## 1: Chemical Reactions and Balancing Reaction Equations

### Chemical Reactions

Chemical reactions can occur if sub-microscopic particles, such as atoms, molecules and ions approach each other closely or collide. Bonds between atoms in the starting molecules are broken and new bonds and compounds appear.

The transformation is represented by a chemical equation like:

A  +  B  →  C

, with A and B as reactants and C as product.

### Balancing Reaction Equations

In this and the following subunit you will learn about the concept of stoichiometry. Stoichiometry is the calculation of the quantities of elements or compounds involved in chemical reactions.

As a direct consequence of the law of conservation of matter, the number of atoms of all elements should be equal on the left and right hand side of a reaction equation. Thus a chemical equation should be balanced with the help of stoichiometric coefficients.

(N.B. As a rule the coefficients in chemical equations should be whole numbers).

### Example: Balancing Reaction Equations

An example is the reaction between oxygen (O2) and the organic compound glucose, C6H12O6, which forms carbon dioxide (CO2) and water (H2O). This reaction takes place when glucose is degraded by microorganisms in the presence of air or when glucose is combusted.

The balancing of the equation for this chemical reaction proceeds as follows:

 Core equation ...C6H12O6 + ...O2  →  ...CO2 + ...H2O This equation is unbalanced. For example, there is 6 C atoms on the left side of the equation but only 1 C atom on the right. Balancing C ...C6H12O6 + ...O2  →  6 CO2 + ...H2O Now, the C atoms are balanced. (There is 6 C atoms on the left and 6 C atoms on the right.) However, the equation is still unbalanced. Looking at the H atoms, we see that there are 12 H atoms on the left but only 2 on the right. Balancing H ...C6H12O6 + ...O2  →  6 CO2 + 6 H2O Now, the H atoms are balanced as well. However, the equation is still not balanced. Looking at the O atoms, we notice that there are 8 O atoms on the left and 18 O atoms on the right. Balancing O C6H12O6 +  6 O2  →  6 CO2 + 6 H2O Finally, the equation is balanced.

One molecule of C6H12O6 reacts with 6 oxygen molecules, to yield 6 carbon dioxide and 6 water molecules.