Chemical Reactions
The following diagrams graphically illustrate two simple chemical reactions. Notice the difference between the subscripts (ex. H2) and the coefficients in these reactions. When balancing reactions, the coefficients can (and usually need to be) changed, but the subscripts present in the molecular formula can not be altered.
H2 + Cl2 ¾® 2 HCl
2 H2 + O2 ¾® 2 H2O
Definitions
- Reactant
- The starting material in a chemical reaction. Shown on the left side of a chemical equation.
- Product
- The resulting material formed from a chemical reaction. Shown on the right side of a chemical equation.
- Symbol
- Chemical compounds are shown symbolically. Each element has a unique one or two letter abbreviation. In compounds, subscripts are used to indicate the number of each type of element in the pure substance.
- Arrow
- In a chemical reaction, an arrow is used to show that a chemical reaction occurs. Catalysts, reaction conditions, etc. are sometimes written over and/or under this arrow.
- Coefficients
- Chemical equations are balanced by changing the coefficients in front of each compound. It is important to remember that only coefficients may be changed: The chemical formula (indicated by subscripts) can NOT be altered.
Law of Conservation of Mass
Matter cannot be created or destroyed a chemical reaction. The mass of starting material must always equal the mass of product. One obvious implication of this law is that it is impossible to 'get rid' of anything. For example, trash can be buried, burned (turned into gases), or recycled. However, it doesn't simple 'go away'. The actual fate is typically determined by a variety of factors, including cost (which is cheapest?), environmental concerns (which is the 'safest' route?), or convenience (ex. we already have a landfill, so lets use it).