USNCO Reaction Writing Study Guide
163 questions from 1999–2025. Filter by year, type, and difficulty. Progressively reveal answers, justifications, and extensions.
Writing reactions requires not only memorizing patterns, but also reasoning from first principles and careful analysis of driving forces, oxidation states, and acid-base hierarchies.
Browse All Questions📋 Official USNCO Instructions
“Write a balanced, net ionic equation for the reaction that occurs. Represent substances in solution as ions if the substances are extensively ionized. Omit formulas for any ions or molecules that are unchanged by the reaction. You need not balance the equations.”
First Principles Reference
| # | Principle | Summary |
|---|---|---|
| FP1 | Driving Forces | Reactions proceed toward precipitates, gases, weak electrolytes (H₂O), stable complexes, or more stable oxidation states. |
| FP2 | Oxidation State Tracking | Assign oxidation states → identify what is oxidized/reduced → electron balance determines stoichiometry. |
| FP3 | Acid-Base Hierarchy | Stronger acid displaces weaker acid; compare Ka values to predict which proton transfers are favorable. |
| FP4 | Lewis Structure / Electron Availability | Lone pairs donate to empty orbitals; nucleophiles attack electrophiles. |
| FP5 | Periodic Trends | Charge density (Z²/r), electronegativity, metallic character, E° predict chemical behavior from periodic position. |
| FP6 | Conservation Laws | Mass, charge, and electrons are conserved; these constrain and verify balanced equations. |
Common Oxidizing & Reducing Agents
Redox is the largest category (50 questions). Knowing what each agent produces is the key to writing these equations.
Oxidizing Agents (get reduced)
| Agent | Condition | Product | e⁻ change | Key detail |
|---|---|---|---|---|
| MnO₄⁻ | acidic | Mn²⁺ | +7→+2 (5e⁻) | Purple → colorless |
| MnO₄⁻ | neutral/basic | MnO₂ | +7→+4 (3e⁻) | Purple → brown ppt |
| Cr₂O₇²⁻ | acidic | Cr³⁺ | +6→+3 (6e⁻/ion) | Orange → green. Oxidizes alcohols |
| NO₃⁻ (HNO₃) | dilute | NO | +5→+2 (3e⁻) | Colorless gas, browns in air |
| NO₃⁻ (HNO₃) | concentrated | NO₂ | +5→+4 (1e⁻) | Brown gas |
| H₂SO₄ | conc., hot | SO₂ | +6→+4 (2e⁻) | Dissolves Cu. Molecular equation |
| H₂O₂ | as oxidizer | H₂O | −1→−2 (2e⁻) | Oxidizes I⁻→I₂ |
| MnO₂ | + conc. HCl | Mn²⁺ | +4→+2 (2e⁻) | Lab prep of Cl₂ (3× on USNCO) |
Reducing Agents (get oxidized)
| Agent | Product | e⁻ change | Key detail |
|---|---|---|---|
| Fe²⁺ | Fe³⁺ | +2→+3 (1e⁻) | Classic titration with MnO₄⁻ or Cr₂O₇²⁻ |
| Sn²⁺ | Sn⁴⁺ | +2→+4 (2e⁻) | Paired with Cr₂O₇²⁻ |
| I⁻ | I₂ | −1→0 (1e⁻) | Oxidized by H₂O₂, Cu²⁺, IO₃⁻ |
| C₂O₄²⁻ | CO₂ | +3→+4 (2e⁻/ion) | Paired with KMnO₄. Gas escapes |
| SO₂/SO₃²⁻ | SO₄²⁻ | +4→+6 (2e⁻) | Mild reductant |
| S₂O₃²⁻ | S₄O₆²⁻ | +2→+2.5 (1e⁻/ion) | Titrant for I₂ (iodometric titration) |
| Cl⁻ (conc.) | Cl₂ | −1→0 (1e⁻) | Needs conc. HCl + strong oxidizer |
| RCH₂OH (1° alcohol) | RCHO / RCOOH | 2e⁻ or 4e⁻ loss | PCC stops at aldehyde; Cr₂O₇²⁻ or KMnO₄ gives full oxidation to acid |
| R₂CHOH (2° alcohol) | R₂C=O | 2e⁻ loss | Stops at ketone — no further oxidation |
Special Redox Patterns
Solubility Rules for Precipitation
Precipitation accounts for 14questions. Knowing what's insoluble is essential.
Generally SOLUBLE (dissolve)
Generally INSOLUBLE (precipitate)
Most-Tested Precipitates on USNCO
| Precipitate | Ksp | Appearances | Notes |
|---|---|---|---|
| BaSO₄ | 1.1×10⁻¹⁰ | 5× | Most common. Often in double ppt with Mg(OH)₂ |
| Mg(OH)₂ | 5.6×10⁻¹² | 4× | Paired with BaSO₄ in double ppt questions |
| AgCl / AgBr | 1.8×10⁻¹⁰ / 5.4×10⁻¹³ | 6× | Dissolved by NH₃ or S₂O₃²⁻ (complexation) |
| PbSO₄ | 2.5×10⁻⁸ | 2× | One of the rare insoluble sulfates |
| PbCrO₄ | 2×10⁻¹⁴ | 1× | Bright yellow “chrome yellow” pigment |
| Ag₂CrO₄ | 1.1×10⁻¹² | 1× | Brick-red. Mohr titration endpoint indicator |
| Hg₂Cl₂ | 1.4×10⁻¹⁸ | 1× | White. Hg(I) always as Hg₂²⁺ dimer |
| CaF₂ | 3.5×10⁻¹¹ | 1× | Precipitate-to-precipitate (CaCO₃ → CaF₂) |
| NiS | ~10⁻²⁰ | 1× | Ppts even in acid (Ksp overwhelms H⁺) |
| CuI | 1.3×10⁻¹² | 1× | Precipitation drives Cu²⁺ + I⁻ redox |
Reaction Type Guide
Proton transfer reactions including strong/weak acid-base, neutralization, amphoteric reactions, acid anhydride hydrolysis, and non-redox thermal decompositions (carbonates, sulfites).
Formation of insoluble products from mixing ionic solutions, including double precipitation and precipitate-to-precipitate conversions.
Electron transfer reactions including displacement, strong oxidizers (KMnO₄, K₂Cr₂O₇, HNO₃), disproportionation, comproportionation, thermal decomposition, and electrolysis.
Lewis acid-base reactions forming coordination complexes, including ammine, cyanide, thiosulfate, hydroxo, halide, and thiocyanate complexes.
Carbon chemistry including electrophilic addition, EAS (nitration/halogenation), elimination (E1/E2), substitution (SN1/SN2), saponification, and polymerization.
Nuclear transformations including alpha decay, beta decay, positron emission, electron capture, fission, and nuclear bombardment.
Hydrolysis reactions of oxides, ionic compounds, covalent halides, and peroxides in water, including thermal decompositions of salts.
How to Approach Unknown Reactions
Read & Decode
Names → formulas. This is where many students get stuck. If you can't write the formula, you can't write the equation.
Watch for keywords that change the product:
Classify & Predict Products
First, two easy checks:
Isotope notation, decay particles (α, β, positron, neutron). Use mass number balance and atomic number balance to find the unknown nuclide.
C–H bonds, functional groups. Identify the mechanism (addition, elimination, EAS, SN1/SN2, saponification). Don't miss the small molecule byproduct (H₂O, HCl, ROH).
For inorganic reactions — the key question: Redox or Non-redox?
Look for common oxidizing agents (MnO₄⁻, Cr₂O₇²⁻, HNO₃, conc. H₂SO₄) or reducing agents (metals, I⁻, Fe²⁺, SO₃²⁻). If oxidation states change → redox.
- Identify the oxidizer and reducer — use the reference table above to know what each produces
- Determine products based on common oxidation states and the medium:Acidic → use H⁺ and H₂O to balance O and H. Basic/neutral → MnO₄⁻ gives MnO₂ not Mn²⁺
- Electron balance — e⁻ lost = e⁻ gained → determines stoichiometric coefficients
- Add H₂O to whichever side needs oxygen atoms balanced
Some reactions involve multiple types (e.g. precipitation + acid-base with no spectators, or redox driven by precipitation as in Cu²⁺ + I⁻ → CuI + I₂). Always check for a second driving force.
Write Net Ionic
As ions: strong acids, strong bases, soluble salts
As molecules: weak acids/bases, H₂O, gases, organic compounds
As formulas: solids (precipitates, undissolved reactants), pure liquids
Omit: spectator ions that appear unchanged on both sides
Balance & Check
Atoms: every element balanced on both sides
Charge: total charge equal on both sides
Electrons: (redox only) e⁻ lost = e⁻ gained
Driving force: at least one present — if not, reconsider
Note: USNCO Part II does not require balancing or phase labels — but including them deepens your understanding and helps catch errors.
Common Mistakes
Concrete wrong → right examples drawn from actual USNCO questions.