SCH3U Synthesis & Decomposition Reactions: Notes and Practice

Chemists sort reactions into recognizable families based on the pattern of how reactants become products. Recognizing the pattern lets you predict products even for a reaction you've never seen before, the same way knowing that verbs take "-ed" in the past tense lets you read a verb you have never met. Synthesis and decomposition are the simplest families and are exact opposites of each other: one builds a compound up from simpler pieces, the other breaks a compound down into simpler pieces.

Fig 1An overview of the major reaction type patterns: synthesis, decomposition, single displacement, double displacement, and combustion.

Synthesis (combination) reactions

A + B \to A B

Two or more simpler substances combine to form one, more complex product. The reactants can be elements or simple compounds; the defining feature is that exactly one product forms. Why do they combine? When a metal meets a nonmetal, the metal atoms lose electrons and the nonmetal atoms gain them, and the oppositely charged ions attract strongly. Forming those ionic bonds releases a great deal of energy, which is why sodium burning in chlorine gives off a brilliant flash. Two nonmetals combine by sharing electrons instead, giving a molecular compound. Common patterns worth memorizing for predicting products:

Reactants	Product pattern	Example
metal + nonmetal	ionic compound	$2 Na (s) + Cl_{2} (g) \to 2 NaCl (s)$
metal + oxygen	metal oxide	$4 Al (s) + 3 O_{2} (g) \to 2 Al_{2} O_{3} (s)$
nonmetal + nonmetal	molecular compound	$N_{2} (g) + 3 H_{2} (g) \to 2 NH_{3} (g)$
metal oxide + water	metal hydroxide (a base)	$CaO (s) + H_{2} O (l) \to Ca (OH)_{2} (aq)$
nonmetal oxide + water	an acid	$CO_{2} (g) + H_{2} O (l) \to H_{2} CO_{3} (aq)$

Oxides and water in the real world

Sulfur oxides released by metal smelters and coal-burning power plants form sulfur trioxide in the air, which combines with rain in a synthesis reaction,

SO_{3} + H_{2} O \to H_{2} SO_{4}

, which is one cause of acid rain that damaged lakes in Ontario's Muskoka and Sudbury regions. Farmers work the other way on purpose: to raise the pH of acidic soil they spread lime, most often ground limestone (

CaCO_{3}

) but sometimes quicklime (CaO), which reacts with soil water to form the base

Ca (OH)_{2}

Decomposition reactions

A B \to A + B

One compound breaks apart into two or more simpler products — often needing an energy input (heat, electricity) to start. This is the reverse pattern of synthesis, and the energy logic runs in reverse too: if forming a compound released energy, pulling it apart must put that energy back in to break the bonds. That is why most decompositions only happen when you heat the substance strongly ( $Δ$ above the arrow) or pass electricity through it. The products are either the elements themselves or smaller, very stable compounds such as $CO_{2}$ , $H_{2} O$ and metal oxides. Some decompositions are useful precisely because they release a gas quickly: early car airbags were inflated by the decomposition of sodium azide, $2 NaN_{3} (s) \to 2 Na (s) + 3 N_{2} (g)$ , which fills the bag with nitrogen in a few hundredths of a second after an electrical spark sets it off.

Reactant	Product pattern	Example
metal carbonate	metal oxide + carbon dioxide gas	$CaCO_{3} (s) Δ CaO (s) + CO_{2} (g)$
metal hydroxide	metal oxide + water	$Mg (OH)_{2} (s) Δ MgO (s) + H_{2} O (g)$
metal chlorate	metal chloride + oxygen gas	$2 KClO_{3} (s) Δ 2 KCl (s) + 3 O_{2} (g)$
water (via electrolysis)	hydrogen gas + oxygen gas	$2 H_{2} O (l) electricity 2 H_{2} (g) + O_{2} (g)$
binary compound	its two elements	$2 HgO (s) Δ 2 Hg (l) + O_{2} (g)$

ASynthesis

Several reactants, exactly one product
Builds a more complex substance
Often releases energy once started
Example: making ammonia for fertilizer

BDecomposition

Exactly one reactant, several products
Breaks into simpler substances
Usually needs heat or electricity
Example: heating limestone in a lime kiln

Predicting products step by step

Count reactants and look at what they are: two elements, an oxide plus water, or one compound on its own.
Match the pattern from the tables above to name the product(s).
Write each ionic product from its ion charges (for example $Al^{3 +}$ and $O^{2 -}$ give $Al_{2} O_{3}$ ), and write any free HOFBrINCl element as a diatomic molecule.
Add state symbols: metals and ionic solids (s), $O_{2}$ , $CO_{2}$ and $H_{2}$ (g), hydroxides and acids formed in water (aq).
Balance with coefficients only, then recount.

Quick classification test

Quick test: if there's only one reactant, it's decomposition. If there's only one product, it's synthesis. This single check resolves most classification questions instantly. Watch for one trap: a hydrocarbon plus oxygen has two reactants and two products, so it is neither; it is combustion.

Synthesis & Decomposition Reactions

Key things to remember

Synthesis (combination) reactions

Decomposition reactions

Predicting products step by step

Worked examples

Where marks get lost

Flashcards 10

Practice