Write balanced half-equations and ionic equations

Atomic structure and the periodic table • Atomic models and isotopes

Flashcards

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Where are electrons placed in an oxidation half-equation?

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Electrons appear on the right-hand side in an oxidation half-equation.

Key concepts

What you'll likely be quizzed about

Definition of a half-equation

A half-equation represents either oxidation or reduction for a single reactant or product and explicitly shows electrons lost or gained. Electrons appear on one side of the arrow to balance charge. Charge balance and atom balance both determine the number of electrons required.

Oxidation and reduction in half-equations

Oxidation involves loss of electrons and places electrons on the right-hand side of the half-equation. Reduction involves gain of electrons and places electrons on the left-hand side. Combining one oxidation half-equation and one reduction half-equation requires equalising the number of electrons transferred so they cancel when added.

Balancing half-equations in acidic and alkaline conditions

In acidic solution, balance atoms other than O and H first, then balance O with H2O and H with H+. Add electrons to balance charge. In alkaline solution, follow acidic balancing then neutralise extra H+ by adding OH− to both sides, forming H2O where appropriate. Finally, cancel identical species on both sides.

Ionic and net ionic equations

A full ionic equation splits all soluble ionic compounds into their constituent ions and shows all species present in solution. Spectator ions appear unchanged on both sides and cancel to give the net ionic equation. The net ionic equation includes only the ions and molecules directly involved in the chemical change.

Common pitfalls and limiting factors

Incorrect placement of electrons or failure to conserve charge leads to incorrect half-equations. Forgetting to convert H+ to OH− in alkaline conditions produces invalid equations. Solubility assumptions dictate which species split into ions; insoluble solids remain as compounds in ionic equations. Always check that atoms and total charge balance after cancellation.

Key notes

Important points to keep in mind

Balance atoms other than H and O first.

Use H2O to add oxygen atoms and H+ to add hydrogen atoms in acidic conditions.

Add electrons last to balance overall charge.

In alkaline conditions, convert H+ to H2O using OH- and then cancel H2O.

Split only soluble ionic species when writing full ionic equations.

Cancel spectator ions to produce the net ionic equation.

Check both mass and charge balance for every equation.

Multiply half-equations to equalise electrons before combining.

Keep insoluble solids and gases as molecules in ionic equations.

Common half-equation templates speed up balancing for frequent redox pairs.