Types of Chemical Reactions

Reactions Have Patterns

Changes in heat, color, precipitate formation, or gas production provide the first clues. The arrangement of reactants and products then determines the reaction type.

Four common examples are combustion, rusting, precipitation, and gas formation. Reactions can also be classified as precipitation, acid-base, and oxidation-reduction reactions.

These four names use two bases of classification. Combustion and rusting are identified by oxidation-reduction processes, while precipitation and gas formation are identified by observable products. One reaction can satisfy more than one description.

Reaction Type Model

Choose one reaction type. Watch the visible change first, then match it with the chemical equation.

Four Examples of Chemical Reactions

The four examples below show different clues: energy release, metal change, solid formation, and gas formation. No single clue settles the question on its own, so each example names the observation that separates a chemical change from a physical one.

Combustion

In the common oxygen-combustion examples studied here, a substance reacts with oxygen and releases energy as heat or light. White phosphorus combustion is represented by:

Phosphorus pentoxide has the molecular formula $\mathrm{P_4O_{10}}$. $\mathrm{P_2O_5}$ is the commonly used empirical formula. The molecular form is used here because the 3D particle model explicitly counts $4$ phosphorus atoms and $10$ oxygen atoms.

Rusting

Rusting is an electrochemical corrosion process involving iron, oxygen, and water. Rust is a mixture of iron oxides and oxyhydroxides, and its composition can change with the conditions. The expression below is a simplified model of that composition.

The letter $x$ makes the variable hydration explicit. Because the product has coefficient $2$, the left side uses $2x\mathrm{H_2O}$ to keep this simplified expression balanced. Corrosion products can contain several iron oxides and oxyhydroxides. Their proportions depend on the conditions.

Precipitation

Precipitation happens when the solution conditions produce a substance with low solubility. For example, limewater can react with dissolved sodium hydrogen carbonate and form calcium carbonate as a white precipitate.

The equation labels $\mathrm{CaCO_3(s)}$ as the solid product. A precipitate is a newly formed product with low solubility. Material that was already present and merely sank does not count.

Gas Formation

In alcoholic fermentation, yeast can convert glucose into ethanol and carbon dioxide. The bubbles are an observable clue, while the equation identifies the gaseous product.

The coefficient $2$ before ethanol and carbon dioxide should not be dropped. Without it, the carbon, hydrogen, and oxygen atoms would not be balanced.

Coefficients Do Not Change Formulas

A coefficient is the number placed before a chemical formula. We may change coefficients so the number of atoms before and after the reaction matches.

In this equation, $5$ before $\mathrm{O_2}$ is a coefficient. The subscripts in $\mathrm{P_4}$, $\mathrm{O_2}$, and $\mathrm{P_4O_{10}}$ are part of the formulas and must not be altered to force a balance.

Determining Reaction Types from Evidence

Names such as combustion, precipitation, or gas formation suggest an initial classification. Confirm that classification with the balanced equation, state symbols, reaction conditions, and observations that apply to the reaction.

Use the following table to check each reaction type.

After identifying the reaction type, write the reactants on the left and the products on the right. Then adjust the coefficients until each type of atom has the same count on both sides.