Higher Tier: conserve charge in ionic and half equations
| English | Français |
|---|---|
| half equation/hɑːf ɪˈkweɪʒn/ | half equation |
| spectator ion/spekˈteɪtə ˈaɪɒn/ | spectator ion |
What would explain this observation?
- Higher Tier: An equation can conserve atoms while failing to conserve electrical charge. Ionic reactions need both checks, and a half equation 半反应方程式 explicitly shows electron transfer.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- An ionic equation represents reacting ions and omits spectator ions 旁观离子 that remain unchanged. For aqueous silver nitrate and sodium chloride, Ag⁺(aq) + Cl⁻(aq) → AgCl(s) describes the precipitate; Na⁺ and NO₃⁻ remain in solution. A half equation shows electron loss or gain: Mg → Mg²⁺ + 2e⁻ is oxidation, while Cu²⁺ + 2e⁻ → Cu is reduction. Electrons carry negative charge.
- half equation: An equation showing electron loss or gain for one oxidation or reduction process; spectator ion: An ion unchanged on both sides of a reaction and omitted from its net ionic equation.
Which half equation balances both Mg atoms and charge?
In the magnesium half equation, the right-hand total charge is +2 − 2 = 0, matching neutral Mg. In copper reduction, +2 − 2 = 0 on the left, matching neutral copper. Neutralization can be represented as H⁺(aq) + OH⁻(aq) → H₂O(l); the net charge is zero on each side. Electron numbers must match when combining half equations so no free electrons remain in the overall reaction.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- In the magnesium half equation, the right-hand total charge is +2 − 2 = 0, matching neutral Mg. In copper reduction, +2 − 2 = 0 on the left, matching neutral copper. Neutralization can be represented as H⁺(aq) + OH⁻(aq) → H₂O(l); the net charge is zero on each side. Electron numbers must match when combining half equations so no free electrons remain in the overall reaction.
- First write the relevant ions and products, then balance atoms and charge separately. For a full aqueous equation, expand soluble strong electrolytes into their ions and cancel only identical species present on both sides. Retain solid precipitates, water and other unchanged molecular forms as appropriate. Use supplied ion charges; the equation is not a licence to invent a charge.
Which two habits make the investigation or model in this case more defensible?
First write the relevant ions and products, then balance atoms and charge separately. For a full aqueous equation, expand soluble strong electrolytes into their ions and cancel only identical species present on both sides. Retain solid precipitates, water and other unchanged molecular forms as appropriate. Use supplied ion charges; the equation is not a licence to invent a charge.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: aluminium oxidation is Al → Al³⁺ + 3e⁻. Two aluminium atoms therefore release six electrons: 2Al → 2Al³⁺ + 6e⁻. Three Cu²⁺ ions accept the same six electrons: 3Cu²⁺ + 6e⁻ → 3Cu. Combined: 2Al + 3Cu²⁺ → 2Al³⁺ + 3Cu. Both sides have total charge +6.
How many electrons are released when three Al atoms each form Al³⁺? Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
How many electrons are released when three Al atoms each form Al³⁺?
The result is 9 electrons. Known: aluminium oxidation is Al → Al³⁺ + 3e⁻. Two aluminium atoms therefore release six electrons: 2Al → 2Al³⁺ + 6e⁻. Three Cu²⁺ ions accept the same six electrons: 3Cu²⁺ + 6e⁻ → 3Cu. Combined: 2Al + 3Cu²⁺ → 2Al³⁺ + 3Cu. Both sides have total charge +6.
Check the conclusion and its limits
- Electrons appear on the product side for oxidation and the reactant side for reduction. Spectator ions are chemically present even though omitted from the net equation. This Higher-only treatment does not make common-tier atom counting, isotope calculations or relative atomic mass Higher-only.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
An ionic equation need only conserve atoms, not charge. This claim is false: Electrons appear on the product side for oxidation and the reactant side for reduction. Spectator ions are chemically present even though omitted from the net equation. This Higher-only treatment does not make common-tier atom counting, isotope calculations or relative atomic mass Higher-only.
Higher Tier: conserve charge in ionic and half equations: In the magnesium half equation, the right-hand total charge is +2 − 2 = 0, matching neutral Mg. In copper reduction, +2 − 2 = 0 on the left, matching neutral copper. Neutralization can be represented as H⁺(aq) + OH⁻(aq) → H₂O(l); the net charge is zero on each side. Electron numbers must match when combining half equations so no free electrons remain in the overall reaction.
An ionic equation need only conserve atoms, not charge.
Electrons appear on the product side for oxidation and the reactant side for reduction. Spectator ions are chemically present even though omitted from the net equation. This Higher-only treatment does not make common-tier atom counting, isotope calculations or relative atomic mass Higher-only.
An equation showing electron loss or gain for one oxidation or reduction process: write the technical term.
half equation means An equation showing electron loss or gain for one oxidation or reduction process.