AP Chemistry · Lesson 1

Balancing and classifying reactions

Know Know it

A chemical reaction rearranges atoms; it never creates or destroys them. A balanced equation has the same number of each kind of atom on both sides, and the same total charge. Balance by changing coefficients only. Changing a subscript would change the substance itself.

Three reaction types appear again and again. In a precipitation reaction, two dissolved ionic compounds trade partners and an insoluble solid forms. In an acid-base reaction, a proton (H⁺) moves from an acid to a base. In an oxidation-reduction (redox) reaction, electrons move: the species that loses electrons is oxidized and its oxidation number rises; the species that gains them is reduced.

In water, many ionic compounds are split into ions. A net ionic equation keeps only the species that change. Ions that are dissolved before and after the reaction are spectator ions, and they cancel. Compounds that contain Na⁺, K⁺, NH₄⁺ or NO₃⁻ are always soluble, so those ions are never the precipitate.

Coefficients give the mole ratio: in 2A + B → 3C, every 2 mol of A that react make 3 mol of C.

How to tell the three reaction types apart
TypeWhat movesWhat to look for
PrecipitationIons pair up into a solidTwo aqueous ionic compounds; a product labeled (s)
Acid-baseA proton (H⁺)A species that donates H⁺ and one that accepts it, often making water
RedoxElectronsAn oxidation number that changes; an element that is uncombined on one side and in a compound on the other usually signals it

Worked example Easy

When the equation for burning butane, C₄H₁₀ + O₂ → CO₂ + H₂O, is balanced with the smallest whole-number coefficients, what is the coefficient of O₂?

  1. 6.5
  2. 13Answer
  3. 9
  4. 26

How to solve it

  1. Balance carbon and hydrogen first: 1 C₄H₁₀ gives 4 CO₂ and 5 H₂O.
  2. Count oxygen on the right: 4(2) + 5(1) = 13 atoms, which would need 6.5 O₂.
  3. Double everything to clear the half: 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O. The coefficient of O₂ is 13.

Why each choice is right or wrong

  • A. Incorrect. 6.5 balances the equation for one butane, but it is not a whole number; doubling every coefficient gives 13.
  • B. Correct. Balancing carbon and hydrogen gives 2C₄H₁₀ → 8CO₂ + 10H₂O, which has 16 + 10 = 26 oxygen atoms, so 13 O₂ are needed.
  • C. Incorrect. 9 comes from counting 2 oxygen atoms in each water molecule; water has only 1, so the oxygen count is 13 per butane, not 18.
  • D. Incorrect. 26 is the number of oxygen atoms on the right, not the number of O₂ molecules; each O₂ carries 2 atoms, so 13 molecules supply them.

Trap Never fix an equation with subscripts

If oxygen does not balance, do not change H₂O to H₂O₂. That is a different compound. Change only the numbers in front of the formulas, and use a fraction for one step if you need it, then multiply through to get whole numbers.

Explain Explain it

Task verbs: identify, explain

To write a net ionic equation: (1) write the balanced equation with states; (2) split every strong electrolyte that is (aq) into its ions; (3) keep solids, liquids, gases and weak electrolytes whole; (4) cancel ions that appear unchanged on both sides; (5) check that atoms and charges balance.

To decide whether a reaction is redox, assign oxidation numbers to each element on both sides. If any of them changes, electrons moved, and the reaction is redox. The total number of electrons lost must equal the total gained.

Worked example Medium

A student mixes clear aqueous solutions of barium nitrate, Ba(NO₃)₂, and sodium sulfate, Na₂SO₄. A white solid forms at once.

Which equation is the net ionic equation for the reaction?

  1. Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)Answer
  2. Ba(NO₃)₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaNO₃(aq)
  3. Na⁺(aq) + NO₃⁻(aq) → NaNO₃(s)
  4. Ba²⁺(aq) + 2NO₃⁻(aq) + 2Na⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) + 2Na⁺(aq) + 2NO₃⁻(aq)

How to solve it

  1. The two possible new pairings are BaSO₄ and NaNO₃. NaNO₃ is always soluble, so BaSO₄ is the solid.
  2. Complete ionic: Ba²⁺ + 2NO₃⁻ + 2Na⁺ + SO₄²⁻ → BaSO₄(s) + 2Na⁺ + 2NO₃⁻.
  3. Cancel the spectators (Na⁺ and NO₃⁻): Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). Atoms and charge (0 on each side) balance.

Why each choice is right or wrong

  • A. Correct. Sodium nitrate contains always-soluble ions, so the white solid is barium sulfate, and only Ba²⁺ and SO₄²⁻ change.
  • B. Incorrect. This is the balanced molecular equation; it still shows the dissolved compounds whole instead of removing the Na⁺ and NO₃⁻ spectators.
  • C. Incorrect. NaNO₃ contains Na⁺ and NO₃⁻, which are always soluble, so it cannot be the white solid.
  • D. Incorrect. This complete ionic equation still lists the Na⁺ and NO₃⁻ spectator ions on both sides; cancelling them leaves the net equation.

Explain Your turn

Look at the reaction 2Al(s) + 3Cu²⁺(aq) → 2Al³⁺(aq) + 3Cu(s). Name the species that is oxidized, explain how its oxidation number shows this, and say how many electrons are transferred in the reaction as written.

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Compare with a model answer

A full answer does this

  • Names aluminum (Al) as the species that is oxidized
  • Explains that aluminum's oxidation number rises from 0 to +3 as it loses electrons
  • Says that 6 electrons are transferred for the reaction as written

Model answer

Aluminum metal is oxidized. Its oxidation number rises from 0 in Al(s) to +3 in Al³⁺, so each aluminum atom loses 3 electrons. As written, 2 aluminum atoms lose 3 electrons each, so 6 electrons are transferred; the 3 copper(II) ions gain 2 electrons each, which is also 6.

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