Is the following a single displacement reaction? 2Na + 2H2O -> 2NaOH + H2 if yes, why?

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The four basic reaction types can be outlined as follows:

Synthesis Reaction:
A + B -> AB

Decomposition Reaction:
AB -> A + B

Note that the synthesis and decomposition reactions are exact opposites of one another; if you run either of them in reverse, you get the other type of reaction.

Single Displacement Reaction:
AB + C -> AC + B

Double Displacement Reaction (also known as a Metathetical Reaction):
AB + CD -> AC + BD

 

The reaction you are asking about is a single displacement reaction because it follows the single displacement pattern; in your example the H2O breaks up, and some (though not all) of the H is replaced by Na, forming the new compound NaOH. The remaining H atoms join to form H2 gas. Since this shows an atom being replaced by another atom and not forming a new compound, it qualifies as a single displacement. Chemists generally consider diatomic gases such as H2, O2, Cl2, and so on as similar to single atoms in this case.

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There exist six types of chemical reactions, one of which is a Single Displacement reaction. That occurs when one reactant element trades places with another of the reactant elements in a compound.

The general formula is:

A + BC --> AC + B

In your example, two sodium atoms react with water to make two sodium hydroxide molecules and hydrogen:

2Na + 2H2O --> 2NaOH +H2

which follows the form exactly, so the reaction is a single displacement.  See the link for a complete description of the other 5 types of reactions:


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