Types of Chemical Reactions

Learning Objective 4.7.A Identify a reaction as acid–base, oxidation–reduction, or precipitation.

Quick Notes

Acid–base reactions: Involve transfer of H⁺ (protons) between species.
Example: HCl + NaOH → NaCl + H₂O
Oxidation–reduction (redox) reactions: Involve transfer of electrons between species.
- Oxidation = loss of electrons (increase in oxidation number)
- Reduction = gain of electrons (decrease in oxidation number)
- Combustion is a key subclass: fuel + O₂ → CO₂ + H₂O
Precipitation reactions: Involve formation of an insoluble solid (precipitate) from aqueous solutions.
- Solubility rules can be used to predict precipitate products.
Oxidation numbers can be used to identify redox processes.

Full Notes

Acid–Base Reactions

These involve the transfer of protons (H⁺ ions) from one species to another.

Acid: Proton (H⁺) donor
Base: Proton (H⁺) acceptor

General form:
HA (acid) + B^- (base) → A^- + HB

Example HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)
H⁺ is transferred from HCl to OH⁻
Water is formed from H⁺ + OH⁻

Oxidation–Reduction (Redox) Reactions

These involve the transfer of electrons.

Oxidation: Loss of electrons → increase in oxidation number
Reduction: Gain of electrons → decrease in oxidation number

Key concept: Electrons are transferred from the species that is oxidized to the species that is reduced.

Example Zn (s) + Cu²⁺ (aq) → Zn²⁺ (aq) + Cu (s)
Zn is oxidized: Zn⁰ → Zn²⁺
Cu²⁺ is reduced: Cu²⁺ → Cu⁰

Combustion Reactions

A type of redox reaction where a substance reacts with oxygen.

Example When carbon combusts and reacts with oxygen, it gets oxidised. This can be shown as an increase in its oxidation state from 0 to +4.
AP Chemsitry Oxidation state changes in combustion of carbon

Precipitation Reactions

These occur when two aqueous ionic solutions react to form an insoluble solid, or precipitate.

General form:
AB (aq) + CD (aq) → AD (s) + CB (aq)

Example AgNO₃ (aq) + NaCl (aq) → AgCl (s) + NaNO₃ (aq)
AgCl is insoluble and forms a white precipitate.
Use a solubility table to determine whether a precipitate forms.

Oxidation Numbers

Assigning oxidation numbers is useful to:

Track electron transfer
Identify redox reactions
Determine which species is oxidized and which is reduced

Basic rules:

Elements in their standard state = 0
Group 1 = +1, Group 2 = +2
Oxygen = usually –2
Hydrogen = +1 with nonmetals, –1 with metals
The sum of oxidation numbers = the charge on the species

Matt’s exam tip

When in doubt, write oxidation numbers for each atom before and after the reaction. If at least one element changes its oxidation number, it’s a redox reaction. If an insoluble product forms, it’s precipitation. If H⁺ is transferred, it’s acid–base.

Summary

Acid–base reactions: involve transfer of protons (H⁺).
Redox reactions: involve transfer of electrons, tracked using oxidation numbers.
Precipitation reactions: involve formation of a solid from aqueous solutions.

Recognizing these patterns allows you to classify reactions, write balanced equations, and predict products with greater confidence.