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Chemistry · 04 / 10

Balance a reaction equation

High school foundations · Worked example and 4 exercises with solutions, at your pace.

UnderstandSee an exampleGuided exercise3 progressive challenges

The principle, simply

Reaction equations conserve each element’s atoms and total charge. Coefficients give amount-of-substance ratios. Change coefficients, not formulas: replacing H₂O with H₂O₂ changes the species. Simplest integer coefficients are a useful convention.

THE PICTURE TO REMEMBERBalance a reaction equation
  1. 01Exact formulas
  2. 02Count atoms
  3. 03Coefficients
  4. 04Check atoms and charge

WORKED EXAMPLE

From principle to reasoning

H₂+O₂→H₂O becomes 2H₂+O₂→2H₂O, with four H and two O on each side.

Solve a problem in this subject

  1. Identify species

    Write formulas and useful states. Distinguish atoms, ions, molecules and amount of substance. Total charge is conserved.

  2. Connect quantities

    Use m = nM and C = n/V with consistent units. V is the final solution volume.

  3. Use coefficients

    Balance before building an extent table. Compare initial amounts divided by coefficients to identify the limiting reactant.

  4. Check context

    Remaining amounts cannot be negative. Check conservation, units and assumptions: dilution, medium, completion or equilibrium.

YOUR TURN

01 · Try before checking

Balance N₂+H₂→NH₃.

One hint

Balance nitrogen first, then count hydrogen.

Reveal and understand the solution

N₂+3H₂→2NH₃ contains 2 N and 6 H on each side. Coefficients also give molar ratios.

Compare method, conditions and answer; identify the exact part to revisit.

IMPROVE THROUGH PRACTICE

Three additional challenges with solutions

Start with the first challenge. Write your method before opening a hint; use the solution to find the first step that needs work.

Reflect after practising
02 · Recognise and understandLink to this exercise

Balance H₂ + O₂ → H₂O.

A hint to get started

Change coefficients, never subscripts.

Step-by-step solution
  1. Two water molecules use the two oxygen atoms.
  2. Four H atoms require two H₂.
  3. 2H₂ + O₂ → 2H₂O.

Which step did you miss? Retry without help, then explain why the method works.

03 · Apply the methodLink to this exercise

Balance CH₄ + O₂ → CO₂ + H₂O.

A hint to get started

Balance C first, then H, then O.

Step-by-step solution
  1. One C gives one CO₂.
  2. Four H atoms give two H₂O.
  3. Four O atoms are needed: CH₄ + 2O₂ → CO₂ + 2H₂O.

Which step did you miss? Retry without help, then explain why the method works.

04 · Go furtherLink to this exercise

Why is changing H₂O to H₂O₂ an invalid balancing method?

A hint to get started

A formula identifies a chemical species.

Step-by-step solution
  1. H₂O and H₂O₂ are different substances.
  2. Changing the subscript changes the reaction product.
  3. Only coefficients adjust amounts of the specified species.

Which step did you miss? Retry without help, then explain why the method works.

The trap to avoid

A coefficient multiplies the whole formula rather than changing a subscript.

Without notes, can you…

  • Explain the principle in your own words?
  • Rebuild the diagram and its connections?
  • Solve the exercise and justify the method?

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