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A decomposition reaction breaks one reactant compound into two or more products. Chemically, decomposition is the reverse of synthesis. One example is the decomposition of carbonic acid, H2CO3, in carbonated water: H2CO3(aq) → H2O(l) + CO2(g). Under pressure (unopened) the carbonic acid stays in solution because one of the products of the reaction is a gas. When pressure is released the reaction splits some of the dissolved carbonic acid into water and carbon dioxide gas. The released gas makes the "fizz."
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One important decomposition reaction is the electrolysis of water. Electric current passing through water causes a decomposition reaction which generates hydrogen and oxygen gas in a 2:1 ratio as seen in the balanced reaction. Hydrogen gas is produced at the negative battery terminal and oxygen gas is produced at the positive terminal. Similar to electrolysis, most decomposition reactions are endothermic. It takes an input of energy to break the chemical bonds in the reactant compound. This is often more than the energy released by forming the products. Energy changes during a reaction can be observed in the form of heat, electrical current, light, or even pressure.
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At first glance you may think a reactant can decompose into any combination of elements or compounds, but there is a predictable pattern associated with some compounds:
- Compounds made of only 2 elements will break up into their component elements like water shown above.
- Carbonates like carbonic acid, H2CO3 shown above break up into the oxide of the cation and carbon dioxide.
- Chlorates like potassium chlorate, KClO3 break down into oxygen gas and a binary salt that contains no oxygen: 2KClO3(s) → 2KCl(s) + 3O2(g).
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| Predict the products and write the balanced reaction for the breakdown of solid barium chlorate, Ba(ClO3)2(s).
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| Relationships |
There is only one reactant so this is a decomposition reaction. Chlorates decompose into a binary salt and oxygen gas. |
| Solve |
- Break the compound up into molecular oxygen and a salt.
- Molecular oxygen is O2(g)
- Barium and chlorine will form the salt. Write the charges for each ion and perform a criss-cross. Write the cation first in the formula.
- Charged ions: Ba2+ and Cl−
- Barium chloride: BaCl2
- Write the reaction with the states: Ba(ClO3)2(s) → BaCl2(s) + O2(g)
- Balance the reaction: Ba(ClO3)2(s) → BaCl2(s) + 3O2(g)
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| Answer |
The decomposition reaction is balanced as:
Ba(ClO3)2(s) → BaCl2(s) + 3O2(g). |
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