KEY2CHEM

Synthesis and Decomposition

Synthesis reactions are those where two or more substances combine to form one substance. The opposite process is decomposition, where a single substance decomposes into two or more substances.

Example 1.

Sodium bicarbonate (baking soda) decomposes by the following chemical equation. If $$5\text{ g } \require{mhchem}\ce{NaHCO3}$$decomposes, how many grams of total products will form?

$$\require{mhchem}\ce{NaHCO3(s) -> Na2CO3(s) + CO2(g) + H2O(l)}$$

A. $$5\text{ g}$$

B. $$10\text{ g}$$

C. $$15\text{ g}$$

Solution

A. $$5\text{ g}$$

Based on the conservation of mass, the total mass of products formed must be equal to the mass of reactant used.

Example 2.

Potassium chlorate ($$\require{mhchem}\ce{KClO3}$$) has many industrial uses, including generation of oxygen gas ($$\require{mhchem}\ce{O2}$$). In addition to $$\require{mhchem}\ce{O2}$$ gas, what ionic compound is formed from the decomposition of $$\require{mhchem}\ce{KClO3}$$

A. $$\require{mhchem}\ce{KCl}$$

B. $$\require{mhchem}\ce{KCl2}$$

C. $$\require{mhchem}\ce{K2Cl}$$

Solution

A. $$\require{mhchem}\ce{KCl}$$

$$\require{mhchem}\ce{ 2KClO3(s) -> 2 KCl(s) + 3 O2(g) }$$

Based on the law of definite proportion, $$\require{mhchem}\ce{K}$$ and $$\require{mhchem}\ce{Cl}$$ combine in a $$1:1$$ ratio as the ionic byproduct of $$\require{mhchem}\ce{KClO3}$$ decomposition.

Example 3.

Which is a synthesis reaction?

A. $$\require{mhchem}\ce{2 H2O(l) -> 2 H2(g) + O2(g) }$$

B. $$\require{mhchem}\ce{2 Na(s) + Cl2(g) -> 2 NaCl(s) }$$

C. $$\require{mhchem}\ce{CH4(g) + 2 O2(g) -> CO2(g) + 2 H2O(g)}$$

Solution

B. $$\require{mhchem}\ce{2 Na(s) + Cl2(g) -> 2 NaCl(s) }$$

A synthesis reaction is one where two or more substances combine to form a single substance. The stoichiometric coefficients do not need to be 1.