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Unit 3: Equilibrium, acids and redox reactions

Quick questions on Oxidation numbers and half-equations (QCE Chemistry Unit 3)

13short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.

What is assigning oxidation numbers?
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1. Free elements (in any allotrope) have oxidation number 0. Na in metallic sodium, O in O2, P in P4 are all 0. 2.
What is identifying oxidation and reduction?
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A species is oxidised if its oxidation number increases (it loses electrons). A species is reduced if its oxidation number decreases (it gains electrons).
What is constructing half-equations?
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A half-equation shows either the oxidation or the reduction half of a redox reaction. The protocol for aqueous solution:
What is half-equations in basic solution?
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When QCAA states the reaction occurs in basic conditions, do the acidic-conditions protocol first, then add OH- to both sides to neutralise the H+, and simplify the resulting H2O on each side.
What is combining half-equations into an overall ionic equation?
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The number of electrons must balance. Multiply each half-equation so that the electrons cancel when added.
What is disproportionation?
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A special case where the same species is both oxidised and reduced. Classic example: copper(I) in acid.
What is common oxidising and reducing agents to recognise?
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Oxidising agents (commonly examined).
What is forgetting that oxygen in peroxides is -1, not -2?
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H2O2 surprises students who apply rule 4 mechanically.
What is counting fluorine as -2 when it bonds to oxygen?
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F always wins; in OF2 oxygen is +2.
What is balancing atoms after balancing charge?
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Always do atom balance first, then charge balance with electrons.
What is calling the oxidising agent "the one that is oxidised"?
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It is the one that is reduced (it causes oxidation in something else).
What is confusing oxidation states of polyatomic ions?
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The oxidation number is per atom, not per ion. In Cr2O7^2-, Cr is +6 each, not +12.
What is leaving electrons in the overall equation?
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Once balanced, electrons must cancel completely.

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