AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/4/4-3)
Unit 4 · Topic 4.3
4.3 Representations of Reactions
You need to turn a balanced equation into a particle diagram, and a particle diagram back into an equation. A correct diagram keeps every atom, shows reactants and products in the ratio set by the coefficients, and shows any reactant left over.
Key terms
- particulate diagram
- coefficient
- mole ratio
- excess reactant
From equation to diagram
A particle diagram draws atoms as circles, with different colors or sizes for different elements. Molecules are circles touching; ions are separate circles, sometimes with charges.
Coefficients give ratios of particles, not exact numbers. 2H₂ + O₂ → 2H₂O means 2 hydrogen molecules react for every 1 oxygen molecule, making 2 water molecules. A diagram can show 4 H₂ + 2 O₂ → 4 H₂O, or 6 H₂ + 3 O₂ → 6 H₂O; the ratio is what matters.
Rules for a correct diagram
- Conserve atoms: count each kind of atom before and after. The totals must match.
- Use the right formulas: H₂O is drawn as one O atom touching two H atoms. Diatomic elements (H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂) are drawn as pairs.
- Match the ratios set by the coefficients.
- Show leftover reactant. If one reactant runs out first (the limiting reactant), the other (the excess reactant) remains unreacted in the after picture.
- Show the phase: separate ions spread out in water for (aq), a packed cluster for a precipitate (s).
From diagram to equation
Going the other way, count the particles that actually changed. Cross out particles that appear unchanged on both sides, since they didn't react (they're excess reactant or spectators). Then reduce the numbers of reacting particles and products to the smallest whole-number ratio. Those are your coefficients.
Example: a before box has 4 NO molecules and 3 O₂ molecules. The after box has 4 NO₂ molecules and 1 O₂ molecule. So 4 NO and 2 O₂ reacted to make 4 NO₂. Dividing by 2 gives 2NO + O₂ → 2NO₂.
Diagrams for reactions in solution
For a precipitation reaction, the before picture shows two solutions of separated ions. The after picture shows the precipitate as a cluster of paired-up ions, and the spectator ions still floating separately. That picture is a net ionic equation in drawing form: only the ions that changed are different between the before and after boxes.
Reading a diagram with ions
Suppose a before box shows 4 Ag⁺ and 4 NO₃⁻ ions from one solution, mixed with 3 Na⁺ and 3 Cl⁻ ions from another. Ag⁺ and Cl⁻ combine 1 : 1 to form solid AgCl, so only 3 pairs can form; Cl⁻ is limiting. The after box should show a cluster of 3 AgCl units, plus 1 leftover Ag⁺, all 4 NO₃⁻ and all 3 Na⁺ still separate in solution.
Count charges as a check: the dissolved ions left over (1 Ag⁺, 3 Na⁺, 4 NO₃⁻) add to zero, as they must.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Finding the product picture
A container holds 3 N₂ molecules and 6 H₂ molecules. They react according to N₂ + 3H₂ → 2NH₃ until one reactant runs out. Describe the particles in the container afterward.
Show the solutionHide the solution
- Step 1: Each N₂ needs 3 H₂. The 6 H₂ molecules can react with only 6 ÷ 3 = 2 N₂ molecules. H₂ is limiting.
- Step 2: 2 N₂ react, making 2 × 2 = 4 NH₃ molecules.
- Step 3: N₂ left over: 3 − 2 = 1 molecule. H₂ left: 0.
- Step 4: Check atoms: before, 6 N and 12 H. After, 4 NH₃ (4 N, 12 H) + 1 N₂ (2 N) = 6 N and 12 H.
Answer: 4 NH₃ molecules and 1 leftover N₂ molecule
- Example 2
Leftover reactant (classic trap)
A before box shows 6 H₂ and 4 O₂ molecules reacting as 2H₂ + O₂ → 2H₂O. A student's after box shows 6 H₂O molecules and nothing else. What's wrong?
Show the solutionHide the solution
- Step 1: 6 H₂ need 6 ÷ 2 = 3 O₂. There are 4 O₂, so O₂ is in excess and H₂ is limiting.
- Step 2: Products: 6 H₂O. Leftover: 4 − 3 = 1 O₂.
- Step 3: Atom check: before, 8 O atoms. The student's box has only 6 O atoms, so 2 O atoms vanished. The leftover O₂ molecule is missing.
Answer: The after box should contain 6 H₂O and 1 unreacted O₂ molecule; the student's drawing loses two oxygen atoms.
Common mistakes
- Drawing exactly the coefficients instead of a matching ratio when the problem gives a specific number of particles.
- Forgetting leftover excess reactant in the product picture.
- Drawing diatomic elements as single atoms.
- Showing a precipitate as separated ions, or spectator ions as part of the solid.
On the exam
- Expect before-and-after boxes on multiple-choice questions. Count atoms of each element first; wrong answers often fail atom conservation.
- Free-response questions may ask you to draw the product side. Label your particles with a key, keep the ratio right and include any excess reactant.
Connected topics
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Check yourself
4 questions on 4.3 Representations of Reactions. Pick an answer to see if you got it, and why.
A particle diagram shows a container that holds 3 X₂ molecules and 6 Y₂ molecules before a reaction. After the reaction, the container holds 4 XY₃ molecules and 1 X₂ molecule. Which balanced equation best represents the reaction?
Which of the following is a correct interpretation of the balanced equation 2H₂(g) + O₂(g) → 2H₂O(g)?
Iron reacts with oxygen to form iron(III) oxide, Fe₂O₃. Which of the following equations correctly represents this reaction?
A particle diagram shows a sealed container holding 6 molecules of N₂ and 6 molecules of H₂. The reaction N₂(g) + 3H₂(g) → 2NH₃(g) goes to completion. Which of the following describes the contents of the container after the reaction?
0 of 4 answered