Key Difference – Combination vs Decomposition Reaction
A chemical reaction is the conversion of one or more chemical compounds into one or more different products, changing the identity of a chemical compound. The starting material of a chemical reaction is called the reactant and the resulting compound is called the product. Breakdown of compounds or combination of compounds and formation of new compounds will occur during a chemical reaction process since the bonds between atoms of the compound are broken and created in a different manner. Chemical reactions can be divided into several broad categories. Redox reactions or oxidation-reduction reactions are very important among them. The oxidation and reduction reactions are called electron transfer reactions since the electrons of reactants are transferred from one compound to the other to cause the reaction. In redox reactions, two parallel reactions, which are called half-reactions, occur at the same time. These half reactions show the transfer of electrons. By balancing these half-reactions, one can guess the overall reaction that has happened at the end. Combination reactions and decomposition reactions are two major types of redox reactions. The main difference between combination and decomposition reaction is that combination reaction involves a combination of reactants to give a single product whereas decomposition reaction involves the breakdown of a single compound into two or more products.
CONTENTS
1. Overview and Key Difference
2. What is a Combination Reaction
3. What is a Decomposition Reaction
4. Side by Side Comparison – Combination vs Decomposition Reaction
5. Summary
What is a Combination Reaction?
A combination reaction, also called as synthesis reaction, is a reaction in which reactant compounds are combined to form a different compound as the product. In other words, the reaction of simple molecules results in a complex molecule. Some or all the bonds between atoms of that particular compound are broken down; at the same time, atoms will combine to form the new compound, which is the product. In decomposition reactions, the same reactant acts as the starting material for both half reactions. Unlike in decomposition reactions, the half reactions in combination reactions have different reactants to begin with. Combination reaction results in a single product. Following is a common example that can be given as combustion reactions.
For example, when Aluminium(Al) is placed in liquid bromide (Br2) a combination reaction occurs and produces Aluminium bromide(AlBr3). Here, the oxidation number is increased in Al and decreased in Br. Therefore, it is a redox reaction and is a combination reaction since two reactants have reacted to give one particular product.
Figure 01: Combination Reaction
What is Decomposition Reaction?
Decomposition reaction is another important reaction in the category of redox reactions. It is basically the opposite of combination reaction. A decomposition reaction is a reaction where reactant compound is broken down into products. Here, half-reactions occur at the same time as the oxidation reaction and the reduction reaction. But unlike in combination reaction, the reactant for both half reactions is the same in decomposition reactions. Decomposition reaction results in several products.
In the electrolysis of water, when direct current is passed through water, water molecules are decomposed to give oxygen and hydrogen gases. Here, the oxidation number is increased in oxygen atom and is decreased in hydrogen atom. Hence, it is a redox reaction and a decomposition reaction due to the breakdown of water molecules into oxygen and hydrogen gases.
Figure 02: Combination vs Decomposition
What is the difference between Combination and Decomposition Reaction?
Combination vs Decomposition Reaction |
|
| Two or more reactant compounds are involved in combination reactions. | A single compound is involved in a decomposition reaction. |
| Products | |
| Combination reaction results in a single product. | Decomposition reactions result in several products. |
| Half Reactions | |
| In combination reactions, two half reactions have two different starting molecules. | In decomposition reactions, a single molecule act as the starting material for both half reactions. |
| Chemical Bonds | |
| Combination reactions result in bonding of atoms to produce the single end product. | In decomposition reactions, chemical bonds are broken down to form two or more end products. |
| Molecules | |
| Combination reactions cause simple molecules to react and produce complex molecules. | Decomposition reactions cause complex molecules to break down into simple molecules. |
Summary – Combination vs Decomposition Reaction
Redox reactions are very much a part of the world around us because the majority of the important chemical reactions are redox reactions. Combination reactions and decomposition reactions are simple reactions which are opposite of each other. The main difference between combination and decomposition reaction is that combination reaction involves the combination of two or more reactant molecules to result in a single end product whereas decomposition reaction involves the breakdown of a single molecule into two or more products.
References:
1.Chang, R., 2010. chemistry. 10th ed. NewYork: McGraw-Hill.
2.Redox. (2009, July 30). Retrieved May 29, 2017, from https://weakinteractions.wordpress.com/primers/redox/
3.Reactions. (n.d.). Retrieved May 29, 2017, from http://www.ric.edu/faculty/ptiskus/reactions/
Image Courtesy:
1. “Miller Ferrocen Synthese” By Bert.Kilanowski – Own work (CC0) via Commons Wikimedia
2. “Chemical reactions” By Daniele Pugliesi – Own work (CC BY-SA 3.0) via Commons Wikimedia [Cropped]