Chemical Equations

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The Language of Chemistry - 03

Chemical Equations

A chemical equation is a standardised short-hand form for depicting a chemical reaction. Substances which take part in a chemical reaction are called reactants, while those which are produced as a result are called products.

In a chemical equation, the reactants are placed on the left side of a right-pointing arrow (), and the products are placed on its right side. The arrow denotes the transformation process. Thus, the chemical equation for the formation of carbon dioxide is: C + O 2 CO 2 .

A chemical equation can pack in more information related to the reaction, by incorporating the following symbols:

  • Using (s) to denote a solid state, (l) for liquid, (g) for gas, and (aq) for aqueous solutions (solutions in water).
  • Using an up-arrow () to indicate a gaseous product of the reaction which is liberated.
  • Using a down-arrow () to indicate an insoluble product of the reaction which precipitates out from the solution (i.e. settles down).
  • Using a triangle above the right-pointing arrow, like Δ , to specify the application of heat for reaction.
Word equation

A chemical equation begins its life as a word equation, in which the reactants and products are written by name. Thus, the formation of common salt, or sodium chloride, from sodium and chlorine gas, is represented as the following word equation:

sodium + chlorine sodium chloride

Skeletal equation

The next step is the penning down of the skeletal equation, which is a simplification of the word equation by expressing it using chemical formulae. Further symbols may be added to enhance the information. So, the skeletal equation for the above becomes:

Na ( s ) + Cl 2 ( g ) NaCl ( s )

So far so good. This is more informative, until you realise that something is wrong. The one atom of sodium on the left matches with one atom of sodium in NaCl on the right, but there are two atoms of chlorine on the left, and only one atom of chlorine on the right in NaCl. A chlorine atom has gone missing!

However, all atoms have to be accounted for, because that is a requirement of the law of conservation of mass, according to which matter is neither created not destroyed in a chemical reaction. This skeletal equation violates the law of conservation of mass, and is what is called an unbalanced chemical equation. (Note that a skeletal equation need not always violate the law of conservation of mass).

Balanced chemical equation

A balanced chemical equation complies with the law of conservation of mass. It is the skeletal equation suitably modified with numeric coefficients in front of the formulas such that that atoms for each element on both side of the equation balance out. Remember that adding numeric coefficients is the only way to balance chemical equations. You cannot change the chemical formulas in any way.

A balanced chemical equation completely and correctly describes a chemical reaction. It tells us the relative amount of all reactants and products in the reaction.

Steps for balancing chemical equations

Balancing a chemical equation is part systematic and part trial-and error process. You attain proficiency with some practise.

Considering the example of reaction of hydrogen with oxygen to yield water, the steps involved in balancing the equation are:

  1. Write the skeletal equation using the correct chemical formula for each reactant and product.
    The formula for water will be H2O, since H has a valency of 1, and O has a valency of 2. So, the skeletal equation for the reaction is:
    H 2 + O 2 H 2 O
    The equation is unbalanced since the oxygen atoms on both sides of the equation do not balance out.
  2. Find suitable coefficients—the numbers placed before formulae to indicate how many formula units of each substance are required to balance the equation. It is usually best to start with the most complex substance, and to deal with one element at a time. Keep up this process of finding coefficients till all kinds of atoms on both sides of the equation balance.
    In our case, the most complex substance is H2O, so we start with it. Although the number of H atoms on both sides of the equation seems to be balanced, the number of O atoms are not. To balance the two O atoms on the left side, a coefficient of 2 will have to be added to the H2O molecule on the right. This gives:
    H 2 + O 2 2 H 2 O
    The above takes care of O, but results in 2 excess atoms of H on the right side. This can easily be balanced by adding a coefficient of 2 to H2 molecule on the left. Thus we have
    2 H 2 + O 2 2 H 2 O
    On inspection, it can be seen that both H and O atoms balance out on the two sides.
  3. Reduce the coefficients to their smallest whole-number values, if necessary, by dividing them by a common divisor.
    This will not be necessary in this particular case.
Propane, C3H8, is a colorless, odorless gas often used as a heating and cooking fuel in campers and rural homes. Write a balanced equation for the combustion reaction of propane with oxygen to yield carbon dioxide, CO2, and water. (McMurray & Fay 2003, Chemistry, p. 77)

The skeletal equation is:

C 3 H 8 + O 2 CO 2 + H 2 O

On inspection, all the three atoms of C, H and O are unbalanced. Starting with propane, the most complex molecule, let us try to balance C first by adding a coefficient of 3 to CO2:

C 3 H 8 + O 2 3 CO 2 + H 2 O        (C balanced)

Next, for balancing H in propane, we add a coefficient of 4 to H2O:

C 3 H 8 + O 2 3 CO 2 + 4 H 2 O        (C and H balanced)

The remaining atom of O has 2 on the LHS and 10 on the RHS. This can be easily balanced thus:

C 3 H 8 + 5 O 2 3 CO 2 + 4 H 2 O        (All atoms balanced)

Reducing the coefficients to their smallest whole number values will not be necessary. So, the final balanced equation is:

C 3 H 8 + 5 O 2 3 CO 2 + 4 H 2 O

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List of References

McMurray, J & Fay, RC, Chemistry, 4th edn, USA: Prentice Hall, 2003.


McMurray, J & Fay, RC, Chemistry, 4th edn, USA: Prentice Hall, 2003.
Mustoe, F et al, Chemistry 11, Canada:McGraw-Hill Reyerson, 2005.