Types of Chemical Reactions

Most chemical reactions you'll encounter on the MCAT fall into one of six recognizable categories: combination, decomposition, combustion, single-displacement, double-displacement, and neutralization. Recognizing which type you're looking at is often the fastest way to predict what an equation's products will be.

Key Takeaways

• Combination (synthesis) reactions: two or more reactants form one product (A + B → C). Example: 2 Mg + O₂ → 2 MgO.

• Decomposition reactions: one reactant breaks into two or more products (A → B + C), usually via heat, radiation, or electrolysis. Example: 2 HgO → 2 Hg + O₂.

• Combustion reactions: a fuel (usually a hydrocarbon) reacts with an oxidant (usually O₂) to typically produce CO₂ and H₂O; inherently a redox reaction.

• Single-displacement reactions: an atom or ion in a compound is replaced by an atom or ion of another element (e.g., Cu + 2 AgNO₃ → Cu(NO₃)₂ + 2 Ag); inherently a redox reaction.

• Double-displacement (metathesis) reactions: elements from two compounds swap places to form two new compounds, driven by formation of a precipitate, a gas, or a weak electrolyte (e.g., CaCl₂ + 2 AgNO₃ → 2 AgCl↓ + Ca(NO₃)₂).

• Neutralization reactions: a specific case of double-displacement where an acid reacts with a base to form a salt and usually water (e.g., HCl + NaOH → NaCl + H₂O).

Combination (Synthesis) Reactions

A combination reaction (also called a synthesis reaction) is one in which two or more reactants combine to form a single product. The general form has more reactants than products:

A + B → C

The formation of water from burning hydrogen gas in air is one example. Another is the reaction of magnesium with oxygen to form magnesium oxide:

2 Mg + O₂ → 2 MgO

Decomposition Reactions

A decomposition reaction is the opposite of a combination reaction: a single reactant breaks down into two or more products, usually as a result of heating, high-frequency radiation, or electrolysis. The general form has more products than reactants:

A → B + C

The breakdown of mercury(II) oxide into mercury and oxygen gas is a classic example:

2 HgO → 2 Hg + O₂

Combustion Reactions

A combustion reaction is a special type of reaction involving a fuel — usually a hydrocarbon — and an oxidant, normally oxygen gas. In its most common form, these reactants form two products: carbon dioxide and water.

MCAT Callout — Combustion and Redox: Combustion involves the oxidation of the fuel. Along with single-displacement reactions, combustion reactions are inherently redox reactions — they involve a transfer of electrons and a change in oxidation state. The mechanics of oxidation numbers and half-reactions are covered in Oxidation and Reduction Reactions (Chapter 11).

Single-Displacement Reactions

A single-displacement reaction occurs when an atom or ion in a compound is replaced by an atom or ion of another element. For example, solid copper metal displaces silver ions from a clear solution of silver nitrate, forming a blue copper nitrate solution and solid silver metal:

Cu + 2 AgNO₃ → Cu(NO₃)₂ + 2 Ag

Because one element is replacing another, single-displacement reactions are also redox reactions, as noted above.

Double-Displacement (Metathesis) Reactions

In a double-displacement reaction, also called a metathesis reaction, elements from two different compounds swap places to form two new compounds. This type of reaction occurs when one of the products leaves the solution as a precipitate or a gas, or when two of the original species combine to form a weak electrolyte that stays undissociated in solution.

For example, when solutions of calcium chloride and silver nitrate are combined, insoluble silver chloride forms as a precipitate in a solution of calcium nitrate:

CaCl₂ + 2 AgNO₃ → 2 AgCl↓ + Ca(NO₃)₂

Neutralization Reactions

A neutralization reaction is a specific type of double-displacement reaction in which an acid reacts with a base to produce a salt — and usually water. Hydrochloric acid reacting with sodium hydroxide is a standard example:

HCl + NaOH → NaCl + H₂O

Identifying Reaction Types: Worked Example

Worked example: Classify the following reaction by type: Zn + CuSO₄ → ZnSO₄ + Cu

Zinc metal is replacing copper in copper sulfate, leaving zinc sulfate and solid copper behind. A single element (Zn) is swapping into a compound in place of another element (Cu) — one reactant compound, one reactant element, one product compound, one product element. That pattern — an atom replacing an atom or ion already in a compound — is the signature of a single-displacement reaction (and, since an element is being oxidized and another reduced, it's a redox reaction as well).

Common MCAT Mistakes

• Confusing combination and decomposition. Combination has more reactants than products (A + B → C); decomposition has more products than reactants (A → B + C) — they're mirror-image patterns.

• Missing that neutralization is a subtype of double-displacement. Every neutralization reaction is a double-displacement reaction, but not every double-displacement reaction is a neutralization — only the ones between an acid and a base.

• Forgetting combustion and single-displacement are redox reactions. Both involve electron transfer and a change in oxidation state, even though they're categorized by their overall reaction pattern rather than explicitly labeled "redox."

• Assuming any element-for-element swap is single-displacement. Single-displacement specifically involves one element replacing an atom or ion already in a compound — not two compounds swapping components with each other, which is double-displacement instead.

MCAT-Style Concept Check

Question: Which reaction type is illustrated by the equation Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2 NaCl?

• A) Combination

• B) Decomposition

• C) Single-displacement

• D) Double-displacement

Answer: D

Explanation: Two compounds (Na₂SO₄ and BaCl₂) swap components to form two new compounds (BaSO₄ and NaCl), with BaSO₄ leaving the solution as a precipitate — the defining pattern of a double-displacement (metathesis) reaction.

FAQ

What's the difference between combination and decomposition reactions?

A combination reaction has more reactants than products, as two or more substances join into one (A + B → C). A decomposition reaction is the reverse pattern: one reactant breaks down into two or more products (A → B + C).

Are combustion reactions always redox reactions?

Yes. Combustion involves the oxidation of a fuel, so it always involves electron transfer and a change in oxidation state, making it inherently a redox reaction — the same is true of single-displacement reactions.

What causes a double-displacement reaction to occur?

A double-displacement reaction happens when swapping components between two compounds produces a precipitate, a gas, or a weak electrolyte that stays undissociated in solution — one of these outcomes is what drives the reaction forward.

Is neutralization its own separate reaction type?

Not entirely — a neutralization reaction is a specific type of double-displacement reaction, one where an acid reacts with a base to form a salt and usually water.