Home Up First Example Second Example Third Example Fourth Example Fifth Example Sixth Example Practice

 

Home

First Example

In the first example potassium plus chlorine becomes potassium chloride. First of all let's take a quick inventory of what elements are involved and what is already shown. There is one potassium on the left and one potassium on the right. No problem there now. It may change, but for the moment we don't have to do anything with the potassium. There are two chlorine atoms shown on the left and one chlorine shown on the right. We will have to balance that. If we start with two chlorine atoms, we are going to finish with two chlorine atoms.
  K +   Cl2 rtarrow.gif (850 bytes)   KCl
1 K, 2 Cl 1 K, 1 Cl
To accomplish that we cannot just change the formula for potassium chloride! We can't put a two in after the Cl and make it KCl2. That would balance the equation, but KCl2 is not the formula of the compound that is formed when these two elements combine. KCl is the formula, and you cannot change the formula. Remember, the formula shows the ratio in which the atoms actually combine. Nature determines the ratio. We cannot change it just to make it convenient to balance the equation. KCl is the formula, and we must leave it as KCl! The atoms combine in a one to one ratio in this compound. What we have to do is put a two in front of the KCl.
  K +   Cl2 rtarrow.gif (850 bytes) 2 KCl
1 K, 2 Cl 2 K, 2 Cl
That gives us two chlorines on each side, but it also gives us two potassiums on the right side. If we end up with two potassiums, we need to start with two potassiums. So, we go back and put a two in front of the K on the left side. That balances the equation (2 K + Cl2 rtarrow.gif (850 bytes) 2 KCl).
 2 K +   Cl2 rtarrow.gif (850 bytes)  2 KCl
2 K, 2 Cl 2 K, 2 Cl

Here is an important thing to note. Notice there is not room to put a two between the K and the Cl in the formula of the potassium chloride, and that is because KCl is the formula of a compound. You don't insert anything into the formula. KCl is a formula unit. You can, however, put a number in front of the formula to show more than one of those units.

 

Top of Page

Back to Course Homepage

 

E-mail instructor: Eden Francis

Clackamas Community College
©1998, 2002 Clackamas Community College, Hal Bender