Synthesis

A synthesis reaction is a reaction in which two or more substances react to form a single substance. The reactants may be either elements or compounds. The product synthesized is always a compound. The general equation for a synthesis reaction is

$\mathrm{A}+\mathrm{B}\to \mathrm{AB}$

Figure 10 shows what happens when sodium reacts with chlorine. The product of this reaction is the compound sodium chloride, which appears as a whitish cloud of solid particles. You are probably more familiar with sodium chloride as table salt. You can describe the synthesis of sodium chloride with the following equation.

$2\mathrm{Na} + {\mathrm{Cl}}_2 \to 2\mathrm{NaCl}$

Another example of a synthesis reaction is hydrogen and oxygen reacting to form water.

$2{\mathrm{H}}_2 + {\mathrm{O}}_{2 }\to 2{\mathrm{H}}_2\mathrm{O}$

This reaction is used to generate electricity for satellites and spacecraft.

For: Links on chemical reactions

Visit: www.SciLinks.org

Web Code: ccn-1076

Decomposition

The opposite of synthesis is decomposition. A decomposition reaction is a reaction in which a compound breaks down into two or more simpler substances. The reactant in a decomposition reaction must be a compound. The products may be elements or compounds. The general equation for a decomposition reaction is

$\mathrm{AB}\to \mathrm{A}+\mathrm{B}$

When electricity passes through water, the water decomposes into hydrogen gas and oxygen gas. You can describe the decomposition of water by writing the following equation.

$2{\mathrm{H}}_2\mathrm{O} \to 2{\mathrm{H}}_2 + {\mathrm{O}}_{2 }$

Notice that this reaction is the opposite of the synthesis of water.

Another example of decomposition occurs in the making of cement. Cement factories use a giant kiln, or oven, to heat a mixture of clay and limestone. The heat causes the calcium carbonate in the limestone to decompose into lime, CaO, and carbon dioxide.

${\mathrm{CaCO}}_3 \to \mathrm{CaO} + {\mathrm{CO}}_2$

The carbon dioxide escapes the kiln through a smokestack. The clayand-lime mixture is cooled and ground into cement powder.

The How It Works box on page 201 describes a decomposition reaction that is used to make automobiles safer.