K12Chemistry

Chemical reactions

Use observations and particle reasoning to represent chemical change, balance equations, recognize common patterns, and connect energy changes to bonds.

Goals and prerequisites

By the end, you should be able to describe evidence for a reaction while naming what the evidence cannot prove, balance equations with coefficients, classify several introductory reaction patterns, and explain why a reaction can release or absorb energy.

Prerequisites: Review atomic structure and bonding. You need to read chemical formulas, count atoms, and distinguish an element from a compound.

Evidence is useful, but limited

A new gas, precipitate, lasting color change, temperature change, light, or odor can be evidence that particles have been rearranged. A fizz can also come from a physical change such as boiling, and a temperature change can come from dissolving. Therefore observations support a model; they do not by themselves identify every product.

At the particle level, a chemical reaction changes which atoms are connected. The atoms themselves are conserved. In a safe classroom observation, record what you can see or measure, avoid tasting or directly smelling substances, and do not infer an invisible gas's identity without appropriate evidence and supervision.

Equations communicate conservation

A chemical equation names reactants on the left and products on the right. An arrow means the reaction is represented in that direction; conditions such as heat or light may be written above it. State symbols, (s), (l), (g), and (aq), add useful information. A coefficient multiplies every atom in a formula. A subscript changes the substance and must never be changed to balance an equation.

2 H2(g) + O2(g) → 2 H2O(l)

This equation has four hydrogen atoms and two oxygen atoms on each side. The coefficients also give a particle or mole ratio: 2 mol H2 reacts with 1 mol O2 to make 2 mol H2O.

Worked example: balance propane combustion

Balance C3H8 + O2 → CO2 + H2O.

  1. Three carbon atoms require 3 CO2.
  2. Eight hydrogen atoms require 4 H2O.
  3. The products contain 3 × 2 + 4 = 10 oxygen atoms, so use 5 O2.

The balanced equation is C3H8 + 5 O2 → 3 CO2 + 4 H2O. Check each element again before using its coefficients in a calculation.

Conservation depends on the system boundary

In a closed system, total mass remains constant because atoms are rearranged rather than created or destroyed. An open flask may appear to lose mass when a gaseous product escapes, or gain mass when oxygen from air enters. The atoms are still accounted for if the system includes the surroundings. Charge is conserved as well, which is why ionic equations must have equal total charge on both sides.

For example, CaCO3(s) → CaO(s) + CO2(g) can leave a solid sample lighter in an open container. The missing mass is carbon dioxide in the air, not vanished matter. A sealed setup would retain it, although sealed reactions require appropriate supervision and equipment.

Recognize useful patterns

Patterns help predict products, but they are starting points and require charge, solubility, and reaction-condition checks.

  • Synthesis: A + B → AB, such as 2 Mg + O2 → 2 MgO.
  • Decomposition: AB → A + B, such as CaCO3 → CaO + CO2 when heated.
  • Single replacement: A + BC → AC + B, if the free element is reactive enough.
  • Double replacement: AB + CD → AD + CB, often driven by a precipitate, gas, or water. For example, AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq).
  • Combustion: A carbon-containing fuel reacts with O2; complete hydrocarbon combustion produces CO2 and H2O.

In aqueous reactions, a net ionic equation removes spectator ions. The precipitation example becomes Ag+(aq) + Cl−(aq) → AgCl(s), which highlights the particles that change.

Energy, bonds, and activation

Breaking attractions and bonds requires energy input. Forming attractions and bonds releases energy. A reaction's net energy change depends on both: if forming new bonds releases more energy than breaking old ones requires, the surroundings may warm (exothermic); if breaking requires more, the surroundings may cool (endothermic). This bond picture is a model, and measured enthalpy data are more reliable than rough average bond energies.

Activation energy is the minimum energy needed to reach a reactive arrangement. A catalyst supplies a different pathway with lower activation energy; it does not change the net energy difference between reactants and products and is not consumed overall. A spark can start combustion without being the source of all released energy.

Worked example: reading energy language

Suppose, for the same defined amount of reaction, breaking bonds requires 460 kJ and forming bonds releases 520 kJ. The approximate net change is 460 − 520 = −60 kJ for that amount of reaction. The negative sign indicates energy is released overall. Both terms matter: saying “bond breaking releases energy” reverses the model.

Practice with explained answers

  1. Balance Fe + O2 → Fe2O3.
    Answer and reasoning

    Use 2 Fe2O3 for six oxygen atoms, 3 O2, and 4 Fe: 4 Fe + 3 O2 → 2 Fe2O3.

  2. Classify 2 KClO3 → 2 KCl + 3 O2.
    Answer and reasoning

    One compound becomes simpler substances, so it is decomposition. The coefficients also conserve two K, two Cl, and six O atoms.

  3. Why cannot a temperature change alone prove a chemical reaction?
    Answer and reasoning

    Dissolving or changing state can transfer energy without making a new substance. Combine temperature evidence with particle, composition, or product evidence.

  4. What happens to activation energy when a catalyst is used?
    Answer and reasoning

    The pathway's activation energy decreases, so more collisions can react at the same temperature. Reactant and product energies and the balanced equation do not change.

  5. In 2 Al + 3 Cl2 → 2 AlCl3, how many moles of Cl2 react with 0.80 mol Al?
    Answer and reasoning

    Multiply by the ratio 3 mol Cl2/2 mol Al: 0.80 × 3/2 = 1.2 mol Cl2. The ratio comes from the balanced equation.

Common misconceptions

Coefficients do not describe mass percentages, and subscripts cannot be adjusted to balance. A visible bubble is not automatically a new product. “Exothermic” describes net heat transfer to surroundings, while “fast” describes rate; a reaction can be slow and exothermic.

Sources for further study