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.