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Chemical Formulae & Valency | Chemical Equations | Introduction to Acids, Bases & Salts |

# The Language of Chemistry - 03

## Chemical Equations

A **chemical equatio**n 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 ($\to $), 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: $C+{O}_{2}\to {\mathrm{CO}}_{2}$ .

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

- Using (
) to denote a solid state, (`s`) for liquid, (`l`) for gas, and (`g`) for aqueous solutions (solutions in`aq``water`). - Using an up-arrow ($\uparrow $) to indicate a gaseous product of the reaction which is liberated.
- Using a down-arrow ($\downarrow $) 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 $\stackrel{\u27f6}{\Delta}$, 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:

$$\mathrm{sodium}+\mathrm{chlorine}\to \mathrm{sodium}\text{\hspace{0.17em}}\mathrm{chloride}$$

##### 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:

$${\mathrm{Na}}_{(s)}+{\mathrm{Cl}}_{2}{}_{(g)}\to {\mathrm{NaCl}}_{(s)}$$

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:

*Write the skeletal equation*using the correct chemical formula for each reactant and product.The formula for`water`will be`H`, since_{2}O`H`has a valency of 1, and`O`has a valency of 2. So, the skeletal equation for the reaction is:$${\u200aH}_{2}+{\u200aO}_{2}\to {\u200aH}_{2}\u200aO$$The equation is unbalanced since the`oxygen`atoms on both sides of the equation do not balance out.*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`H`, so we start with it. Although the number of_{2}O`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`H`molecule on the right. This gives:_{2}O$${\u200aH}_{2}+{\u200aO}_{2}\to 2{\u200aH}_{2}\u200aO$$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`H`molecule on the left. Thus we have_{2}$$2{\u200aH}_{2}+{\u200aO}_{2}\to 2{\u200aH}_{2}\u200aO$$On inspection, it can be seen that both`H`and`O`atoms balance out on the two sides.*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`**, 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**

`C`_{3}H_{8}`propane`with

`oxygen`to yield

`carbon dioxide`,

`CO`, and

_{2}`water`. (McMurray & Fay 2003, Chemistry, p. 77)

The skeletal equation is:

$${\u200aC}_{3}{\u200aH}_{8}+{\u200aO}_{2}\to {\mathrm{CO}}_{2}+{\u200aH}_{2}O$$

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 `CO _{2}`:

`C`balanced)

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

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

$${\u200aC}_{3}{\u200aH}_{8}+5{\u200aO}_{2}\to 3{\mathrm{CO}}_{2}+4{\u200aH}_{2}O$$

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# List of References

*Chemistry*, 4th edn, USA: Prentice Hall, 2003.

# Bibliography

*Chemistry*, 4th edn, USA: Prentice Hall, 2003.

*Chemistry 11*, Canada:McGraw-Hill Reyerson, 2005.

Chemical Formulae & Valency | Chemical Equations | Introduction to Acids, Bases & Salts | |||

The Language of Chemistry | $$ |