Types of Chemical Reactions

Reactions Have Patterns

When substances react, we do more than notice clues such as heat, color, precipitates, or gas. We can also read the reaction type from the pattern of reactants and products.

This lesson uses four common examples: combustion, rusting, precipitation, and gas formation. OpenStax Chemistry 2e also groups reactions through examples such as precipitation, acid-base reactions, and oxidation-reduction in openstax.org.

Reaction Type Lab

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

Four Examples to Recognize

The four examples below show different clues: energy release, metal change, solid formation, and acid-base neutralization.

Combustion

Combustion happens when a substance reacts with oxygen and usually releases energy as heat or light. For burning phosphorus, a common school-level equation is:

Some chemistry sources write the phosphorus oxide as $\mathrm{P_4O_{10}}$. That does not contradict the equation above because $\mathrm{P_2O_5}$ is its empirical formula. For coefficient practice here, $\mathrm{2P_2O_5}$ keeps the numbers of $\mathrm{P}$ and $\mathrm{O}$ atoms balanced.

Rusting

Rusting is an example of oxidation in metals. Iron reacts with oxygen and water, then forms rust, often written as hydrated iron(III) oxide.

The letter $x$ means the amount of water in rust is not always one fixed number. Because the product has coefficient $2$, the water on the left is written as $2x\mathrm{H_2O}$ so the equation stays balanced. The key pattern here is that iron changes into a new reddish-brown substance.

Precipitation

Precipitation happens when a newly formed substance does not dissolve well in the mixture. For example, limewater can react with dissolved baking soda and form calcium carbonate as a white precipitate.

The product $\mathrm{CaCO_3}$ matters because this compound appears as the solid. A precipitate is a newly formed product with low solubility, not just material that falls to the bottom.

Gas Formation

In some reactions, one product escapes as a gas. Fermentation of glucose can produce ethanol and carbon dioxide.

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 $\mathrm{O_2}$ and $2$ before $\mathrm{P_2O_5}$ are coefficients. The formula $\mathrm{O_2}$ must not be changed into $\mathrm{O_5}$, because that would change the substance.

The Name Helps, the Equation Decides

Names such as combustion, precipitation, or gas formation help us predict the reaction pattern. The final check still comes from the balanced chemical equation.

Use this quick check.

Once the reaction type is readable, the next move is to write the reaction neatly. Put reactants on the left, products on the right, then adjust coefficients until the atoms balance.