Balancing Equations

When teaching to balance chemical equations, it is essential to make students understand first what's going on. That the atoms from the reactants side are rearranged into different configurations and make different compounds on the products side, but that all the "atom balls" from the reactants are used on the "products side".  This can be done visually, with drawings.  But even better, if you can do it with actual models, actual "balls" representing the atoms.  This will really drive the idea that atoms are three dimensional.  Also this will show the students that you can really only use the "atoms" of the reactants to make the products.

You can buy really expensive model kits from places such as Borealis or other science supply stores/suppliers.  However, you can use Fruit Loops, Smarties, M&M's, Lego pieces, marbles, balls, etc. just as easily.  I wouldn't suggest playdough as kids can actually take one playdough atom and make it into several - not allowed in Balancing equations.

After spending a class on exploration of such a visual and individual understanding of atoms and how they rearrange in a chemical reaction, the practice of balancing reactions can begin. But before you give out the many worksheets, numerous homework assignments to practice this important concept, try to give some hints and ways of solving balancing equations.  Make sure you start with easy problems first, continue with harder ones, and then show some really difficult ones for the bright students in the class.  Here are some hints I try to incorporate into all of my balancing equations lessons:

1.  The only numbers that you can touch / change are the coefficients.  (You cannot manipulate the subscripts at all!) Coefficients are the numbers written before the compound.  Initially I usually draw blank spots to indicate where I can put my numbers.

__Mg +  __HCl --> __MgCl2 + __ H2

2.  The coefficients multiply the subscript in order to get the right amount of atoms.  If there is no coefficient, this means 1.

 

3.  The subscripts only apply to the atom preceding it.  Therefore in P4O10 there aer 4 P's and 10 O's.  Unless there are brackets - then the subscript applies to all the atoms in the bracketted section.  For example:  Ca(NO3)-- Here there are 2N's and 6O's (remember that the two subscripts multiply each other to get the right amount of atoms.

 

4.  Under the reactants and products seperately, write all the atoms present, stating beside it how many of these atoms are there.  Remember, some atoms can occur in multiple compounds.

example:

__Mg +  __HCl --> __MgCl2 + __ H2

Mg - 1                       Mg - 1

H - 1                          H - 2

Cl - 1                          Cl  -2

 

5.  Then try to make all the atoms have the same amounts by manipulating only the coefficients. 

In this example, the reactants has 1 Cl and the products have 2 Cl's.  To balance that, you need to change the coefficient in front of HCl.  But make sure to change the value of H here also!

 

__Mg +  _2_HCl --> __MgCl2 + __ H2

Mg - 1                       Mg - 1

H - 2                          H - 2

Cl - 2                          Cl  -2

 

This magically solved the problem also for the H's.  Yey, we're done.

6.  If you can't multiply the compound by a whole number coefficient to make it work, try multiplying both the reactant compound and product compound to get Lowest Common Multiple (like lowest common denominator in math).

example:

__Na3N +__S --> __Na2S + __N2

Na -3                        Na - 2

N - 1                         N - 2

S -  1                       S -  1

 

The reactants have 3 Na's and the products have 2 Na's.  The lowest common multiple is 6.  To get 6 Na's, we need to place a 2 in front of Na3N and a 3 in front of Na2S resulting in:

 

_2_Na3N +__S --> _3_Na2S + __N2

Na -6                        Na - 6

N - 2                         N - 2

S -  1                       S -  3

The last thing that needs to be balanced is the S:

2_Na3N +_3_S --> _3_Na2S + __N2

Na -6                        Na - 6

N - 2                         N - 2

S -  3                       S -  3

And there it is!

Now practicing is what will really make the students realize all the tricks, and think of their own tricks!

 

BALANCING EQUATION WORKSHEETS:

Here are some links to free balancing equation worksheets:

 

Worksheets for Middle School Teachers

Balancing Equations Worksheet-1

Balancing Equations Worksheet - 2

Chem 130: Balancing-Equations

Worksheet to teach Balancing Equations

CHEMISTRY 101 
BALANCING EQUATIONS WORKSHEET

Chemical Reactions

Balancing Chemical Reactions Worksheet A

 

 

 

 

 

 

 

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Submitted by bogusia on Fri, 10/10/2008 - 20:19

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