AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/4/4-7)
Unit 4 · Topic 4.7
4.7 Types of Chemical Reactions
Most reactions you'll meet are one of three types: acid-base (a proton moves), redox (electrons move, shown by changing oxidation numbers) or precipitation (ions join to form an insoluble solid). The only solubility rule to memorize: any compound containing Na⁺, K⁺, NH₄⁺ or NO₃⁻ dissolves in water.
Key terms
- acid-base reaction
- redox reaction
- precipitation reaction
- oxidation number
- combustion
- solubility
Acid-base reactions
An acid-base reaction transfers one or more protons (H⁺ ions) from one particle to another. HF(aq) + OH⁻(aq) → F⁻(aq) + H₂O(l) is an example: HF gives a proton to OH⁻. Topic 4.8 covers the details.
Redox reactions and oxidation numbers
A redox (oxidation-reduction) reaction transfers electrons. The species that loses electrons is oxidized; the one that gains electrons is reduced. Electrons always move from the species oxidized to the species reduced.
Oxidation numbers track electrons. Assign them with these rules, in order of priority:
- An atom in a pure element (Fe, O₂, Cl₂) is 0.
- A monatomic ion equals its charge (Na⁺ is +1, S²⁻ is −2).
- Fluorine is −1 in compounds.
- Oxygen is usually −2 (in peroxides like H₂O₂ it's −1).
- Hydrogen is +1 when bonded to nonmetals (−1 in metal hydrides like NaH).
- The oxidation numbers in a neutral compound add to 0; in an ion they add to the ion's charge.
Spotting redox, and combustion
If any element's oxidation number changes, the reaction is redox. An increase means oxidation; a decrease means reduction. Reactions involving an element in its pure form (like a metal dropping into a solution, or O₂ reacting) are usually redox.
Combustion is an important kind of redox reaction in which a substance reacts with oxygen gas. Complete combustion of a hydrocarbon (a compound of only C and H) produces CO₂ and H₂O. For example, CH₄ + 2O₂ → CO₂ + 2H₂O. Carbon goes from −4 to +4 (oxidized) and oxygen from 0 to −2 (reduced).
Another common redox reaction is a metal dissolving in acid: Zn(s) + 2H⁺(aq) → Zn²⁺(aq) + H₂(g). Zinc goes from 0 to +2 (oxidized) and hydrogen from +1 to 0 (reduced). The H₂ bubbles are the visible evidence.
Precipitation reactions
A precipitation reaction happens when ions in two solutions combine to form an insoluble (or only slightly soluble) ionic solid. Oxidation numbers don't change, and no proton is transferred.
You need to memorize only one solubility rule: every compound containing Na⁺, K⁺, NH₄⁺ or NO₃⁻ is soluble in water. For other compounds, the question will tell you or give you data. This rule lets you rule out precipitates: mixing NaNO₃ and KCl solutions produces nothing new, because every possible product (KNO₃, NaCl) is soluble.
Not tested
You don't need the terms 'oxidizing agent' and 'reducing agent', and you won't be asked to recall other solubility rules from memory.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Classifying reactions
Classify each reaction as acid-base, redox or precipitation: (a) Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s); (b) Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s); (c) HF(aq) + OH⁻(aq) → F⁻(aq) + H₂O(l).
Show the solutionHide the solution
- Step 1: (a) Zn goes from 0 to +2 (oxidized); Cu goes from +2 to 0 (reduced). Redox.
- Step 2: (b) Pb stays +2 and I stays −1. Ions combine into a solid. Precipitation.
- Step 3: (c) HF loses H⁺ to OH⁻. Oxidation numbers: H is +1, F is −1 and O is −2 throughout. Acid-base.
Answer: (a) redox; (b) precipitation; (c) acid-base
- Example 2
Oxidation numbers in a polyatomic ion
Find the oxidation number of chromium in the dichromate ion, Cr₂O₇²⁻.
Show the solutionHide the solution
- Step 1: Oxygen is −2, so seven O atoms contribute 7 × (−2) = −14.
- Step 2: The ion's charge is −2, so the two Cr atoms must add up to −2 − (−14) = +12.
- Step 3: Each Cr is +12 ÷ 2 = +6.
Answer: +6
- Example 3
Redox without a metal (classic trap)
Is the decomposition of hydrogen peroxide, 2H₂O₂(aq) → 2H₂O(l) + O₂(g), a redox reaction?
Show the solutionHide the solution
- Step 1: In H₂O₂, H is +1, and the two O atoms must add to −2, so each O is −1 (a peroxide).
- Step 2: In H₂O, O is −2 (reduced from −1). In O₂, O is 0 (oxidized from −1).
- Step 3: Oxygen is both oxidized and reduced, so this is redox, even though there's no metal and no ions are visible.
Answer: Yes. Oxygen goes from −1 to −2 in H₂O (reduced) and from −1 to 0 in O₂ (oxidized).
Common mistakes
- Assuming oxygen is always −2. In peroxides it's −1, and in O₂ it's 0.
- Mixing up oxidation and reduction. Oxidation is losing electrons, so the oxidation number goes up.
- Calling a reaction redox because a metal is present. Check whether oxidation numbers change; Pb²⁺ + 2I⁻ → PbI₂ isn't redox.
- Predicting a precipitate when all possible products contain Na⁺, K⁺, NH₄⁺ or NO₃⁻.
On the exam
- Expect to classify a reaction and justify it: for redox, give the oxidation number changes; for acid-base, name the proton donor and acceptor; for precipitation, name the solid formed.
- Combustion questions often ask for a balanced equation. Balance C first, then H, then O.
Connected topics
Videos
Check yourself
5 questions on 4.7 Types of Chemical Reactions. Pick an answer to see if you got it, and why.
Which of the following is an oxidation-reduction reaction?
Aqueous solutions of KNO₃ and NaCl are mixed. Which of the following correctly predicts what happens?
Which of the following correctly classifies each reaction? I. NH₃(g) + HCl(g) → NH₄Cl(s) II. 2Na(s) + Cl₂(g) → 2NaCl(s) III. Pb(NO₃)₂(aq) + 2NaI(aq) → PbI₂(s) + 2NaNO₃(aq)
Propane, C₃H₈, is used as fuel in gas grills. When it burns completely in excess oxygen, it produces only carbon dioxide and water.
C₃H₈(g) + __O₂(g) → __CO₂(g) + __H₂O(g)
Described reaction
When the equation is balanced with the lowest whole-number coefficients, what is the coefficient of O₂?
Which of the following statements about the combustion of propane is correct?
0 of 5 answered