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Credit Sprint's Guide to Chemistry Unit 4: Chemical Reactions

AP® weight 7–9%CLEP® weight 20–25%

Balanced equations conserve atoms and charge, and stoichiometry turns them into predictions of how much product forms. Net ionic equations show what really reacts, and oxidation numbers track electrons in redox reactions.

The mole mapGrams A Moles A Moles B ÷ molar massmole ratio Grams B × molar mass Always go through moles. The balanced equation gives the ratio.

What you need to know

  • Balance by atoms and charge. Types: synthesis, decomposition, single and double replacement, combustion, acid-base and redox.
  • Stoichiometry path: grams → moles → mole ratio → moles → grams. The limiting reactant runs out first and sets the maximum yield.
  • Percent yield = actual ÷ theoretical × 100.
  • Solubility: nitrates, group 1 and ammonium salts dissolve; most carbonates, phosphates and hydroxides do not; AgCl, PbI₂ and BaSO₄ precipitate.
  • Write net ionic equations by splitting strong electrolytes and cancelling spectator ions. Weak acids and bases stay as molecules.
  • Oxidation is loss of electrons (OIL), reduction is gain (RIG). The oxidizing agent is reduced; the reducing agent is oxidized.
  • Acid-base titration: moles acid × H⁺ per acid = moles base × OH⁻ per base at the equivalence point.
  • Heating a hydrate to constant mass finds the water of hydration.
  • Descriptive chemistry: alkali metals react with water to give H₂ and a hydroxide; halogens displace weaker halogens; flame tests identify cations; carbonates fizz with acid; ammonium releases NH₃ with base.

Equations and rules

percent yield = (actual ÷ theoretical) × 100
M_acid V_acid (n_H⁺) = M_base V_base (n_OH⁻)
sum of oxidation numbers = charge on the species

Exam traps

Watch forUse the limiting reactant, not whichever amount is smaller in grams.
Watch forSpectator ions don't appear in the net ionic equation.
Watch forA diprotic acid needs two moles of OH⁻ per mole.
Watch forDon't split weak electrolytes or insoluble solids into ions.

Key terms

  • Limiting reactant: The reactant that runs out first and limits the amount of product.
  • Percent yield: Actual yield ÷ theoretical yield × 100.
  • Net ionic equation: An equation showing only the species that actually change, with spectator ions removed.
  • Spectator ion: An ion present in solution but unchanged by the reaction.
  • Precipitate: An insoluble solid formed when solutions are mixed.
  • Oxidation: Loss of electrons; oxidation number increases.
  • Reduction: Gain of electrons; oxidation number decreases.
  • Oxidizing agent: The species that is reduced and takes electrons from another.
  • Titration: A technique that finds an unknown concentration by reacting it with a solution of known concentration.
  • Equivalence point: The point at which moles of acid and base are stoichiometrically equal.
  • Primary standard: A pure, stable solid of known formula used to prepare a solution of exactly known concentration.

Practice questions

Five of the 65 Unit 4 questions in Credit Sprint. Every choice is explained in the app.

1. What are the coefficients that balance Fe + O₂ → Fe₂O₃ (in that order)?
  1. 2, 3, 1
  2. 2, 1, 1
  3. 4, 3, 2
  4. 1, 1, 1
Show answer

4, 3, 2. 4 Fe + 3 O₂ → 2 Fe₂O₃ has 4 Fe and 6 O on each side.

2. What mass of CO₂ forms when 10.0 g of CH₄ (16.04 g/mol) burns completely? (CH₄ + 2 O₂ → CO₂ + 2 H₂O; CO₂ = 44.01 g/mol)
  1. 10.0 g
  2. 27.4 g
  3. 54.8 g
  4. 6.30 g
Show answer

27.4 g. 0.623 mol CH₄ gives 0.623 mol CO₂ = 27.4 g.

3. A 25.0 mL sample of H₂SO₄ is neutralized by 30.0 mL of 0.200 M NaOH. What is the molarity of the acid?
  1. 0.0600 M
  2. 0.120 M
  3. 0.240 M
  4. 0.167 M
Show answer

0.120 M. 0.00600 mol OH⁻ neutralizes 0.00300 mol H₂SO₄ (diprotic); 0.00300 ÷ 0.0250 L = 0.120 M.

4. Which pair of solutions forms a precipitate when mixed?
  1. HCl and NaOH
  2. NaCl and KNO₃
  3. K₂SO₄ and NaNO₃
  4. Pb(NO₃)₂ and KI
Show answer

Pb(NO₃)₂ and KI. Yellow PbI₂ forms.

5. What is the oxidation number of manganese in KMnO₄?
  1. +7
  2. +2
  3. +4
  4. +5
Show answer

+7. K is +1, four O total −8, so Mn = +7.