rules: an uncombined element is 0; a monatomic ion equals its charge; oxygen is usually −2; hydrogen usually +1; the numbers sum to 0 in a compound, or to the ion charge. Example — Mn in : gives Mn = +7. Oxidation INCREASES the number, reduction DECREASES it.
Drawn from real examiner reports.
Do not swap oxidation and reduction
The commonest redox error is giving the opposite definition — reduction when oxidation is asked, or vice versa. Fix the Core pair as a unit: GAINS oxygen = oxidised; LOSES oxygen = reduced. Vague answers like oxygen is produced or exposed to oxygen earn nothing. The Extended electron definitions are also accepted at Core.
W22 P31 Q4(b)(ii); W22 P32 Q4(b)(ii) — opposite definition given; vague answers (oxygen is produced, exposed to oxygen) gain no credit.
Halogen displacement: which is oxidised
In , the bromide ion is OXIDISED (−1 to 0, loses electrons) — the reducing agent. Chlorine is REDUCED (0 to −1, gains electrons) — the oxidising agent. Candidates explain why correctly but pick the wrong species; decide it from the oxidation-number change.
S22 P41 Q7(d)(ii) — students struggle to identify which species is oxidised in halogen displacement; the explanation is better recalled than the species.
(Extended) Names need the Roman numeral
For a variable-valency metal, the Roman numeral is a compulsory part of the name — omit it or get it wrong and the name scores zero. is iron(III) chloride (Fe = +3), is iron(II) chloride, and is vanadium(V) oxide (not vanadium oxide). Find the number by making the oxidation states sum to zero.
W22 P41 Q2 general; S23 P41 Q6(a)(ii) — vanadium(V) oxide given with a wrong/missing state; iron(III) confused with iron(II).
The oxidising agent is itself reduced
An oxidising agent causes another species to be oxidised but is ITSELF reduced (it accepts electrons, or gives oxygen). Likewise a reducing agent is itself oxidised. Writing the oxidising agent is oxidised is a common self-contradiction that loses the mark. The species that gains electrons or loses oxygen is reduced — and is the oxidising agent.
Name or formula — read the command
A persistent error is answering with a formula when the question says state the NAME, or a name when it asks for the FORMULA. Re-read the command: State the name wants words (vanadium(V) oxide); Write the formula wants . The wrong form usually scores nothing, so match the answer to the command.
W22 P41 general; S23 P41 general — name given when a formula/equation is asked for, and vice versa; most acute in redox and electrochemistry.
(Extended) OIL RIG is not a definition
OIL RIG (Oxidation Is Loss, Reduction Is Gain — of electrons) is only a memory aid. Writing OIL RIG alone, or oxidation is loss without saying loss of ELECTRONS, does not earn the definition mark. State it in full: oxidation is loss of electrons, reduction is gain of electrons. The same applies to the oxygen definitions — name what is gained or lost, not just the direction.
(Extended) Identify, then half-equation
Extended redox marks split into identify and explain. Step 1: find each species' oxidation number before and after — an increase is oxidised, a decrease reduced. Step 2: confirm with a half-equation, e.g. .
Name the species before explaining
Students often explain correctly but identify the wrong species. State which named substance is oxidised or reduced BEFORE the reason. Write zinc is oxidised, then justify it — a reason with no species attached earns no identify mark.
(Extended) Use O = -2, H = +1 to decide
To decide oxidation or reduction fast, assign oxidation numbers: oxygen −2, hydrogen +1, summing to zero. Compare each element before and after — an increase is oxidation, a decrease reduction. It also names metals: Fe in is +3, so iron(III) oxide.
Oxidation (Core definition) — a reaction in which a substance gains oxygen.
Reduction (Core definition) — a reaction in which a substance loses oxygen.
(Extended) Oxidation — loss of electrons by a species (oxidation number increases).
(Extended) Reduction — gain of electrons by a species (oxidation number decreases).
Redox reaction — a reaction in which oxidation and reduction occur simultaneously; one species gains oxygen (or loses electrons) while another loses oxygen (or gains electrons).
Oxidising agent — a substance that oxidises another species. The oxidising agent itself is reduced during the reaction (it provides the oxygen, or accepts the electrons).
Reducing agent — a substance that reduces another species. The reducing agent itself is oxidised during the reaction (it removes the oxygen, or donates the electrons).
(Extended) Oxidation number (oxidation state) — the charge an atom would carry if the compound were fully ionic; assigned by a set of rules. An increase in oxidation number = oxidation; a decrease = reduction.
Define oxidation (Core definition, in terms of oxygen).
Iron(III) oxide is reduced by carbon monoxide in the blast furnace:
(a) Balance this equation.
(b) Identify the oxidising agent and state why.
