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General Chemistry/Reactions in Chemistry/Balancing Chemical Reactions

Reactions in Chemistry

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Balancing Chemical Reactions

To balance a chemical equation, both atoms and charge must be balanced. Balancing atoms can be done by following certain procedural steps.
Balancing a chemical equation involves balancing both the atoms and the charge. In molecular equations charge is usually ignored. Balancing a molecular equation involves balancing the atoms. To balance the atoms, each element should have equal numbers of atoms in the reactants side and the products side. A coefficient is a multiplier that can be placed in front of the chemical symbol of a substance in a chemical reaction.

Balanced Chemical Equation

The equation describes the reaction of nitrogen gas (N2) and hydrogen gas (H2) to produce ammonia (NH3). A coefficient of 3 must be placed before H2 and a coefficient of 2 must be placed before NH3 to balance the equation. The number of each type of atom is the same on both sides of the equation.
Step-By-Step Example
Balancing the Chemical Equation for the Reaction of Hydrogen and Water
Balance the chemical equation for the reaction of hydrogen gas (H2) and oxygen gas (O2).
Step 1
Write the unbalanced equation.
Hydrogen+Oxygen→WaterH2(g)+O2(g)→H2O(l)(unbalanced)\begin{gathered}\text{Hydrogen}&+&\text{Oxygen}&\rightarrow&\text{Water}\\{\rm {H_2}}(g)&+&{\rm {O_2}}(g)&\rightarrow&{\rm H_2\rm O}(l)&(\text{unbalanced})\end{gathered}
Step 2

Count the number of atoms on each side of the arrow.

Hydrogen atoms (H) Oxygen atoms (O)
Reactants side 2 2
Products side 2 1

The number of oxygen atoms is not the same on the reactants and products sides. This equation is considered unbalanced.

Step 3

For one of the unbalanced elements, add a coefficient to balance it on both sides.

There are fewer oxygen atoms on the right side of the arrow (products side). If a coefficient of 2 is placed in front of the H2O term, it means there are now two H2O molecules on the right side of the arrow.
H2(g)+O2(g)→2H2O(l)(unbalanced){\rm H}_2(g)+{\rm O}{\rm_2}(g)\rightarrow{\rm2H\rm_2O}(l)\hspace{10pt}\text{(unbalanced)}
Solution

Make sure that adding a coefficient in front of H2O did not result in more unbalanced elements. If there are unbalanced elements, add another coefficient in front of the unbalanced element.

In this case, there are now four hydrogen atoms on the right side of the arrow but only two on the left side of the arrow. To balance the number of hydrogen atoms, add a coefficient of two on the H2 term on the left side of the arrow:
2H2(g)+O2(g)→2H2O(l)2{\rm H}_2(g)+{\rm O\rm_2}(g)\rightarrow2{\rm H}{\rm_2\rm O}(l)
There are four hydrogen atoms and two oxygen atoms on each side of the arrow. Since the number of each atom on both sides of the equation is equal, the reaction is considered balanced.
Step-By-Step Example
Balancing the Chemical Equation for the Reaction of Aluminum and Hydrochloric Acid
Balance the chemical equation for the reaction of metallic aluminum (Al) with concentrated hydrochloric acid (HCl) to produce aluminum chloride (AlCl3) and hydrogen gas (H2).
Step 1
Write the unbalanced equation.
Al(s)+HCl(aq)→AlCl3(aq)+H2(g)(unbalanced){\rm{Al}}(s)+{\rm{HCl}}(aq)\rightarrow{\rm{AlCl}}_3(aq)+{\rm H}_2(g)\hspace {25pt}(\text{unbalanced})
Step 2

Count the number of atoms on each side of the arrow.

Aluminum atoms (Al) Chlorine atoms (Cl) Hydrogen atoms (H)
Reactants side 1 1 1
Products side 1 3 2

The numbers of chlorine and hydrogen atoms are not the same on the reactants and products sides. This equation is considered unbalanced.

Step 3

Choose one unbalanced element. Add a coefficient to balance it.

Generally, leaving hydrogen to balance last is a good idea. Many reactions involve oxygen gas, hydrogen gas, or water. Once other elements are balanced, balancing these elements is usually easy. Start with chlorine, and add a coefficient of 3 to the reactants side of the arrow.
Al(s)+3HCl(aq)→AlCl3(aq)+H2(g)(unbalanced){\rm{Al}}({s})+{3\rm{HCl}}({aq})\rightarrow{\rm{AlCl}}_3({aq})+{\rm H}_2({g})\hspace {25pt}{\text{(unbalanced)}}
Step 4
Adding a coefficient in front of HCl not only changes the number of chlorine atoms but also changes the number of hydrogen atoms. There are now three hydrogen atoms on the left and two hydrogen atoms on the products side of the arrow. Unfortunately, three is not divisible by two. Since both numbers are factors of six, multiply the HCl term by two and the H2 term by three to make them equal:
Al(s)+6HCl(aq)→AlCl3(aq)+3H2(g)(unbalanced){\rm{Al}}({s})+6{\rm{HCl}}({aq})\rightarrow{\rm{AlCl}}_3({aq})+3{\rm H}_2({g})\hspace {25pt}\text{(unbalanced)}
Step 5
Balancing the number of hydrogen atoms affected the balance of the chlorine atoms again. Adding a coefficient of two on the right side will balance them:
Al(s)+6HCl(aq)→2AlCl3(aq)+3H2(g)(unbalanced){\rm{Al}}({s})+6{\rm{HCl}}({aq})\rightarrow2{\rm{AlCl}}_3({aq})+3{\rm H}_2({g})\hspace {25pt}\text{(unbalanced)}
Solution
Now only aluminum is unbalanced. Adding a coefficient of two to the left will balance aluminum and make the entire equation balanced:
Al(s)+6HCl(aq)→2AlCl3(aq)+3H2(g){\rm{Al}}({s})+6{\rm{HCl}}({aq})\rightarrow2{\rm{AlCl}}_3({aq})+3{\rm H}_2({g})
<Chemical Reactions and Chemical Equations>Ionic Equations