Lesson 10
Home Up (10) Equation for Synthesis (11) Equation for Decomposition

 

(10) Balanced Equation for Synthesis from the Elements

Obj. 10.  From the name of a compound, write the balanced equation for its synthesis from the elements.

This is a somewhat involved process, but very manageable if you take it step by step. First, note that synthesis means the formation of that compound. Since you are to start from the elements, you will need to figure out the proper formulas for the elements as well as the formula for the compound. Then follow through by balancing the equation.

 

Exercises

Write the balanced equation for synthesizing each of the following compounds, starting with the appropriate elements.

a. potassium bromide

b. aluminum oxide

c. ferric chloride

d. ferrous chloride

e. dinitrogen pentoxide

f. chromium(VI) oxide

g. hydrogen bromide

 

Worked-Out Examples (a,b)

(a) Potassium bromide. In order to write a balanced equation that represents the synthesis of potassium bromide, we need the formulas of the elements potassium and bromine on the left and the formula of potassium bromide on the right side of the equation.

Synthesis of potassium bromide

We need to know the formula for potassium bromide. Well, potassium bromide consists of potassium and bromine; potassium is a metal, bromine is a nonmetal. We can figure that potassium will lose electrons. By looking at the periodic table we can see that potassium has one valence electron, so it will lose one electron and form a +1 charge. Bromine is a nonmetal which will gain electrons. By looking at its position on the periodic table, we can see that it will gain one electron and form a -1 charge. So with a +1 charge and a -1 charge, the correct formula for potassium bromide is KBr. So we write that as the product on the right side of the arrow.
rtarrow.gif (850 bytes) KBr
Now what is potassium bromide made from? It's made from potassium and bromine. On the left side of the arrow we have to have the proper formulas for potassium and bromine. Potassium is a metal with metallic bonding and therefore it is a network material. So we will just use the symbol to represent potassium and write down K.
K + rtarrow.gif (850 bytes) KBr
Bromine, on the other hand, is one of those diatomic elements and so we need to write down Br2. There are a couple ways of coming up with this. One is to just memorize that bromine is one of the diatomic elements. The other is to go through the thought process that since bromine is a nonmetal, it might form a network or molecules. It does form molecules (because it is not one of the few covalent networks) and the molecules that are formed consist of two bromine atoms hooked together. So Br2 is the correct formula.
K + Br2 rtarrow.gif (850 bytes) KBr
So we know have the skeleton equation of  K + Br2 rtarrow.gif (850 bytes)KBr. In order to balance that, we have to make note of what is not balanced and as written. We have one potassium on each side of the arrow, and we have two bromines on the left and one on the right. If we start with two bromines we need to finish with two bromines, so we put a 2 in front of the KBr. That gives us two potassiums on the right, so we also need to put a 2 in front of the K on the left side of the equation. So that balances the equation and gives us 2 K + Br2 rtarrow.gif (850 bytes)2 KBr.
2 K + Br2 rtarrow.gif (850 bytes) 2 KBr

 

 

(b) Aluminum oxide. The formation of aluminum oxide will start from aluminum and oxygen so we have aluminum plus oxygen gives aluminum oxide.

Synthesis of aluminum oxide

First, let's figure out the formula of aluminum oxide. Aluminum oxide is an ionic material because aluminum is a metal and oxygen is a nonmetal. Aluminum will lose electrons, and since it has three valence electrons it will lose three electrons and form an Al+3 ion. Oxygen is a nonmetal and will gain electrons. From its position on the periodic table, you can tell that it has room to gain two electrons. So when it gains electrons, it will form an oxide ion with a -2 charge. Because we have a +3 charge and a -2 charge, the correct formula for aluminum oxide is Al2O3.
rtarrow.gif (850 bytes) Al2O3
On the left side of the equation we have aluminum, which is represented simply by Al. We also have O2 because oxygen  is one of those diatomic elements that you need to remember. We now have Al + O2 rtarrow.gif (850 bytes)Al2O3 as the unbalanced skeleton equation.
Al + O2 rtarrow.gif (850 bytes) Al2O3
Let's balance the oxygen first because it is the most complicated. If we start with oxygen in pairs and we end up with oxygen in triplets, the least number of oxygen atoms that we can do this with will be six oxygen atoms. So we need a 3 in front of O2 and a 2 in front of Al2O3. That gives six oxygens on both sides. The 2 in front of Al2O3 also gives us four aluminum atoms on the right, so we need a 4 in front of the Al on the left. That gives us 4 Al + 3 O2 rtarrow.gif (850 bytes)2 Al2O3.
4 Al + 3 O2 rtarrow.gif (850 bytes) 2 Al2O3

 

Answers to Exercises

Write the balanced equation for synthesizing each of the following compounds, starting with the appropriate elements.

a. potassium bromide     2 K + Br2 rtarrow.gif (850 bytes)2 KBr

b. aluminum oxide     4 Al + 3 O2 rtarrow.gif (850 bytes)2 Al2O3

c. ferric chloride     2 Fe + 3 Cl2 rtarrow.gif (850 bytes)2 FeCl3

d. ferrous chloride     Fe + Cl2 rtarrow.gif (850 bytes)FeCl2

e. dinitrogen pentoxide     2 N2 + 5 O2 rtarrow.gif (850 bytes)2 N2O5

f. chromium(VI) oxide     2 Cr + 3 O2 rtarrow.gif (850 bytes)2 CrO3

g. hydrogen bromide     H2 + Br2 rtarrow.gif (850 bytes)2 HBr

 

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