Type of Reactions

Synthesis (or combination)

A synthesis reaction is a reaction in which two or more substances combine to produce a single, more complex substance. The components involved in the reaction may be elements, compounds or both.

Several examples of synthesis reactions are:

2H2 + O2 à 2H2O * The burning of hydrogen to form water

C + O2 à C O2 * The burning of carbon to form carbon dioxide

2CO + O2 à 2CO2 * The burning of carbon monoxide to form carbon dioxide

CaO + H2O à Ca(OH)2 * The reaction of calcium oxide and water to form calcium hydroxide

2Mg + O2 à 2MgO * The oxidation of magnesium (see the animation)

Decomposition (or analysis)

In a decomposition reaction a single substance is broken down into two or more simpler substances. These simpler substances may either be elements or compounds. Most decomposition reactions are endothermic (absorb energy during the reaction). The energy for the endothermic reactions is usually supplied as heat or electricity. Sometimes, however, the reaction can be exothermic. An example of this is the breakdown of unstable compounds.

Several examples of decomposition are:

2H2O à 2H2 + O2 * Decomposition of water by an electric current, a reaction also called

 

Electrolysis (see the animation)

2HgO + HEAT à 2Hg + O2 (g) *Mercury(II) oxide decomposed by heat

Single Replacement Reactions

A single replacement reaction is a reaction in which a free element takes the place of a less active element in a compound, setting the less active element free.

Some examples of single replacement reactions are:

Zn + H2SO4 à Zn SO4 + H2

* This is the replacement of hydrogen in an acid with and active metal, in

 

this case, zinc. (an animation of a similar experiment can be seen).

Cl2 + 2NaBr à Br2 + 2NaCl * The replacement of bromide ions in a compound, by the element chlorine

Double Replacement Reactions

In this type of reaction one ionic compound is added to another ionic compound. The anions of one compound react with the cations of the other. The result of this reaction can be a precipitate, water or a gas.

A precipitate is a solid substance formed by a physical or chemical change in a liquid medium. In a double replacement reaction, the anion of one compound and the cation of the other compound react to form a substance that is insoluble in the liquid medium. This solid is denser than the solution and therefore separates by settling to the bottom.

Some examples of double replacement reactions are:

NaCl + AgNO3 à NaNO3 + AgCl(s) * The formation of solid silver chloride in an aqueous solution

3NaOH + FeBr3 à 3NaBr + Fe(OH)3 (s)

* The formation of solid iron hydroxide in an aqueous

 

solution (an animation of a similar experiment can be seen)