Types of Chemical Reactions

In short: Most reactions fit a few types: synthesis (A + B → AB), decomposition (AB → A + B), single displacement, double displacement and combustion. A single displacement only works if the element is higher in the activity series, and solubility rules predict when a double displacement forms a precipitate.

Five main types

Combustion has its own chapter next. This chapter looks at the other four.

Synthesis

Two or more substances join to make one product.

$$\mathrm{Fe}(\mathrm{s})+\mathrm{S}(\mathrm{s})\to \mathrm{FeS}(\mathrm{s})$$ $$2Na(\mathrm{s})+\mathrm{C}{\mathrm{l}}_2(\mathrm{g})\to 2NaCl(\mathrm{s})$$

Metal + non-metal gives an ionic compound. Metal + oxygen gives a metal oxide.

Decomposition

One compound breaks down into two or more simpler substances. It usually needs energy: heat (thermal decomposition), electricity (electrolysis) or light.

$$\mathrm{CaC}{\mathrm{O}}_3(\mathrm{s})\to \mathrm{CaO}(\mathrm{s})+\mathrm{C}{\mathrm{O}}_2(\mathrm{g})$$

Heating limestone (calcium carbonate) makes lime for cement. Passing electricity through water (electrolysis) splits it into hydrogen and oxygen.

Hydrogen peroxide slowly decomposes in sunlight, which is why it is sold in brown bottles:

$$2{\mathrm{H}}_2{\mathrm{O}}_2(\mathrm{l})\to 2{\mathrm{H}}_2\mathrm{O}(\mathrm{l})+{\mathrm{O}}_2(\mathrm{g})$$

Single displacement and the activity series

A more reactive element displaces (pushes out) a less reactive element from its compound.

The activity series ranks metals from most to least reactive:

$$\text{K}>\text{Na}>\text{Ca}>\text{Mg}>\text{Al}>\text{Zn}>\text{Fe}>\text{Sn}>\text{Pb}>(\text{H})>\text{Cu}>\text{Ag}>\text{Au}$$

Metals above hydrogen react with acids to release hydrogen gas:

$$\mathrm{Mg}(\mathrm{s})+2HCl(\mathrm{aq})\to \mathrm{MgC}{\mathrm{l}}_2(\mathrm{aq})+{\mathrm{H}}_2(\mathrm{g})$$

Copper, silver and gold are below hydrogen, so they do not react with dilute acids. That is why they are used for coins and jewellery.

Halogens follow the same idea, with their own order: ${\text{F}}_2>{\text{Cl}}_2>{\text{Br}}_2>{\text{I}}_2$. Chlorine displaces bromine from potassium bromide, but iodine cannot displace chlorine.

Double displacement

Two ionic compounds in solution swap partners. The positive ion of one pairs with the negative ion of the other.

$$\mathrm{AB}+\mathrm{CD}\to \mathrm{AD}+\mathrm{CB}$$

A double displacement only happens if one of the products is:

1. A precipitate (an insoluble solid that appears in the solution), or
2. Water (a neutralisation), or
3. A gas.

To predict whether a product is a precipitate, use solubility rules:

Neutralisation is a double displacement that makes water:

$$\mathrm{HCl}(\mathrm{aq})+\mathrm{NaOH}(\mathrm{aq})\to \mathrm{NaCl}(\mathrm{aq})+{\mathrm{H}}_2\mathrm{O}(\mathrm{l})$$

Quick summary

• Synthesis: A + B → AB. Decomposition: AB → A + B.
• Single displacement: an element replaces another in a compound. It only works if the element is higher in the activity series.
• Double displacement: two compounds swap ions. It needs a precipitate, water or a gas to form.
• Use solubility rules to predict precipitates. Sodium, potassium, ammonium and nitrate compounds always dissolve.
• Combustion: fuel + oxygen gives oxides, heat and light.

Quiz