Balancing Chemical Equations ( The Complete Guide )

When an equation is written , it must obey the law of conservation of mass, matter cannot be created or destroyed. Specifically , the number of atoms of each different element in an equation must be the same on the left and right sides of the arrow. After a chemical change , the same type and the number of atoms are present ; They are merely rearranged.

To obey the law conservation of mass, an equation must be balanced . Balancing an equation involves placing coefficients in front of all reactants and products so that the same number of atoms of each element appears on either side of the equation.

Simple equation are balanced by using the inspection method, which involves equalizing the number of element of each type by placing coefficients in front of all element and compounds in chemical equation.

Lets ilustrate the balancing of equations by the inspection method. We shall use the combination reaction in which hydrogen gas , H2 (g), combines with oxygen gas, O2 ( g) , to yield water vapor , H2O ( g ). first write the unbalanced equation , including all reactant and products.

H2 ( g ) + O2 ( g ) → H2O ( g )          ( unbalanced )

After writing the unbalanced equation , we readily see that the number of oxygen atoms is not the same on both sides of the equation. Two oxygen atoms appear on the left side, and only one oxygen atom is found on the right. To balance the oxygen atoms, we place a 2 in front of the formula of water. Two water molecules contain two oxygen atoms.

We cannot place a 2 as a subscript next to the oxygen in water because this will change water's composition . Never change subscripts when you are balancing a chemical equation . Incorrectly placing the 2 as a subscript next to oxygen gives a new product of the reaction , H2O2 -hydrogen peroxide . Hydrogen Peroxide's properties are vastly different from those of water !

After balancing the oxygen atom , we are left with the following partially balanced equation :

H2 ( g ) + O2  ( g ) →  2H2O ( g )

In balancing the oxygen atoms, we now know the number of hydrogen atoms that are needed. Two water molecules contain four hydrogen atoms ( 2 x H2O = 4 H and 2 O ). Therefore, we place a 2 in front of H2 ( g ) on the left side of the equation , giving four hydrogen atoms.

2H2 ( g ) + O2 ( g ) → 2H2O ( g )

The last step , an important one, is to check to see that you have correctly balanced all atoms. if the same number of atoms of each type appear on both sides, the equation is properly balanced. If you are off by even one atom, then the equation is not balanced . In our example equation , we find that 4 H atoms and 2 O atoms are on either side of the arrow , indicating a correctly balanced equation. It is convenient to check off or underline each atom as you are verifying that the equation is balanced :

2H2 ( g ) + O2 ( g ) → 2H2O ( g ) ( Balanced )



When the equation is balanced, the coefficients indicate the ratio in which the reactants and products combine and form. Our equation  now reads "two molecules of hydrogen gas combine with one molecule of oxygen gas to yield two molecules of water vapor.

Lets tackle a slightly more complex equation to furthe illustrate balancing techniques. balance the equation for the reaction in which methane gas, CH4 ( g ) , combine with oxygen gas , O2 ( g ) to yield Carbon Dioxide gas CO2 ( g ), and water vapor , H2O ( g ) . As before , write the unbalanced equation :

CH4 ( g )  + O2 ( g ) → CO2 ( g ) + H2O ( g )        ( Unbalanced )

When you are balancing equations where one element appears in many of the reactants and products , it is best to balance that atom last. Here, and in many equations, this atom is oxygen. Oxygen frequently is the last atom balanced ; Hydrogen is normaly balanced next to last. thus, we should start by balancing carbon atoms. There is 1 C on each side of the arrow ; therefore , carbons are balanced as written :

CH4 ( g ) + O2 ( g ) → CO2 ( g ) + H2O ( g )

Moving to H, we see that 4 H atoms are located on the left side, and 2 H atoms are on the right. A 2 is placed in front of H2O to produce four H's On the right :

CH4 ( g ) + O2 ( g ) → CO2 ( g ) + 2H2O ( g )

All that remains is to balance the oxygen atoms. Since we have balanced all of the product , we now know the total number of oxygen atoms. One CO2 molecule contains 2 O atoms, and 2 H2O molecules contain 2 O atoms, giving a total of 4 O atoms; therefore , there must be 4 atoms O in the reactants. Looking at the left side of the equation , we find only two oxygen atoms.Ask yourself, what number times 2 gives 4 ? The answer is 2 , so place a 2 in front of the O2 to complete the balancing of the equation.

CH4 ( g )  +  2O2 ( g )  →  CO2 ( g ) + 2H2O ( g )

Finally, checkto see that all atoms are balanced : 1 C atom , 4 H atoms, and 4 O atoms are located on each side of the equation. Our equation states one molecule of methane gas combines with two molecules of oxygen gas to produce one molecule of carbon dioxide gas and two molecule of water vapor.

A summary of the rules that can be followed to successfully balance simple chemical equations follows :

Rule 1. write an unbalanced equation including the correct formulas of all reactants and products.

This first step is most significant , since one incorrect formula alters the way the equation is balanced or in some instances makes it impossible to balance. Always double-check to see that you have written the unbalanced equation properly.

Rule 2. Determine a logical sequence for balancing each atom in the equation , leaving those atoms that appear in more than one compound on each side for last.

It is often best to start with metals , especially those that appear only in one molecule on each side of the equation. next proceed to nonmetals that also occur in one molecule on each side; then balance the remaining nonmetals that are found in a number of molecules. Step 2 is a planning step ; it simplifies balancing equations and gives an orderly approach to the problem.

Rule 3. Balance each atom , one at time, by placing appropriate coefficients in front of the atoms and molecules in the equation . If possible, proceed in the predetermined order arrived at in step 2.

After balancing an atom, underline it on both side of the equation to indicate to yourself that it has been balanced. When all atoms are underlined . The equation should be balanced. If after balancing a couple of atoms you see that your predetermined order for balancing the atoms is not the most efficient, drop it, and proceed in the most efficient way to successfully balance the equation.

Rule 4. Check to see that all atoms are balanced in the equation. if an equal number of atoms are on each side of the equation, for each different atom, then the equation is balanced ; if not repeat step 3, these steps for balancing equation are summarized in figure 1.

Figure 1 : Start→Write Unbalanced Equation→Determine Logical Sequence→Balance Atoms One At A Time→Check To See That Each Atom Is Balanced

Some other helpful hints to consider when balancing equations are :

1. Although whole numbers are generally preferred when balancing equations, it is not incorrect to use fractional coefficients occasionally . Certain equation are more easily balanced with one fraction. Consider the following correctly balanced equation :

Na ( s ) + H2O ( l ) → NaOH ( aq ) + 1/2 H2 ( g )

To eliminate the fraction , the coefficient of each substance in the equation is multiplied by 2.

2Na (s) + 2H2O (l) → 2NaOH (aq) + H2 (g)

2. If you find that all of the coefficients are divisible by a small whole number , divide by that number to reduce to the lowest set of coefficients. Balanced equations are correct when they are expressed in the lowest possible multiple of coefficients. For example :

4C (s) + 2O2 (g) → 4CO (g)

is incorrect. Dividing by 2 gives the correctly balanced equation :

2C (s) + O2 (g) → 2CO (g)

3. If polyatomic ions are found as reactants and are unchanged after the reaction, they can be balanced as a unit. For instance, if two nitrate ions , NO3-, are found within a reactant , you can place a 2 in front of the product containing NO3- to balance the nitrates.

4. If an odd number of atoms appear on one side and an even number of atoms are found on the other side , multiply the odd number by 2 in order to give an even number.

Okay thats all about how to balance chemical equation, The next article we will discuss about Balancing Chemical Equations Example.

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