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

 

(11) Balanced Equation for Decomposition into the Elements

Obj. 11.  From the name of a compound, write the balanced equation for its decomposition into the elements.

This objective is very much the same as the previous one. What's different is that we start with the compound and end up with the elements.

 

Exercises

Write the balanced equation for decomposing each of the following compounds into the appropriate elements.

a. sodium chloride

b. sulfur hexachloride

c. dinitrogen trioxide

d. aluminum sulfide

e. magnesium nitride

f. potassium iodide

g. chromium(III) oxide

 

Worked-Out Examples (a,b)

(a) Sodium chloride is an ionic compound because it contains sodium, a metal and chlorine, a nonmetal. By this point you should be able to just write down a formula for sodium chloride, but let me go through the process again.

Decomposition of sodium chloride
Sodium, being a metal, will lose electrons; it has one valence electron so it loses one electron and forms a +1 charged ion. Chlorine is a nonmetal, and will gain electrons. It has room to gain one electron, so when it does that, it forms a -1 charged ion called the chloride ion. So with a +1 charge and a - 1 charge, the formula for sodium chloride is NaCl.
NaCl rtarrow.gif (850 bytes)
The elements that it will decompose into are sodium and chlorine. For sodium we just use its symbol Na. Because chlorine is one of the diatomic elements, we write down the formula for its molecule, Cl2. So now we have the skeleton equation, NaCl rtarrow.gif (850 bytes)Na + Cl2.
NaCl rtarrow.gif (850 bytes) Na + Cl2
What's not balanced about this? The chlorine. We have Cl2 on the right so we need to have a 2 in front of NaCl on the left. That also gives us two sodiums so we need to put a 2 in front of the Na on the right. And that gives us the balanced equation 2 NaCl rtarrow.gif (850 bytes)2 Na + Cl2.
2 NaCl rtarrow.gif (850 bytes) 2 Na + Cl2

 

(b) Sulfur hexachloride. This is a prefix name, so we don't have to sort out the charges or oxidation states.

Decomposition of sulfur hexachloride
Sulfur hexachloride has the formula SCl6. It decomposes into the elements sulfur and chlorine. Sulfur is represented by the symbol S; chlorine is one of the diatomic elements so it is represented by its formula Cl2.
SCl6 rtarrow.gif (850 bytes) S + Cl2
To balance this equation, we need to put a 3 in front of Cl2. That gives us six chlorines on the right as well as the left. So the balanced equation for the decomposition of sulfur hexachloride is SCl6 rtarrow.gif (850 bytes)S + 3 Cl2.
SCl6 rtarrow.gif (850 bytes) S + 3 Cl2

 

Answers to Exercises

Write the balanced equation for decomposing each of the following compounds into the appropriate elements.

a. sodium chloride     2 NaCl rtarrow.gif (850 bytes)2 Na + Cl2

b. sulfur hexachloride     SCl6 rtarrow.gif (850 bytes)S + 3 Cl2

c. dinitrogen trioxide     2 N2O3 rtarrow.gif (850 bytes)2 N2 + 3 O2

d. aluminum sulfide     Al2S3 rtarrow.gif (850 bytes)2 Al + 3 S

e. magnesium nitride     Mg3N2 rtarrow.gif (850 bytes)3 Mg + N2

f. potassium iodide     2 KI rtarrow.gif (850 bytes)2 K + I2

g. chromium(III) oxide     2 Cr2O3 rtarrow.gif (850 bytes)4 Cr + 3 O2

 

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