Unit 4 · Topic 4.7 Beta

Types of Chemical Reactions

Reactions can be sorted by what changes between particles: ions combining into an insoluble solid (precipitation), an H⁺ moving (acid-base), electrons moving (electron transfer, including every combustion).

Practice 1: Models and RepresentationsPractice 4: Model Analysis

Question set for this topic

Part 1 · Hook

Why this matters

Thousands of reactions happen in labs, kitchens and engines, and no one memorizes them all. Chemists sort them by what moves between the particles: ions that pair off and fall out of solution, a hydrogen ion passed from one particle to another, or electrons handed over. Spot which one is happening and you can predict the products.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. Mixing two solutions forms an insoluble solid. What is that solid called?

  1. A precipitate
  2. A spectator
  3. An electrolyte
  4. A solvent
Show the answer

An insoluble solid formed from solutions is a precipitate.

  • Correct: A precipitate:
  • A spectator:
  • An electrolyte:
  • A solvent:

2. What is the charge on a magnesium ion?

  1. 2+
  2. 1+
  3. 2−
  4. 0
Show the answer

Magnesium is in group 2 and loses its two valence electrons, forming Mg²⁺.

  • Correct: 2+:
  • 1+:
  • 2−:
  • 0:

3. Which formula is empirical?

  1. CH₂O
  2. C₆H₁₂O₆
  3. C₂H₄O₂
  4. H₂O₂
Show the answer

CH₂O has subscripts with no common factor; the others can be divided.

  • Correct: CH₂O:
  • C₆H₁₂O₆:
  • C₂H₄O₂:
  • H₂O₂:

Part 4 · See it

See it first

Four cards. Precipitation: ions from two solutions form a solid, for example Ag⁺(aq) + Cl⁻(aq) → AgCl(s). Acid-base: an H⁺ moves from one particle to another, for example H⁺(aq) + OH⁻(aq) → H₂O(l). Electron transfer: electrons move, look for a free element, for example Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). Combustion: a fuel combines with O₂, for example CH₄ + 2O₂ → CO₂ + 2H₂O. A note says every combustion is also an electron transfer, because O₂ goes from a free element to compounds.
Four kinds of reaction, sorted by what changes between the particles: ions pair off into a solid, a proton (H⁺) moves, electrons move, or a fuel combines with oxygen. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Ions from two solutions meet a partner they cannot stay dissolved withan insoluble solid forms: a precipitation reaction
  2. An H⁺ passes from one particle to anotherthe reaction is an acid-base reaction, such as H⁺ + OH⁻ → H₂O
  3. An element goes from uncombined to combined, or the reverseelectrons have moved between particles
  4. A hydrocarbon burns in plenty of oxygenits carbon ends up in CO₂ and its hydrogen in H₂O

Part 6 · Key ideas

Key ideas

  • Precipitation reaction: two solutions give an insoluble solid. Acid-base: an H⁺ moves. Electron transfer: electrons move from one particle to another.
  • Clue for electron transfer: an element appears uncombined on one side and in a compound on the other.
  • Combustion of a hydrocarbon in excess O₂ gives CO₂ and H₂O; with too little O₂, toxic CO can form.
  • Older names describe the pattern: synthesis, decomposition reaction, single replacement, double replacement.

Part 7 · Misconception

A common mistake

The wrong idea: Every reaction belongs to exactly one type.

What actually happens: The categories overlap. Every combustion is also an electron-transfer reaction, and magnesium dissolving in acid is both a single replacement and an electron transfer. Classify by what changes between the particles, and name every description that fits.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Data table

Five reactions

A student lists five reactions seen in lab.

Balanced equations
ReactionEquation
1Pb(NO₃)₂(aq) + 2NaCl(aq) → PbCl₂(s) + 2NaNO₃(aq)
2Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
3HNO₃(aq) + KOH(aq) → KNO₃(aq) + H₂O(l)
4C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(g)
52H₂O₂(aq) → 2H₂O(l) + O₂(g)

1. Which reaction is a precipitation reaction?

  1. Reaction 1
  2. Reaction 2
  3. Reaction 3
  4. Reaction 5
Show the answer

In reaction 1 two solutions are mixed and an insoluble solid, PbCl₂(s), forms from ions that were dissolved.

  • Correct: Reaction 1: Right: two solutions give an insoluble solid.
  • Reaction 2: Reaction 2 makes a gas from a metal and acid; no solid forms from the solutions.
  • Reaction 3: Reaction 3 makes water and a soluble salt; every product stays dissolved or is liquid water.
  • Reaction 5: Reaction 5 is one substance breaking into two; no solid forms.

2. In which reactions are electrons transferred? Select all that apply. (Hint: look for an element that is uncombined on one side and part of a compound on the other.)

  1. Reaction 1
  2. Reaction 2
  3. Reaction 3
  4. Reaction 5
Show the answer

Reaction 2: Mg metal becomes Mg²⁺ and H⁺ becomes H₂. Reaction 5: oxygen in H₂O₂ ends up partly as O₂. In reactions 1 and 3, every ion keeps its charge; ions change partners or an H⁺ moves. (Reaction 4, a combustion, also transfers electrons.)

  • Reaction 1: In reaction 1 the ions Pb²⁺, NO₃⁻, Na⁺ and Cl⁻ keep their charges; they only change partners, so no electrons move.
  • Correct: Reaction 2: Right: neutral Mg atoms become Mg²⁺ ions and H⁺ ions become neutral H₂.
  • Reaction 3: In reaction 3 an H⁺ moves from HNO₃ to OH⁻; no element changes from uncombined to combined, so no electrons move.
  • Correct: Reaction 5: Right: some oxygen leaves the compound as the element O₂.

3. Which is the net ionic equation for reaction 3?

  1. H⁺(aq) + OH⁻(aq) → H₂O(l)
  2. K⁺(aq) + NO₃⁻(aq) → KNO₃(aq)
  3. HNO₃(aq) + OH⁻(aq) → NO₃⁻(aq) + H₂O(l) + K⁺(aq)
  4. H⁺(aq) + KOH(aq) → K⁺(aq) + H₂O(l)
Show the answer

HNO₃ and KOH are fully ionized in water, and K⁺ and NO₃⁻ are spectators. What changes is an H⁺ joining an OH⁻ to make water: the acid-base reaction in its simplest form.

  • Correct: H⁺(aq) + OH⁻(aq) → H₂O(l): Right: only H⁺ and OH⁻ change.
  • K⁺(aq) + NO₃⁻(aq) → KNO₃(aq): KNO₃(aq) is just the spectator ions dissolved; nothing changes there.
  • HNO₃(aq) + OH⁻(aq) → NO₃⁻(aq) + H₂O(l) + K⁺(aq): Unbalanced (K⁺ appears from nowhere) and it keeps a spectator.
  • H⁺(aq) + KOH(aq) → K⁺(aq) + H₂O(l): KOH is a soluble ionic compound, so it is written as K⁺ and OH⁻, and K⁺ is a spectator.

4. Reaction 4 is a combustion. Which statement best explains why it is also an electron-transfer reaction?

  1. O₂, an uncombined element, ends up combined in CO₂ and H₂O
  2. It gives off heat and light, which shows that electrons moved
  3. It produces gases, which carry electrons away from the flame
  4. Ethanol already holds oxygen, which passes to the CO₂ formed
Show the answer

Use the clue for electron transfer: an element uncombined on one side and combined on the other. Oxygen is the free element O₂ before and part of the compounds CO₂ and H₂O after, so electrons have moved. That is true of every combustion.

  • Correct: O₂, an uncombined element, ends up combined in CO₂ and H₂O: Right: oxygen goes from the free element to compounds, so electrons move.
  • It gives off heat and light, which shows that electrons moved: Many processes give off heat without electron transfer, such as an acid neutralizing a base; heat and light are not the test.
  • It produces gases, which carry electrons away from the flame: Forming a gas says nothing about electrons; reaction 2 makes a gas by electron transfer, but boiling makes gas with none.
  • Ethanol already holds oxygen, which passes to the CO₂ formed: Oxygen already in ethanol stays combined; the clue is the O₂ that starts uncombined.

Particle view

Magnesium in hydrochloric acid

Cl−Cl−H+Cl−H+Cl−H+H+BeforeMgMgMgHHMg2+Cl−HHCl−Mg2+Cl−Cl−AfterMg

Key: green Mg, magnesium atom (in the strip) or Mg²⁺ ion (labeled 2+); small white H with +, H⁺ ion; green Cl with −, chloride ion; two joined white H, H₂ molecule. Water molecules are not drawn.

5. Which net ionic equation matches the diagram?

  1. Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g)
  2. Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
  3. Mg(s) + H⁺(aq) → Mg²⁺(aq) + H(g)
  4. Mg²⁺(aq) + 2Cl⁻(aq) → MgCl₂(s)
Show the answer

Two Mg atoms and four H⁺ ions were used; two Mg²⁺ ions and two H₂ molecules formed. That is a 1 : 2 : 1 : 1 ratio. Cl⁻ is unchanged (4 before, 4 after), so it is a spectator.

  • Correct: Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g): Right: atoms and charge balance (+2 each side) and Cl⁻ is left out.
  • Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g): This is the molecular equation; it keeps the spectator chloride ions.
  • Mg(s) + H⁺(aq) → Mg²⁺(aq) + H(g): Unbalanced in atoms and charge, and hydrogen forms H₂ molecules, not single atoms.
  • Mg²⁺(aq) + 2Cl⁻(aq) → MgCl₂(s): No solid MgCl₂ forms; the Mg²⁺ and Cl⁻ ions stay dissolved in the After box.

6. Which reactant was limiting in this mixture?

  1. H⁺, because none is left while some magnesium remains
  2. Mg, because there were fewer Mg atoms than H⁺ ions
  3. Cl⁻, because it does not appear in the equation
  4. Mg, because one atom remains in the strip
Show the answer

H⁺ ran out (4 before, 0 after). One Mg atom is left, so magnesium was in excess.

  • Correct: H⁺, because none is left while some magnesium remains: Right: the reactant used up is H⁺.
  • Mg, because there were fewer Mg atoms than H⁺ ions: Fewer particles does not mean limiting: each Mg needs two H⁺, and H⁺ ran out first.
  • Cl⁻, because it does not appear in the equation: A spectator ion is not a reactant, so it cannot be limiting.
  • Mg, because one atom remains in the strip: A reactant that is left over is in excess, not limiting.

Part 9 · Summary

Summary

Reactions can be sorted by what changes between particles: ions combining into an insoluble solid (precipitation), an H⁺ moving (acid-base), electrons moving (electron transfer, including every combustion). A hydrocarbon burning in excess oxygen gives carbon dioxide and water.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections