Balance equations by conserving each element
| English | Français |
|---|---|
| reactant/rɪˈæktənt/ | réactif |
| coefficient/ˌkəʊɪˈfɪʃənt/ | coefficient |
What would explain this observation?
- Magnesium burns in oxygen to form magnesium oxide. Writing Mg + O₂ → MgO names the substances correctly, but it does not yet account for every atom.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- A word equation gives reactant · réactif 反应物 and product names: magnesium + oxygen → magnesium oxide. A symbol equation uses correct formulae. Balance it as 2Mg + O₂ → 2MgO: both sides now contain two magnesium atoms and two oxygen atoms. Atoms are rearranged in a reaction, not created or destroyed. The numbers before formulae are coefficients 系数 describing relative particle numbers.
- coefficient: A number before a chemical formula multiplying the entire formula; reactant: A substance used up or changed in a chemical reaction.
Which equation conserves Mg and O atoms?
Begin with the actual substance formulae and make an element-count table. Change coefficients to equalize the count for each element. Never change MgO to MgO₂ just to balance oxygen: that would identify a different substance. For hydrogen burning, 2H₂ + O₂ → 2H₂O conserves four hydrogen atoms and two oxygen atoms. State symbols, when required, distinguish solid (s), liquid (l), gas (g) and aqueous solution (aq).
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Begin with the actual substance formulae and make an element-count table. Change coefficients to equalize the count for each element. Never change MgO to MgO₂ just to balance oxygen: that would identify a different substance. For hydrogen burning, 2H₂ + O₂ → 2H₂O conserves four hydrogen atoms and two oxygen atoms. State symbols, when required, distinguish solid (s), liquid (l), gas (g) and aqueous solution (aq).
- Use coloured counters to represent atoms and construct the reactants from the stated formulae. Rearrange the same counters into product particles, then record the smallest whole-number coefficients. Confirm each element separately. Models count atoms and bonds schematically; they do not show the detailed collision mechanism or actual particle size.
Which two habits make the investigation or model in this case more defensible?
Use coloured counters to represent atoms and construct the reactants from the stated formulae. Rearrange the same counters into product particles, then record the smallest whole-number coefficients. Confirm each element separately. Models count atoms and bonds schematically; they do not show the detailed collision mechanism or actual particle size.
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 reacts with oxygen to form Al₂O₃. Six oxygen atoms are supplied by 3O₂ and appear in 2Al₂O₃. Those product units contain four aluminium atoms, so 4Al + 3O₂ → 2Al₂O₃. Final count: Al 4→4 and O 6→6. The coefficients 4:3:2 are a particle ratio, not a ratio of masses in grams.
In 4Al + 3O₂ → 2Al₂O₃, how many oxygen atoms are represented on either side? Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
In 4Al + 3O₂ → 2Al₂O₃, how many oxygen atoms are represented on either side?
The result is 6 atoms. Known: aluminium reacts with oxygen to form Al₂O₃. Six oxygen atoms are supplied by 3O₂ and appear in 2Al₂O₃. Those product units contain four aluminium atoms, so 4Al + 3O₂ → 2Al₂O₃. Final count: Al 4→4 and O 6→6. The coefficients 4:3:2 are a particle ratio, not a ratio of masses in grams.
Check the conclusion and its limits
- A balanced equation cannot be obtained by changing a product formula or deleting an element. A coefficient of one is normally omitted. Equal atom counts do not mean equal numbers of molecules, and a balanced equation alone does not establish that a reaction occurs under all conditions.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Balancing an equation allows the product formula to change. This claim is false: A balanced equation cannot be obtained by changing a product formula or deleting an element. A coefficient of one is normally omitted. Equal atom counts do not mean equal numbers of molecules, and a balanced equation alone does not establish that a reaction occurs under all conditions.
Balance equations by conserving each element: Begin with the actual substance formulae and make an element-count table. Change coefficients to equalize the count for each element. Never change MgO to MgO₂ just to balance oxygen: that would identify a different substance. For hydrogen burning, 2H₂ + O₂ → 2H₂O conserves four hydrogen atoms and two oxygen atoms. State symbols, when required, distinguish solid (s), liquid (l), gas (g) and aqueous solution (aq).
Balancing an equation allows the product formula to change.
A balanced equation cannot be obtained by changing a product formula or deleting an element. A coefficient of one is normally omitted. Equal atom counts do not mean equal numbers of molecules, and a balanced equation alone does not establish that a reaction occurs under all conditions.
A number before a chemical formula multiplying the entire formula: write the technical term.
coefficient means A number before a chemical formula multiplying the entire formula.