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How do you recognize and predict the products of the five reaction types?

Types of chemical reactions: classify reactions as synthesis, decomposition, single replacement, double replacement or combustion, and predict their products.

A focused Virginia SOL Chemistry answer on reaction types under CH.3: the five categories (synthesis, decomposition, single replacement, double replacement, combustion), how to recognize each, and how to predict the products including using an activity series.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this topic is asking
  2. Synthesis and decomposition
  3. Single and double replacement
  4. Combustion
  5. Try this

What this topic is asking

Standard CH.3 asks you to classify reactions into five types and to predict their products. Virginia expects you to recognize synthesis, decomposition, single replacement, double replacement and combustion from the pattern of reactants and products, and to use an activity series to decide whether a single replacement happens. Recognizing the type tells you what products to expect and how to write the equation.

Synthesis and decomposition

These two are mirror images: synthesis builds up, decomposition breaks down. Recognizing which way the change runs (fewer substances to one, or one to several) tells you the type immediately.

Single and double replacement

For a single replacement, use the activity series: a metal will replace another metal (or hydrogen) only if it is more active. Zinc replaces copper in copper sulfate (Zn+CuSO4→ZnSO4+Cu\text{Zn} + \text{CuSO}_4 \rightarrow \text{ZnSO}_4 + \text{Cu}) because zinc is more active, but copper cannot replace zinc. A double replacement runs to completion when one product leaves the solution as an insoluble precipitate, a gas, or water; otherwise the ions stay mixed and no net reaction occurs.

Combustion

A combustion reaction is the rapid reaction of a fuel with oxygen, releasing energy. Complete combustion of a hydrocarbon always gives carbon dioxide and water: CH4+2 O2→CO2+2 H2O\text{CH}_4 + 2\,\text{O}_2 \rightarrow \text{CO}_2 + 2\,\text{H}_2\text{O}. Because the products are fixed (carbon dioxide and water), combustion is easy to spot from a hydrocarbon plus oxygen, and it is also exothermic, releasing heat.

Try this

Q1. Classify the reaction 2 KClO3→2 KCl+3 O22\,\text{KClO}_3 \rightarrow 2\,\text{KCl} + 3\,\text{O}_2. [1 point]

  • Cue. Decomposition; one compound breaks into two simpler substances.

Q2. Will copper replace silver in silver nitrate, given that copper is more active than silver? [1 point]

  • Cue. Yes; a more active metal (copper) replaces a less active one (silver), so Cu+2 AgNO3→Cu(NO3)2+2 Ag\text{Cu} + 2\,\text{AgNO}_3 \rightarrow \text{Cu(NO}_3)_2 + 2\,\text{Ag}.

Exam-style practice questions

Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

SOL (multiple choice)1 marksThe reaction 2 H2+O2→2 H2O2\,\text{H}_2 + \text{O}_2 \rightarrow 2\,\text{H}_2\text{O} is best classified as (A) decomposition (B) synthesis (C) single replacement (D) double replacement
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The answer is (B) synthesis.

Two simpler substances (hydrogen and oxygen) combine to form a single more complex product (water). That is the pattern of a synthesis (combination) reaction, A+B→AB\text{A} + \text{B} \rightarrow \text{AB}. Decomposition is the reverse, single and double replacement involve swapping ions or atoms, and none of those fit here.

The trap is calling any reaction with oxygen a combustion; combustion produces carbon dioxide and water from a fuel, which this is not.

SOL (tech-enhanced, drag and drop)3 marksClassify each reaction: (a) CaCO3→CaO+CO2\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2; (b) Zn+2 HCl→ZnCl2+H2\text{Zn} + 2\,\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2; (c) AgNO3+NaCl→AgCl+NaNO3\text{AgNO}_3 + \text{NaCl} \rightarrow \text{AgCl} + \text{NaNO}_3.
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A 3-point classification item.

(a) Decomposition (1 point): one compound breaks into two simpler substances, AB→A+B\text{AB} \rightarrow \text{A} + \text{B}.
(b) Single replacement (1 point): an element (zinc) replaces another (hydrogen) in a compound, A+BC→AC+B\text{A} + \text{BC} \rightarrow \text{AC} + \text{B}.
(c) Double replacement (1 point): two compounds swap ions, AB+CD→AD+CB\text{AB} + \text{CD} \rightarrow \text{AD} + \text{CB}, here forming the precipitate AgCl\text{AgCl}.

Markers reward matching each reaction to its general pattern.

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