K.+Soo+Decomposition+reaction.

=** Decomposition Reaction. **=

Essentially, a decomposition reaction is the opposite of a synthesis reaction. In a decomposition reaction, a single compound reacts to produce two or more simpler substances.

** Formula. **
AB —› A + B

** Example of a decomposition reaction. **
Mercury (II) oxide is heated to produce mercury and oxygen.

** Changing the word equation into a formula equation… **
The first step in changing the word equation into a formula equation is always to figure out the elements’ symbol. By checking the periodic table of elements, you know that mercury is shown with “Hg” and oxygen is represented by “O”. Using the atomic symbols, write a formula equation. Because you are dealing with a compound, you need to make sure that the total charge of the compound is zero, because all compounds must be neutral.

The Roman numeral II after the mercury in the word equation indicates that the mercury used in this reaction is a +2 ion, and because oxygen is in group 16, you know that it has a charge of -2. This means that mercury (II) oxide is properly written as HgO.

HgO —› Hg + O

Because O is diatomic, you know that you need to add a subscript 2 after the O.

HgO —› Hg + O 2

Now, that all the reactants and products are properly written, you can balance your formula equation. The number of mercury atoms on each side of the reaction is equal, but there is only one oxygen on the left while there are two oxygen atoms on the right. To correct this, you can add a coefficient of 2 before the HgO, which multiplies everything in the compound by two.

2 HgO —› Hg + O2

Now the oxygens are correct but there is too many mercury atoms on the reactant side of the equation. To balance this, you put a coefficient of 2 before the Hg produced on the right so that there are 2 mercury atoms on both sides of the equation.

2 HgO —› 2 Hg + O2

Now the equation is balanced! Always be sure to remember that you cannot change the subscripts to balance an equation, only add coefficients.

Back to Reaction Teaching Wiki..