4.7 Types of Chemical Reactions

Acid-base: a proton (H+) is transferred between species—think Bronsted-Lowry. Look for H+ moving from an acid to a base (e.g., HCl + NH3 → NH4+ + Cl−). Redox: electrons are transferred; identify by changes in oxidation numbers. If an atom’s oxidation number increases it’s oxidized (lost e−); if it decreases it’s reduced (gained e−). Combustion (hydrocarbons → CO2 + H2O) is a redox subclass. Precipitation: two aqueous ionic solutions mix and form an insoluble (or sparingly soluble) solid—a precipitate. Write the net ionic equation to remove spectator ions; remember Na+, K+, NH4+, and NO3− salts are always soluble (so won’t precipitate). On the AP exam you should identify these by proton transfer (acid-base), oxidation-number changes (redox), or formation of an insoluble solid/net ionic equation (precipitation). For practice and a quick Topic 4.7 review, see Fiveable’s study guide (https://library.fiveable.me/ap-chemistry/unit-4/types-chemical-reactions/study-guide/0VTaPH2MhqYc3Azc3xJz) and extra practice problems (https://library.fiveable.me/practice/ap-chemistry).

Short answer: look for H+ moving vs. oxidation-number changes. Acid–base (Bronsted–Lowry) = proton transfer: if a species donates H+ to another (HCl + H2O → H3O+ + Cl− or net ionic H+ + OH− → H2O), it’s an acid–base reaction. Redox = electron transfer: assign oxidation numbers to atoms before and after—if any change, electrons moved (Zn + Cu2+ → Zn2+ + Cu is redox: Zn goes 0 → +2, Cu goes +2 → 0). Precipitation reactions you’ll spot by formation of an insoluble solid from ions (use solubility rules in the CED). For AP exam work, the CED explicitly says identify acid–base vs. redox vs. precipitation (4.7.A); use net ionic equations to reveal H+ transfer or oxidation-number changes. Want more practice? Check the Topic 4.7 study guide (https://library.fiveable.me/ap-chemistry/unit-4/types-chemical-reactions/study-guide/0VTaPH2MhqYc3Azc3xJz) and the AP practice problems (https://library.fiveable.me/practice/ap-chemistry).

You assign oxidation numbers because they’re a bookkeeping tool that tells you whether electrons are lost or gained in a reaction—which is exactly what defines a redox reaction in the AP CED (4.7.A.2–4.7.A.4). By comparing oxidation numbers of each atom before and after, you can (1) spot which species is oxidized (its oxidation number increases) and which is reduced (its oxidation number decreases), (2) write balanced half-reactions and balance electrons, and (3) identify combustion reactions (a subclass of redox) and predict products. Oxidation numbers make electron transfer explicit even when electrons aren’t shown in the equation, so they’re essential for identifying redox vs. acid–base or precipitation reactions (learning objective 4.7.A). For practice identifying and balancing redox problems, see the Topic 4.7 study guide (https://library.fiveable.me/ap-chemistry/unit-4/types-chemical-reactions/study-guide/0VTaPH2MhqYc3Azc3xJz) and try problems at https://library.fiveable.me/practice/ap-chemistry. Note: the CED says “reducing/oxidizing agent” terms won’t be assessed, but knowing oxidation-number changes is.

Quick rule: oxidation = loss of electrons (oxidation number increases); reduction = gain of electrons (oxidation number decreases). To tell which species does which, assign oxidation numbers to each atom in reactants and products (CED 4.7.A.4–4.7.A.3). Whichever atom’s oxidation number goes up was oxidized (it lost e–); whichever goes down was reduced (it gained e–). Fast method: 1) Write balanced equation (or half-reactions). 2) Assign oxidation numbers to the atoms of interest. 3) Compare reactant vs product values: increase = oxidation, decrease = reduction. Example: Zn + Cu2+ → Zn2+ + Cu: Zn: 0 → +2 (oxidized); Cu: +2 → 0 (reduced). If you prefer, split into half-reactions to see electron transfer explicitly. For AP exams, this oxidation-number method is what they expect for identifying redox (Topic 4.7; see the CED). For extra practice, check the Topic 4.7 study guide (https://library.fiveable.me/ap-chemistry/unit-4/types-chemical-reactions/study-guide/0VTaPH2MhqYc3Azc3xJz) and try practice problems (https://library.fiveable.me/practice/ap-chemistry).

A precipitation reaction happens when two aqueous ionic solutions mix and an insoluble (or very sparingly soluble) ionic solid forms and falls out of solution. To predict a precipitate, write the possible product ions, then use solubility rules from the CED: all Na+, K+, NH4+, and NO3− salts are soluble, so they never precipitate (4.7.A.5). If a product is insoluble (like AgCl, PbSO4, BaSO4, etc.), a precipitate forms. Practically: (1) write the molecular equation, (2) break strong electrolytes into ions to make the complete ionic equation, (3) cancel spectator ions to get the net ionic equation showing the solid (s). If the net ionic equation includes a solid (e.g., Ag+(aq) + Cl−(aq) → AgCl(s)), a precipitate forms. Net ionic work and identifying spectator ions are tested on the AP (4.7.A). Need more practice? Check the Topic 4.7 study guide (https://library.fiveable.me/ap-chemistry/unit-4/types-chemical-reactions/study-guide/0VTaPH2MhqYc3Azc3xJz) and plenty of practice problems (https://library.fiveable.me/practice/ap-chemistry).

Short answer: combustion reactions are chemical reactions where a substance reacts with O2 (usually making CO2 and H2O for hydrocarbons). Per the AP CED, combustion is an important subclass of oxidation–reduction reactions (4.7.A.2), so yes—combustion reactions are redox. Why: in combustion you change oxidation numbers. Example: in complete combustion of methane, CH4 + 2 O2 → CO2 + 2 H2O, C goes from −4 to +4 (oxidized, loses electrons) and O (in O2, 0) goes to −2 (reduced, gains electrons). Assigning oxidation numbers to atoms before and after the reaction is the AP-approved way to spot the oxidized and reduced species (4.7.A.4). For AP review, see the Topic 4.7 study guide (https://library.fiveable.me/ap-chemistry/unit-4/types-chemical-reactions/study-guide/0VTaPH2MhqYc3Azc3xJz) and Unit 4 overview (https://library.fiveable.me/ap-chemistry/unit-4). For lots of practice problems, try the Fiveable practice page (https://library.fiveable.me/practice/ap-chemistry).

Short rule to memorize: all salts containing Na+, K+, NH4+ (ammonium), and NO3− (nitrate) are always soluble in water. A quick mnemonic: “No Kittens Need Ammonia” → Nitrate, Potassium, Sodium, Ammonium (or use “NKSN” if you prefer). Tips to make it stick: - Practice by writing a few ionic equations and crossing out spectator ions—you’ll see Na+, K+, NH4+, NO3− never precipitate. - Remember the AP CED: only those families are required to be memorized for the exam (you won’t be tested on a full solubility table)—so focus your effort there. - Use net ionic problems: when you mix two aqueous salts, if one product would contain one of these ions, it stays soluble (a spectator). For extra practice, check the Topic 4.7 study guide (https://library.fiveable.me/ap-chemistry/unit-4/types-chemical-reactions/study-guide/0VTaPH2MhqYc3Azc3xJz) and the AP Chem practice bank (https://library.fiveable.me/practice/ap-chemistry).

Oxidation and reduction are just electron bookkeeping. Oxidation = loss of electrons (oxidation number increases); reduction = gain of electrons (oxidation number decreases). In any redox reaction electrons flow from the species that’s oxidized (it loses electrons) to the species that’s reduced (it gains electrons). A reliable AP method to ID them is to assign oxidation numbers to atoms in reactants and products (CED 4.7.A.2–4.7.A.4). If an atom’s oxidation number goes up, it was oxidized; if it goes down, it was reduced. Remember combustion is a common redox type (reacting with O2). Note: the CED says the specific meanings of “reducing agent” and “oxidizing agent” won’t be tested, but knowing electron transfer and oxidation-number changes is. For more practice and examples, check the Topic 4.7 study guide (https://library.fiveable.me/ap-chemistry/unit-4/types-chemical-reactions/study-guide/0VTaPH2MhqYc3Azc3xJz), the Unit 4 overview (https://library.fiveable.me/ap-chemistry/unit-4), and AP practice problems (https://library.fiveable.me/practice/ap-chemistry).

When two clear aqueous solutions mix, their dissolved ions can pair up to form an ionic compound that’s insoluble (or sparingly soluble) in water—that solid is the precipitate. For example, AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq). The net ionic equation is Ag+(aq) + Cl−(aq) → AgCl(s); Na+ and NO3− are spectator ions. Precipitation depends on solubility: most Na+, K+, NH4+, and NO3− salts stay dissolved (CED 4.7.A.5), so precipitates form only when the new ion pair violates solubility expectations. On the AP Exam you should be able to identify precipitation reactions, write complete and net ionic equations, and point out spectator ions (CED Topic 4.7). For worked examples and guided practice, see the Topic 4.7 study guide (https://library.fiveable.me/ap-chemistry/unit-4/types-chemical-reactions/study-guide/0VTaPH2MhqYc3Azc3xJz) and try extra problems at (https://library.fiveable.me/practice/ap-chemistry).

Think of a Bronsted–Lowry acid as a proton (H+) donor and a base as a proton acceptor. Example (net ionic): HCl(aq) + NH3(aq) → NH4+(aq) + Cl−(aq). Here HCl donates H+ and NH3 accepts it—that proton transfer makes this an acid-base reaction (CED 4.7.A.1). You can also write HCl + H2O → H3O+ + Cl− to show water acting as the base. A quick way to spot these on the AP: look for H+ moving from one species to another (not electron transfer—that’s redox). On free-response or multiple choice you might be asked for the net ionic equation or to identify the acid/base by proton transfer (keep that CED language in mind). For more examples and practice, see the Topic 4.7 study guide (https://library.fiveable.me/ap-chemistry/unit-4/types-chemical-reactions/study-guide/0VTaPH2MhqYc3Azc3xJz) and the AP Chem practice set (https://library.fiveable.me/practice/ap-chemistry).

Step-by-step method to assign oxidation numbers (useful for spotting redox changes on the AP exam): 1. Know the common rules: - Element in pure form = 0 (O2, H2, Fe). - Monoatomic ion = its charge (Na+ = +1, Cl- = -1). - H is usually +1 (except in hydrides like NaH where H = -1). - O is usually -2 (except in peroxides where O = -1). - Group 1 metals = +1, group 2 = +2. 2. Write the formula and count atoms. 3. Assign oxidation numbers for atoms with fixed rules (H, O, alkali/alkaline earth). 4. Let unknowns be x and use the rule: sum of oxidation numbers = overall charge (0 for neutral compounds, ion charge for polyatomic ions). 5. Solve for x, then check by summing to the total charge. Quick example: Fe2O3. O = -2 × 3 = -6. Sum = 0, so 2·(Fe) + (-6) = 0 → 2Fe = +6 → Fe = +3. This is exactly how you identify oxidized vs reduced species (look for increase/decrease in oxidation numbers). For extra practice and AP-aligned problems, see the Topic 4.7 study guide (https://library.fiveable.me/ap-chemistry/unit-4/types-chemical-reactions/study-guide/0VTaPH2MhqYc3Azc3xJz) and 1000+ practice questions (https://library.fiveable.me/practice/ap-chemistry).

Those ions are “always soluble” because their ionic interactions with water are strong enough that the lattice energy of any salt they form is overcome by hydration (ion–dipole) energy. Na+, K+, and NH4+ are small, singly charged cations and NO3– is a delocalized anion—together they dissolve easily, so salts containing them stay aqueous. For AP Chem this matters because they’re usually spectator ions in precipitation (net ionic) problems: if a salt contains one of these ions, it won’t be the solid that precipitates. The CED explicitly lists “All sodium, potassium, ammonium, and nitrate salts are soluble in water” (useful for identifying precipitation in Topic 4.7)—see the Topic 4.7 study guide (https://library.fiveable.me/ap-chemistry/unit-4/types-chemical-reactions/study-guide/0VTaPH2MhqYc3Azc3xJz). For more practice on net ionic and precipitation questions, check the Unit 4 overview (https://library.fiveable.me/ap-chemistry/unit-4) and AP practice problems (https://library.fiveable.me/practice/ap-chemistry).

No—a reaction involving O2 isn’t automatically a “combustion” reaction. On the AP CED, combustion is a specific subclass of oxidation–reduction reactions in which a species reacts with O2 (and for hydrocarbons, complete combustion gives CO2 + H2O). Key checks: does O2 act as the oxidizing agent (oxidation numbers change)? Is the process rapid and exothermic (flame or heat released)? If yes, it’s combustion. If O2 simply forms an oxide slowly (corrosion/rusting) or participates without the characteristic heat/combustion products, we’d call it a redox or synthesis/oxidation reaction instead. For more examples and AP-style practice, review Topic 4.7 (Types of Chemical Reactions) study guide (https://library.fiveable.me/ap-chemistry/unit-4/types-chemical-reactions/study-guide/0VTaPH2MhqYc3Azc3xJz) and try related practice problems (https://library.fiveable.me/practice/ap-chemistry).

Short answer: assign oxidation numbers to every element on both sides and look for any changes. If an atom’s oxidation number increases, it’s oxidized (lost electrons); if it decreases, it’s reduced (gained electrons). Any net change in oxidation numbers = electron transfer → a redox reaction. Quick tips: metals that go from 0 to a positive number are oxidized (e.g., Zn → Zn2+); oxygen is usually −2 and hydrogen +1 (use CED rules). Combustion reactions are a common redox subclass (reacting with O2 to give CO2 and H2O). Don’t confuse this with acid–base: those transfer H+ (protons), not electrons. For practice, walk through a few balanced equations assigning oxidation numbers (see Topic 4.7 study guide: https://library.fiveable.me/ap-chemistry/unit-4/types-chemical-reactions/study-guide/0VTaPH2MhqYc3Azc3xJz). Want more practice problems? Fiveable has a big set here: https://library.fiveable.me/practice/ap-chemistry.

In the lab a precipitation reaction usually looks like instant cloudiness where two clear solutions meet, then a solid (precipitate) forms as tiny particles. Example: mix AgNO3(aq) and NaCl(aq)—both clear—and you’ll see a white, grainy AgCl(s) precipitate almost immediately. Color varies by compound: AgI is yellow, PbS is brown/black, many sulfate/carbonate precipitates are white. Sometimes the solid settles to the bottom (you can filter it), sometimes it stays suspended as a milky/colloidal mix. Rate depends on solubility—very insoluble salts form instantly; sparingly soluble ones form more slowly. On the AP side, treat these as ionic mixes: write the molecular, complete ionic, and net ionic equations (e.g., Ag+ + Cl– → AgCl(s)), and identify spectator ions. Remember the CED solubility hint: Na+, K+, NH4+, and NO3– salts are soluble (so those are usually spectators). For a quick refresher and practice (including writing net ionic equations), check the Topic 4.7 study guide (https://library.fiveable.me/ap-chemistry/unit-4/types-chemical-reactions/study-guide/0VTaPH2MhqYc3Azc3xJz) and AP Chem practice problems (https://library.fiveable.me/practice/ap-chemistry).