Chemistry-only: balance insoluble-hydroxide formation equations
| English | 中文 | Pinyin |
|---|---|---|
| formation equation | 生成反应方程式 | shēng chéng fǎn yìng fāng chéng shì |
| hydroxide formula | 氢氧化物化学式 | qīng yǎng huà wù huà xué shì |
What would explain this observation?
- Iron(II) chloride and iron(III) chloride need different quantities of sodium hydroxide in their balanced formation equations 生成反应方程式. The metal-ion charge affects the hydroxide formula 氢氧化物化学式.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- For a supplied soluble metal chloride and sodium hydroxide, metal hydroxide precipitates while sodium chloride remains dissolved. Magnesium and calcium form Mg(OH)₂ and Ca(OH)₂. Copper(II) and iron(II) form Cu(OH)₂ and Fe(OH)₂. Aluminium and iron(III) form Al(OH)₃ and Fe(OH)₃. OH inside brackets is a hydroxide group; its subscript multiplies both oxygen and hydrogen. Do not change the compound formula while balancing.
- hydroxide formula: The stated metal-to-hydroxide-group composition of a metal hydroxide; formation equation: A balanced equation describing production of the stated hydroxide precipitate.
Which formula is correct for iron(III) hydroxide?
The divalent examples have form MCl₂(aq) + 2NaOH(aq) → M(OH)₂(s) + 2NaCl(aq), with M the stated Mg, Ca, Cu or Fe(II). For Al and Fe(III), MCl₃(aq) + 3NaOH(aq) → M(OH)₃(s) + 3NaCl(aq). These are full symbol equations, suitable for the both-tier requirement. Separate HT ionic-equation work may simplify them, but it is not needed to recognise the common-tier formation equations here.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- The divalent examples have form MCl₂(aq) + 2NaOH(aq) → M(OH)₂(s) + 2NaCl(aq), with M the stated Mg, Ca, Cu or Fe(II). For Al and Fe(III), MCl₃(aq) + 3NaOH(aq) → M(OH)₃(s) + 3NaCl(aq). These are full symbol equations, suitable for the both-tier requirement. Separate HT ionic-equation work may simplify them, but it is not needed to recognise the common-tier formation equations here.
- Write actual names and formulae first, then count metal, chlorine, sodium, oxygen and hydrogen on both sides. Keep the precipitate solid and the stated starting solutions aqueous. Use the visible precipitate observation to justify (s), rather than assuming every product is solid. Compare the six formation equations with the identification table. The aluminium excess-alkali dissolution observation is separate; the specification does not require a sodium-aluminate production equation.
Which two habits make the investigation or model in this case more defensible?
Write actual names and formulae first, then count metal, chlorine, sodium, oxygen and hydrogen on both sides. Keep the precipitate solid and the stated starting solutions aqueous. Use the visible precipitate observation to justify (s), rather than assuming every product is solid. Compare the six formation equations with the identification table. The aluminium excess-alkali dissolution observation is separate; the specification does not require a sodium-aluminate production equation.
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: FeCl₃ + 3NaOH → Fe(OH)₃ + 3NaCl conserves Fe 1, Cl 3, Na 3, O 3 and H 3 on each side. Two stated FeCl₃ formula units correspond to six NaOH units and two Fe(OH)₃ units in this particle ratio. For FeCl₂ the analogous hydroxide coefficient is two, not three, because the compound formula is different.
In FeCl₃ + 3NaOH → Fe(OH)₃ + 3NaCl, four FeCl₃ formula units require how many NaOH units? Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
In FeCl₃ + 3NaOH → Fe(OH)₃ + 3NaCl, four FeCl₃ formula units require how many NaOH units?
The result is 12 units. Known: FeCl₃ + 3NaOH → Fe(OH)₃ + 3NaCl conserves Fe 1, Cl 3, Na 3, O 3 and H 3 on each side. Two stated FeCl₃ formula units correspond to six NaOH units and two Fe(OH)₃ units in this particle ratio. For FeCl₂ the analogous hydroxide coefficient is two, not three, because the compound formula is different.
Check the conclusion and its limits
- A coefficient before NaOH counts complete formula units; a subscript in Fe(OH)₃ gives composition. Do not write FeOH₃ or remove sodium/chloride from a full symbol equation without explicitly changing to an ionic representation. These particle counts do not require a mole calculation or imply that any unknown chloride should be handled without the approved school method.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Changing a hydroxide subscript is an acceptable way to balance any equation. This claim is false: A coefficient before NaOH counts complete formula units; a subscript in Fe(OH)₃ gives composition. Do not write FeOH₃ or remove sodium/chloride from a full symbol equation without explicitly changing to an ionic representation. These particle counts do not require a mole calculation or imply that any unknown chloride should be handled without the approved school method.
Chemistry-only: balance insoluble-hydroxide formation equations: The divalent examples have form MCl₂(aq) + 2NaOH(aq) → M(OH)₂(s) + 2NaCl(aq), with M the stated Mg, Ca, Cu or Fe(II). For Al and Fe(III), MCl₃(aq) + 3NaOH(aq) → M(OH)₃(s) + 3NaCl(aq). These are full symbol equations, suitable for the both-tier requirement. Separate HT ionic-equation work may simplify them, but it is not needed to recognise the common-tier formation equations here.
Changing a hydroxide subscript is an acceptable way to balance any equation.
A coefficient before NaOH counts complete formula units; a subscript in Fe(OH)₃ gives composition. Do not write FeOH₃ or remove sodium/chloride from a full symbol equation without explicitly changing to an ionic representation. These particle counts do not require a mole calculation or imply that any unknown chloride should be handled without the approved school method.
The stated metal-to-hydroxide-group composition of a metal hydroxide: write the technical term.
hydroxide formula means The stated metal-to-hydroxide-group composition of a metal hydroxide.