Final answer:
A combustion reaction involves a substance reacting with oxygen, releasing energy, and is always a redox reaction; whereas a single-replacement reaction consists of one element replacing another element in a compound and may or may not involve oxygen.
Step-by-step explanation:
The difference between a combustion reaction and a single-replacement reaction is as follows: A combustion reaction involves a substance reacting with oxygen gas, releasing energy as light and heat, and is always a redox reaction. In contrast, a single-replacement reaction involves one element being replaced by another in a compound and can also be a redox reaction, as the oxidation states of elements change, but it does not necessarily involve oxygen like combustion does.
Combustion reactions are always redox reactions because they involve the oxidizing agent oxygen (O₂), which gets reduced. A classic example of a combustion reaction is the burning of methane (CH₄ + 2O₂ → CO₂ + 2H₂O).
Single-replacement reactions are characterized by one element replacing another in a compound, and a change in oxidation state indicates they are also redox reactions. However, these reactions do not always involve oxygen. An example of a single-replacement reaction is Zn + 2HCl → ZnCl₂ + H₂, where zinc replaces hydrogen in hydrochloric acid.