2.2 Conservation of Mass in Chemical Equations

chemistry

Conservation of Mass

The Law of Conservation of Mass

  • In chemical reactions, atoms are neither created nor destroyed
  • Developed by Antoine Lavoisier and his wife Maire-Anne in the 1700s
  • Mass of reactants = Mass of products

Balancing chemical equations is a "trial and error" method; you systematically change the coefficients in front of the chemical formulas until you "balance" the number of each atom on each side of the equation.

Chemical equations do not come already balanced. This must be done before the equation can be used in a chemically meaningful way. All chemical calculations to come must be done with a balanced equation. A balanced equation has equal numbers of each type of atom on each side of the equation.

There is no apparent change in mass during a chemical reaction. This is what is expected from the balanced equation for a reaction. Atoms and mass are conserved.

2 Mg + O2 → 2 MgO

There are 2 Mg atoms + 2 O atoms to start with and 2 Mg atoms + 2 O atoms in the end. There is no gain or loss of mass.

In a chemical reaction:

  • mass is conserved, and
  • atoms are conserved
  • molecules are not necessarily conserved

There are 2 molecules on the reactant side (2 MgO) and 3 particles on the product side (2 Mg atoms and 1 O2 molecule) of the equation.