2.2 Conservation of Mass in Chemical Equations

Chemical Reactions

Chemical reactions result in chemical changes.

• Chemical changes occur when new substances are created.
• The original substance(s), called reactants, change into new substance(s) called products.

Chemical change means new compounds are created.

• BUT no new matter is created or destroyed; atoms are just rearranged.
• All of the matter in the reactants = all of the matter in the products
• John Dalton, 200 years ago, realized that atoms simply rearrange themselves during chemical reactions.
• Number of each atom in reactants = number of each atom in products

Chemical reactions can be written in different ways.

• A word equation:
  • Nitrogen monoxide + oxygen --> nitrogen dioxide
• A symbolic equation:
  • 2NO(g) + O₂(g) --> 2NO₂(g)

Coefficients - Big numbers in front of each compound - indicate the ratio of compounds in the reaction - here, there is twice as much NO and NO₂ than there is O₂.