H2 (g) + I2 (g)A lot of students are confused about the meaning of the coefficients in a chemical equation. For example, consider the equation for the dissociation of hydrogen iodide gas:
2 HI (g) H2 (g) + I2 (g)
The coefficient in front of HI is obviously 2, and in front of H2 and I2 it is 1. However, this doesn't mean that if you put two moles of HI into a container and let it react that you will get 1 mole of H2 and 1 mole of I2. Nor does it mean that you have to put two moles of HI into the container in order to get the reaction to occur. You can actually put anything into the container that you want (provided you have it, and that it will fit).
| The coefficients in a chemical equation show you the ratio in which the molecules react. They indicate nothing about the amount of either the starting reactants or resulting products there will be in the reaction container. |
| Consider the following analogy to a chemical reaction -- the equation for a dance: 1
male + 1 female |
 This spreadsheet simulation shows how the number of people at a dance depends on the equation for the reaction. |
Obviously this equation tells us nothing about how many people are at the dance, nor how many are dancing. Nor does it mean that you can't have a dance unless there are equal numbers of males and females present. It does mean that one male and one female can form a dancing couple (and since the reaction is reversible, one couple can separate to become one male and one female). In other words the coefficients are the ratio of the way in which the reactants (males and females) react to form products (dancing couples).