Redox and ionic equations: identification skills
Chemical changes • Reactivity of metals
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Key concepts
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Definition of oxidation and reduction
Oxidation is the process in which an atom, ion or molecule loses electrons. Loss of electrons causes an increase in oxidation state. Reduction is the process in which an atom, ion or molecule gains electrons. Gain of electrons causes a decrease in oxidation state. Clear identification relies on tracking electron flow and changes in oxidation numbers. Oxidation and reduction always occur together in a redox reaction. Electrons lost by the oxidised species travel to the reduced species. Balanced electron transfer between half-equations ensures overall charge conservation.
Oxidation numbers and identification
Oxidation numbers assign a formal charge to atoms in compounds to track electron shifts. More positive oxidation number indicates loss of electron density; more negative indicates gain. Compare oxidation numbers before and after reaction to identify which element gains or loses electrons. Limiting factor: oxidation numbers are a bookkeeping tool and require correct assignment rules (e.g., O usually −2, H usually +1). Change in oxidation number for a particular element equals number of electrons lost or gained. Positive change indicates oxidation; negative change indicates reduction. Elements that show increased oxidation number are oxidised; those with decreased oxidation number are reduced.
Half-equations
Half-equations represent either the oxidation or the reduction step individually and include electrons to show electron transfer. Example format for oxidation: M -> M^n+ + n e-. Example format for reduction: n e- + A^m+ -> A. Half-equations make it straightforward to identify which species lose electrons and which gain electrons. Balancing half-equations requires balancing atoms and charges. For reactions in solution, add H+ and H2O to balance hydrogen and oxygen when necessary. Combine oxidation and reduction half-equations by cancelling electrons to produce the full redox equation.
Ionic equations and spectator ions
Ionic equations show only the ions and molecules that participate directly in a reaction. Spectator ions remain unchanged and cancel out when writing ionic equations. Removing spectator ions clarifies the actual chemical change and the species that undergo redox or displacement. Limiting factor: only soluble strong electrolytes dissociate into ions in ionic equations. Include state symbols (s, aq, l, g) when needed to identify which species dissociate and which remain molecular or solid.
Displacement reactions and ionic equations
Metal displacement occurs when a more reactive metal displaces ions of a less reactive metal from solution. Reaction causes metal atoms to lose electrons and form cations, while the displaced metal ions gain electrons and form atoms. Ionic equations for displacement remove spectator anions and show only the metal and metal ion involved. Example pattern: M + N^n+ -> M^n+ + N. Write half-equations for M (oxidation) and N^n+ (reduction), balance electrons, then combine to give the ionic equation. State symbols indicate whether the metal is solid and the ions are aqueous.
Identifying oxidised and reduced species in equations
Symbol equations and half-equations both allow identification of oxidised and reduced species. In a symbol equation, assign oxidation numbers to track changes. In half-equations, the species releasing electrons is oxidised and the species consuming electrons is reduced. Clear labelling of electrons in half-equations directly indicates the direction of electron flow. HT identification requires stating the species that loses electrons (oxidised) and the species that gains electrons (reduced). In displacement reactions, the solid metal is usually oxidised and the aqueous metal ion is reduced if the solid is higher in the reactivity series.
Balancing redox equations using half-equations
Balance changes in atoms and charge by writing separate oxidation and reduction half-equations. Multiply half-equations so the number of electrons lost equals electrons gained. Add the half-equations and cancel electrons to obtain the balanced equation. Check mass and charge balance to ensure correctness. In alkaline or acidic solutions, include H+ and H2O (acid) or OH- and H2O (alkali) when balancing. Spectator ions are excluded from the final ionic equation after electron balance.
Key notes
Important points to keep in mind