SNC2D Synthesis and Decomposition Reactions: Notes and Practice

Roughly half of the world's people are fed by crops grown with synthetic nitrogen fertilizer, and that fertilizer starts as ammonia made by one reaction: nitrogen from the air combining with hydrogen. When a car crashes, an airbag inflates in a few hundredths of a second because a solid compound suddenly breaks apart into a large volume of gas, mostly nitrogen. These two reactions are opposites. The first is a synthesis reaction, where substances join to make one product. The second is a decomposition reaction, where one compound splits into simpler substances. There are thousands of different reactions, but most of the ones you meet in this course fit one of five patterns, and recognizing the pattern lets you predict the products.

Fig 1The five reaction patterns drawn with coloured shapes for A, B, C and D: synthesis A + B → AB, decomposition AB → A + B, single displacement A + BC → AC + B, double displacement AB + CD → AD + CB, and combustion, fuel + O₂ → CO₂ + H₂O.

Synthesis: A + B → AB

In a synthesis (or combination) reaction, two or more reactants combine to form a single product. The clue is on the right side of the arrow: only one product. The simplest synthesis reactions join two elements into a binary compound. When a metal reacts with a non-metal, the product is an ionic compound, and you can predict its formula from the ion charges. Compounds can also combine: a metal oxide and water form a base (a metal hydroxide), while a non-metal oxide and water form an acid, which is one step in making acid rain.

Magnesium burning with a brilliant white light: $2 Mg (s) + O_{2} (g) \to 2 MgO (s)$
Sodium and chlorine making table salt: $2 Na (s) + Cl_{2} (g) \to 2 NaCl (s)$
Iron rusting (simplified; real rust also contains water): $4 Fe (s) + 3 O_{2} (g) \to 2 Fe_{2} O_{3} (s)$
Making ammonia for fertilizer: $N_{2} (g) + 3 H_{2} (g) \to 2 NH_{3} (g)$
A metal oxide and water form a base: $CaO (s) + H_{2} O (l) \to Ca (OH)_{2} (aq)$
A non-metal oxide and water form an acid: $SO_{3} (g) + H_{2} O (l) \to H_{2} SO_{4} (aq)$

Predicting a synthesis product

For a metal plus a non-metal: (1) write each ion with its charge, (2) criss-cross to get the product's formula, (3) write the skeleton equation with diatomic elements where needed, (4) balance. Calcium + oxygen:

Ca^{2 +}

and

O^{2 -}

give CaO, and

2 Ca + O_{2} \to 2 CaO

Decomposition: AB → A + B

In a decomposition reaction, a single compound breaks down into two or more simpler substances: elements, simpler compounds, or both. The clue is on the left side of the arrow: only one reactant. Most decompositions need energy added, such as heat, electricity or light, because the compound's bonds must be broken. Using electricity to decompose a compound is called electrolysis. When water is split by electrolysis, the hydrogen gas collected has twice the volume of the oxygen gas, matching the 2 to 1 ratio of the coefficients in the balanced equation.

Decomposition	Energy source	Where it matters
2H₂O(l) → 2H₂(g) + O₂(g)	electricity (electrolysis)	making hydrogen fuel
2NaCl(l) → 2Na(l) + Cl₂(g)	electricity, with the salt melted	producing sodium metal and chlorine
CaCO₃(s) → CaO(s) + CO₂(g)	strong heat	making lime and cement; a large source of industrial CO₂
2H₂O₂(aq) → 2H₂O(l) + O₂(g)	a catalyst or light	why hydrogen peroxide is sold in dark bottles
2NaN₃(s) → 2Na(s) + 3N₂(g)	an electrical igniter heats it	inflating airbags in older cars (simplified; newer airbags use less toxic compounds)
2KClO₃(s) → 2KCl(s) + 3O₂(g)	heat	producing oxygen in the lab

Common decomposition reactions. Metal carbonates usually decompose on heating into a metal oxide and carbon dioxide.

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Balance synthesis and decomposition equations such as 2H₂O → 2H₂ + O₂ by changing coefficients and watching the atom counts match.

ASynthesis

Two or more reactants → one product
Pattern: A + B → AB
Often releases energy (burning magnesium gives off heat and light)
Example: 2H₂ + O₂ → 2H₂O

BDecomposition

One reactant → two or more products
Pattern: AB → A + B
Usually needs energy added (heat, electricity or light)
Example: 2H₂O → 2H₂ + O₂

Mirror images

A decomposition is the reverse of a synthesis. Hydrogen and oxygen combine to make water and release energy; putting that energy back in, as electricity, splits water into hydrogen and oxygen again. Classify by counting: one product means synthesis, one reactant means decomposition.

Chemistry at industrial scale

Chemical engineers design and run the plants that carry out reactions like ammonia synthesis and limestone decomposition on a huge scale, choosing temperatures, pressures and catalysts that make the reaction fast, safe and economical. Because heating limestone releases carbon dioxide, cement makers are now testing ways to capture that gas instead of releasing it.

Synthesis and Decomposition Reactions

Key things to remember

Synthesis: A + B → AB

Decomposition: AB → A + B

Worked examples

Where marks get lost

Flashcards 7

Practice