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Chemistry: Atoms to AP Chemistry · Module 3 of 15

Chemical Reactions & Equations

Writing, balancing, classifying, and predicting the products of chemical reactions.

In this module
  1. Writing & Balancing Chemical Equations
  2. Types of Chemical Reactions
  3. Predicting Products
  4. Net Ionic Equations
  5. Check yourself

Writing & Balancing Chemical Equations

  • A balanced equation has the same number of atoms of each element on both sides, satisfying the law of conservation of mass.
  • Balance by adjusting coefficients in front of formulas — never subscripts — usually balancing the most complex molecule first.
Worked example

Balance: __ H₂ + __ O₂ → __ H₂O.

Solution. Balancing oxygen first needs 2 H₂O (2 oxygens); that requires 4 H, so 2 H₂. Balanced: 2H₂ + O₂ → 2H₂O.

Common mistake: Never change a subscript to balance an equation — that changes what substance the formula represents.

Types of Chemical Reactions

  • Synthesis (A + B → AB), decomposition (AB → A + B), single replacement (A + BC → AC + B), double replacement (AB + CD → AD + CB), combustion (a hydrocarbon + O₂ → CO₂ + H₂O).
Worked example

Classify: CH₄ + 2O₂ → CO₂ + 2H₂O.

Solution. This is a combustion reaction — a hydrocarbon reacting with oxygen to produce carbon dioxide and water.

Predicting Products

  • An activity series ranks metals by reactivity; a more reactive metal can displace a less reactive one from a compound in a single replacement reaction.
  • Solubility rules help predict whether a double replacement reaction forms a precipitate (insoluble solid) — if no precipitate, gas, or water forms, no reaction occurs.
Worked example

Will zinc metal react with a copper(II) sulfate solution: Zn + CuSO₄ → ?

Solution. Yes — zinc is more reactive than copper (higher on the activity series), so it displaces copper: Zn + CuSO₄ → ZnSO₄ + Cu.

Net Ionic Equations

  • In aqueous double replacement reactions, "spectator ions" appear unchanged on both sides and are removed to write the net ionic equation.
  • The net ionic equation shows only the ions or molecules that actually participate in forming the precipitate, gas, or water.
Worked example

Write the net ionic equation for: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq).

Solution. Na⁺ and NO₃⁻ are spectator ions, unchanged on both sides. Net ionic equation: Ag⁺(aq) + Cl⁻(aq) → AgCl(s).

Check yourself

2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.

1. What law does balancing a chemical equation satisfy?

2. When balancing an equation, what are you allowed to change?

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