Conservation of Mass

1
Conservation of Mass

• Lavoisier observed that mass is conserved in a
  chemical reaction.
• Matter cannot be lost in chemical reactions.
• The products have to account for all atoms
  present in the reactants.
• Equations are balanced by changing the
  stoichiometric coefficients.

2
Stoichiometry

• Greek stoicheion element
• Greek metron measure

3
Chemical Equations

• Law of conservation of mass
• Atoms are neither created nor destroyed (in
  ordinary chemical reactions).
• number of atoms on left number of atoms on
  right
• The correct formula must be shown for all
  reactants and products.
• Use oxidation states and ionic charges to
  correctly write a formula
• In balancing, do not split up molecules or change
  the formula.
• Remember common polyatomic ions and diatomic
  molecules.

4
Symbols used in Equations

• Yields
• Reversible reaction
• Reactants are heated
• Catalyst added
• (s) solid state or precipitate
• (l) liquid state
• (g) gaseous state
• (aq) aqueous state, or dissolved in water

catalyst
5
Order for Balancing

• MINOH method (Me know chemistry, said Tarzan as
  he climbed the stoichiome-tree.)
• M - metals Balance metals first.
• I - ions Balance polyatomic ions.
• N nonmetals Balance Cl, S,N.
• O oxygen
• H hydrogen
• An odd on one side and even on other will
  require you to multiply the odd side to make it
  even!

6
Steps to balance a chemical equation

• Write the formulas and symbols.
• Cu(s) Ag(NO3)(aq) Ag(s) Cu(NO3)2 (aq)
• Count the atoms on each side of the arrow.
• 1 Cu 1 Cu
• 1 Ag 1 Ag
• 1 N 2 N
• 3 O 6 O
• Balance by using coefficients.
• Do not change subscripts!
• Cu(s) 2Ag(NO3)(aq) 2Ag(s) Cu(NO3)2 (aq)
• Check work by counting each element.
• 1 Cu 1 Cu
• 2 Ag 2Ag
• 2 N 2 N
• 6 O 6 O

7
Two Important Principles

• Every chemical compound has a formula that cannot
  be altered.
• A chemical reaction must account for every atom
  used.
• (Law of Conservation of Matter)