Synthesis Reactions

Synthesis Reactions tutorial

In a synthesis (or combination) reaction you start out with two separate elements on the reactants side and combine them to form a compound on the products side. The general pattern of a synthesis reaction is:
A + B --> AB
Note that in order for the reaction to occur and a compound to form, one of the elements must be able to give electrons (cation) and the other element must be able to gain electrons (anion). Look at the two examples below.
Example #1:
2Na(s) + Cl2(g)--> 2NaCl(cr)
Sodium reacts with chlorine to form sodium chloride. Note that chlorine is written as Cl2 because it is a diatomic molecule when it is not bonded to another element.
Example #2:
Mg(s) + Al(s) --> N.R.
The above reaction can not occur because magnesium and aluminum both form cations. Two cations can not combine to form an ionic compound. Thus, the reaction is labeled N.R. for no reaction.

Now let’s go step by step.
Predict the products when solid iron reacts with oxygen gas.

Description of Action	Action
1. Write the formulas for the given reactants. Don't forget to write a subscript of two with elements that are diatomic.	Fe(s) + O2(g) --> Remember oxygen is diatomic.
2. On the products side of the equation, write the cation with its charge and the anion with its charge. If a multivalent element is being used, you can use whatever charge you want.	Fe(s) + O2(g) --> Fe3+O2- Iron is multivant and can have a 2+ or a 3+ charge. I used the 3+ charge.
3. Cross the charges between the cation and the anion, remove + signs, - signs and ones.	Fe(s) + O2(g) --> Fe2O3(s)
4. Balance the equation.	4Fe(s) + 3O2(g) --> 2Fe2O3(s)