IMAT Chemistry · Stoichiometry and Reactions

Types of Reaction, Displacement and Precipitates

Most reactions fit a few patterns (joining, splitting, swapping partners, burning, neutralising), and the pattern tells you what the products will be.

All Stoichiometry and Reactions notes

Why this matters

Classifying reactions was asked in 2012, as a type missing from a list of equations. The products of a reactive metal with water came up in 2020, and a metal displacing another from its salt sits behind a 2014 yield calculation.

Concept 1 of 3: The main types of chemical reaction

Rather than memorise every reaction, learn the patterns. Two substances joining, one splitting, an element pushing another out of a compound, or two compounds swapping partners: once you see the pattern you can predict the products and balance the equation.

Definition

Synthesis (combination): two or more substances form one product.
Decomposition: one substance breaks into two or more, usually on heating (thermal decomposition) or by electricity (electrolysis).
Single displacement: an element takes the place of another element in a compound. It is always a redox reaction.
Double displacement (metathesis): two ionic compounds swap partners. Precipitation and neutralisation are double displacements.
Combustion: a substance burns in oxygen, giving out heat.
One reaction can belong to two types. Substitution, addition and elimination are the names used for organic reactions (see Organic Chemistry).

Type	Pattern	Example	How to spot it
Synthesis	A + B → AB	$2\mathrm{Mg} + \mathrm{O_2} \rightarrow 2\mathrm{MgO}$	Several reactants, one product
Decomposition	AB → A + B	$\mathrm{CaCO_3} \rightarrow \mathrm{CaO} + \mathrm{CO_2}$ (heated)	One reactant, several products
Single displacement	A + BC → AC + B	$\mathrm{Fe} + \mathrm{CuSO_4} \rightarrow \mathrm{FeSO_4} + \mathrm{Cu}$	An element in, a different element out
Double displacement	AB + CD → AD + CB	$\mathrm{AgNO_3} + \mathrm{NaCl} \rightarrow \mathrm{AgCl} + \mathrm{NaNO_3}$	Two compounds swap ions
Precipitation	ions (aq) → insoluble solid	$\mathrm{Fe^{3+}} + 3\mathrm{OH^-} \rightarrow \mathrm{Fe(OH)_3(s)}$	A solid appears when two solutions mix
Neutralisation	acid + base → salt + water	$\mathrm{HCl} + \mathrm{NaOH} \rightarrow \mathrm{NaCl} + \mathrm{H_2O}$	Net change is $\mathrm{H^+} + \mathrm{OH^-} \rightarrow \mathrm{H_2O}$
Combustion	fuel + oxygen → oxides	$\mathrm{CH_4} + 2\mathrm{O_2} \rightarrow \mathrm{CO_2} + 2\mathrm{H_2O}$	Oxygen is a reactant; heat is given out

Practice this conceptself-check · 4 quick reps

A reaction can have more than one label

Burning magnesium is both synthesis and combustion; neutralisation is also a double displacement; every single displacement is also a redox reaction. An option is not wrong just because another label also fits.

Concept 2 of 3: Displacement reactions and the reactivity series

Metals differ in how readily they give away electrons. A more reactive metal can take the place of a less reactive one in its compounds, but never the other way round. The same ranking decides which metals react with water or acids and what they make.

Definition

Reactivity series (most to least reactive): K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Ag > Au.
A metal displaces any metal below it from a solution of its salt.
Metals above hydrogen release hydrogen gas from dilute acids; copper, silver and gold do not.
With cold water, the most reactive metals give the hydroxide and hydrogen. With steam, less reactive metals give the oxide and hydrogen.
Among the halogens, a more reactive halogen displaces a less reactive one from its salt: $\mathrm{F_2} > \mathrm{Cl_2} > \mathrm{Br_2} > \mathrm{I_2}$ .

Reaction	General result	Example
Group 1 metal or calcium + cold water	hydroxide + hydrogen	$2\mathrm{Na} + 2\mathrm{H_2O} \rightarrow 2\mathrm{NaOH} + \mathrm{H_2}$ Magnesium reacts the same way with cold water, but only very slowly.
Magnesium, zinc or iron + steam	oxide + hydrogen	$\mathrm{Mg} + \mathrm{H_2O(g)} \rightarrow \mathrm{MgO} + \mathrm{H_2}$
Metal above hydrogen + dilute acid	salt + hydrogen	$\mathrm{Zn} + 2\mathrm{HCl} \rightarrow \mathrm{ZnCl_2} + \mathrm{H_2}$
Metal + salt solution of a less reactive metal	the less reactive metal is deposited	$\mathrm{Fe} + \mathrm{CuSO_4} \rightarrow \mathrm{FeSO_4} + \mathrm{Cu}$
Halogen + salt of a less reactive halogen	the less reactive halogen is released	$\mathrm{Cl_2} + 2\mathrm{NaBr} \rightarrow 2\mathrm{NaCl} + \mathrm{Br_2}$

The formula of each product follows the ion charges:

\mathrm{Mg^{2+}}

with

\mathrm{OH^-}

gives

\mathrm{Mg(OH)_2}

, never MgOH.

Practice this conceptself-check · 4 quick reps

Cold water gives a hydroxide, steam gives an oxide

A reactive metal in cold water makes the metal hydroxide and hydrogen. The oxide forms when a less reactive metal is heated in steam. An option pairing cold water with an oxide is wrong.

Concept 3 of 3: Solubility rules for predicting a precipitate

A precipitate forms when two solutions mix and one possible pairing of their ions is insoluble. A handful of rules tells you which salts dissolve, so you can predict whether a solid appears and what it is.

Definition

A precipitate is an insoluble solid formed when two solutions are mixed.
To predict one, swap the partners of the two compounds and check whether either new pairing is insoluble.
Salts of sodium, potassium and ammonium, and all nitrates, are soluble. So these ions are never in the precipitate.
The colour of some precipitates identifies the ion: $\mathrm{AgCl}$ white, $\mathrm{AgI}$ and $\mathrm{PbI_2}$ yellow, $\mathrm{Cu(OH)_2}$ blue, $\mathrm{Fe(OH)_3}$ red-brown.

Compounds of	Usually	Main exceptions
Sodium, potassium, ammonium	Soluble	None at this level
Nitrates	Soluble	None at this level
Chlorides, bromides, iodides	Soluble	Silver and lead(II) salts are insoluble
Sulfates	Soluble	Barium and lead(II) insoluble; calcium only slightly soluble
Carbonates	Insoluble	Sodium, potassium and ammonium carbonates dissolve
Hydroxides	Insoluble	Group 1 hydroxides dissolve; calcium hydroxide slightly

Practice this conceptself-check · 4 quick reps

The precipitate never contains sodium, potassium or nitrate

Their compounds all dissolve, so they stay in solution as spectator ions. An option showing sodium chloride or potassium nitrate as the solid formed is wrong.

Summary — formulas & gotchas at a glance

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Reference tables (3)

The main types of chemical reaction7 rows

Type	Pattern	Example	How to spot it
Synthesis	A + B → AB	$2\mathrm{Mg} + \mathrm{O_2} \rightarrow 2\mathrm{MgO}$	Several reactants, one product
Decomposition	AB → A + B	$\mathrm{CaCO_3} \rightarrow \mathrm{CaO} + \mathrm{CO_2}$ (heated)	One reactant, several products
Single displacement	A + BC → AC + B	$\mathrm{Fe} + \mathrm{CuSO_4} \rightarrow \mathrm{FeSO_4} + \mathrm{Cu}$	An element in, a different element out
Double displacement	AB + CD → AD + CB	$\mathrm{AgNO_3} + \mathrm{NaCl} \rightarrow \mathrm{AgCl} + \mathrm{NaNO_3}$	Two compounds swap ions
Precipitation	ions (aq) → insoluble solid	$\mathrm{Fe^{3+}} + 3\mathrm{OH^-} \rightarrow \mathrm{Fe(OH)_3(s)}$	A solid appears when two solutions mix
Neutralisation	acid + base → salt + water	$\mathrm{HCl} + \mathrm{NaOH} \rightarrow \mathrm{NaCl} + \mathrm{H_2O}$	Net change is $\mathrm{H^+} + \mathrm{OH^-} \rightarrow \mathrm{H_2O}$
Combustion	fuel + oxygen → oxides	$\mathrm{CH_4} + 2\mathrm{O_2} \rightarrow \mathrm{CO_2} + 2\mathrm{H_2O}$	Oxygen is a reactant; heat is given out

Displacement reactions and the reactivity series5 rows

Reaction	General result	Example
Group 1 metal or calcium + cold water	hydroxide + hydrogen	$2\mathrm{Na} + 2\mathrm{H_2O} \rightarrow 2\mathrm{NaOH} + \mathrm{H_2}$ Magnesium reacts the same way with cold water, but only very slowly.
Magnesium, zinc or iron + steam	oxide + hydrogen	$\mathrm{Mg} + \mathrm{H_2O(g)} \rightarrow \mathrm{MgO} + \mathrm{H_2}$
Metal above hydrogen + dilute acid	salt + hydrogen	$\mathrm{Zn} + 2\mathrm{HCl} \rightarrow \mathrm{ZnCl_2} + \mathrm{H_2}$
Metal + salt solution of a less reactive metal	the less reactive metal is deposited	$\mathrm{Fe} + \mathrm{CuSO_4} \rightarrow \mathrm{FeSO_4} + \mathrm{Cu}$
Halogen + salt of a less reactive halogen	the less reactive halogen is released	$\mathrm{Cl_2} + 2\mathrm{NaBr} \rightarrow 2\mathrm{NaCl} + \mathrm{Br_2}$

The formula of each product follows the ion charges:

\mathrm{Mg^{2+}}

with

\mathrm{OH^-}

gives

\mathrm{Mg(OH)_2}

, never MgOH.

Solubility rules for predicting a precipitate6 rows

Compounds of	Usually	Main exceptions
Sodium, potassium, ammonium	Soluble	None at this level
Nitrates	Soluble	None at this level
Chlorides, bromides, iodides	Soluble	Silver and lead(II) salts are insoluble
Sulfates	Soluble	Barium and lead(II) insoluble; calcium only slightly soluble
Carbonates	Insoluble	Sodium, potassium and ammonium carbonates dissolve
Hydroxides	Insoluble	Group 1 hydroxides dissolve; calcium hydroxide slightly

Watch out for (3)

A reaction can have more than one label
→ The main types of chemical reaction
Cold water gives a hydroxide, steam gives an oxide
→ Displacement reactions and the reactivity series
The precipitate never contains sodium, potassium or nitrate
→ Solubility rules for predicting a precipitate

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