How to Write & Balance a Decomposition Reaction

A student is writing the products of a decomposition reaction.
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A decomposition reaction is a type of chemical reaction in which a compound is separated into its component parts. It is important to understand how to write and balance decomposition reactions because they occur within many types of chemical experiments. In performing more complicated calculations, you will need to write a balanced equation in order to gather the values that you need. Such a process may appear intimidating, but knowing the nature of the compounds and how the reaction proceeds will help you to learn.

    Know the general form for a decomposition reaction. The standard format for a decomposition is AB gives you A + B, where AB is the compound and A and B are the elements that make up that compound. An example of a decomposition reaction is 2HgO gives you 2Hg + O2.

    Write down the formula for the compound that you are starting with, and determine the elements that form that compound. For example, if you are starting with the compound sodium chloride, then the chemical formula is NaCl. The elements that compose sodium chloride are sodium, Na, and chlorine, Cl2. The chlorine has a two at the end of it because it is a diatomic element.

    Write the chemical formula. based on the information that you know. For example, you know that the reactant is NaCl, and that the products are Na and Cl2. Therefore, you may write NaCl gives you Na + Cl2.

    Count the number of moles of each element on both sides of the decomposition reaction. When balancing a decomposition reaction, the number of moles should be equal on both sides. For example, in the equation NaCl gives you Na + Cl2, there is one mole of Na on both sides, but there is one mole of Cl on the left side and two moles on Cl on the right side.

    Put coefficients before the elements and compounds so that the equation is balanced. For example, for the sodium chloride decomposition reaction, putting a 2 before the NaCl gives you 2 moles of Na and 2 moles of Cl. This makes the moles of Cl equal on both sides, but not for Na. Therefore, you can put a 2 before the Na on the right side. The final decomposition reaction gives you 2NaCl gives you 2Na + Cl2.

    Things You'll Need

    • Computer
    • Chemistry book
    • Calculator
    • Periodic table
    • Pen
    • Paper

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