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Synthesis reactions are also known as combination reactions. The word "combination" is a hint about how the reactions proceed. Materials combine to form a new product. The most basic general form is expressed like this:
A + B → AB
Just like Angelina (A) and Brad (B) have combined to form Brangelina, in synthesis reactions two reactants become one to form a new product. How romantic. We've already seen one real life example of a combination example in this guide: sodium chloride synthesis.
2 Na (s) + Cl2 (g) → 2 NaCl (s)
The simpler substances of sodium and chlorine have combined to from the more complex sodium chloride molecule.
These reactions are not just limited to two starting materials. Two, three, four, or even more elements may react to form a single product in these types of reactions. To be honest, though, synthesis reactions involving four or more reactants are pretty darn rare.
Common examples of synthesis reactions are when a metal or non-metal reacts with oxygen to form an oxide. Synthesis reactions can also occur when a metal or non-metal combine to form a binary compound. Binary what? Binary compound is just a fancy way of saying a compound contains exactly two elements.
3 Fe (s) + O2 (g) → Fe2O3 (s)
2 K (s) + Cl2 → 2 KCl (s)
Of course, synthesis reactions go beyond the scope of just elements. As with all things in science, things can get complicated fast.
Compounds can combine with other compounds to form a new single compound. A simple example would be the formation of hydrogen sulfate, which is a major contributor to nasty and toxic acid rain.
SO3 (g) + H2O (l) → H2SO4 (aq)
The main key to recognize a synthesis reaction is to observe two (or more) reactants combining to form one product.
Need some extra help with synthesis reactions? Check out this video.
Did you hear that oxygen and magnesium were dating? Like OMg.