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Chemistry Unit 5: Chemical reactions

Reactions rearrange atoms. Recognizing the type of reaction helps you predict products and write balanced equations.

Covers: Reaction types, net ionic equations, precipitation, oxidation-reduction, oxidation numbers

Study guide

Chemical reactions rearrange atoms into new substances. Common types include synthesis (A + B → AB), decomposition (AB → A + B), single replacement (A + BC → AC + B), double replacement (AB + CD → AD + CB) and combustion (a fuel reacting with O₂; hydrocarbons produce CO₂ and H₂O). Evidence of a reaction includes color change, gas, a precipitate, or a temperature change.

In water, ionic compounds dissociate into ions. A complete ionic equation shows all dissolved species as ions; removing the spectator ions (those unchanged on both sides) gives the net ionic equation, which shows only what actually reacts.

Precipitation reactions are double replacements that form an insoluble solid. Solubility rules predict them: compounds of group 1 metals, ammonium and nitrate are always soluble, while many carbonates, phosphates and hydroxides are not.

Oxidation-reduction (redox) reactions transfer electrons. Oxidation is loss of electrons and reduction is gain (OIL RIG). The substance oxidized is the reducing agent, and the substance reduced is the oxidizing agent. Combustion, corrosion, batteries and respiration are all redox reactions.

Oxidation numbers track electrons. Free elements are 0, monatomic ions equal their charge, oxygen is usually −2, hydrogen is usually +1, and the numbers in a compound add to its overall charge. An increase in oxidation number means oxidation and a decrease means reduction. Redox equations can be balanced by the half-reaction method, matching electrons lost and gained.

A metal replaces another metal's ion in solution only if it is more active, which means higher on the activity series. Zinc metal displaces Cu²⁺, for example, but copper metal can't displace Zn²⁺. Metals above hydrogen react with acids to release H₂ gas. Only strong electrolytes are written as separate ions: soluble ionic compounds, strong acids and strong bases. Solids, liquids, gases, weak acids and weak bases (like acetic acid and NH₃) and molecular compounds are written as complete formulas.

Some double-replacement reactions produce a gas. An acid and a carbonate produce CO₂ and water, and an acid and a sulfide produce H₂S. Bubbling is evidence of these reactions.

Key ideas

• Main types: synthesis, decomposition, single replacement, double replacement, combustion.
• Precipitation reactions form an insoluble solid.
• Net ionic equations leave out spectator ions.
• Oxidation is loss of electrons; reduction is gain (OIL RIG).
• Combustion of hydrocarbons makes CO₂ and H₂O.

Formulas and rules

Common mistakes

• Thinking reduction means losing electrons. It means gaining them.
• Including spectator ions in a net ionic equation.

Words to know

OIL RIG

Oxidation Is Loss, Reduction Is Gain (of electrons).

Combustion

A reaction with O₂; hydrocarbons make CO₂ and H₂O.

Spectator ion

An ion that appears unchanged on both sides.

Oxidation number of an element by itself

0

Double replacement reaction

AB + CD → AD + CB.

Oxidation number rules

Free elements = 0; O is usually −2; H is usually +1.

Synthesis reaction

A + B → AB

Decomposition reaction

AB → A + B

Single replacement

A + BC → AC + B

Precipitate

An insoluble solid formed from a solution reaction.

Net ionic equation

Shows only the species that actually react.

Solubility rules

Group 1, ammonium and nitrate compounds are always soluble.

Oxidizing agent

The substance that is reduced.

Reducing agent

The substance that is oxidized.

Half-reaction method

Balancing redox by matching electrons lost and gained.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.