Download Law of Conservation of Mass

Law of Conservation of Mass
The Law of Conservation of Mass
• Chemist Antoine Lavoisier’s work in the 1700s
resulted in the Law of Conservation of Mass.
• It states that: In a chemical reaction, the total
mass of the products is always the same as
the total mass of the reactants.
The Law of Conservation of Mass
Reactant A + Reactant B  Product C + Product D
45 g
25 g
50 g
?
Calcium + hydrochloric acid  calcium chloride + hydrogen gas
16 g
33 g
28 g
?
Consider the Electrolysis of Water
• H2O  H2 + O2 is NOT a balanced chemical equation. Why?
• Because the number of H and O atoms in the reactants does not
equal the number of H and O atoms in the products.
• How can we make a balanced chemical equation?
Example
The electrolysis of water yields oxygen gas and
hydrogen gas.
• Write the word equation
Water  oxygen + hydrogen
• Write the skeleton equation
H2O(l)  O2(g) + H2(g)
• Write the balanced chemical equation
2 H2O(l)  1 O2(g) + 2 H2(g)
REVIEW: How to Count Atoms
• The symbol of an element represents one atom
of that element.
– E.g. Ca = 1 atom of calcium
• A subscript indicates more than one atom of the
element.
– E.g. N2 = 2 atoms of nitrogen
• A subscript outside a bracket multiplies all the
elements inside the brackets.
– E.g. Ba3(PO4)2 = 3 atoms of barium, 2 atoms of
phosphorous and 8 atoms of oxygen
REVIEW: How to Count Atoms …cont’d
• A coefficient is a number written in front of a
chemical symbol and indicates the number of
atoms of that element.
– E.g. 3 C = 3 atoms of carbon
• A coefficient written in front of a chemical
formula multiplies the number of atoms of
each element in the formula.
– E.g. 4 Cu(NO3)2 = 4 atoms of copper, 8 atoms of
nitrogen and 24 atoms of oxygen
Steps to Balancing Chemical Equations
1. Write out the skeleton equation.
2. Balance atoms that occur in the largest number
first. Leave oxygen and hydrogen until the end.
3. Balance polyatomic ions next (only if they are
the same on both sides) and balance them as an
ion unit (do not split into individual atoms).
4. Balance hydrogen and oxygen last.
5. Make sure you have the same number of each
element on both sides of the equation.
Example
Word equation:
Methane + oxygen gas  water + carbon dioxide
Skeleton equation:
CH4(g) + O2(g)  H2O(g) + CO2(g)
• The chemical equation must obey the law of conservation of mass.
Thus the total number of one type of atom must be the same on
both sides of the equation.
• To ensure this one balances the equation by putting COEFFICIENTS
in front of the substances.
Balanced Chemical Equation:
1 CH4(g) + 2 O2(g)  2 H2O(g) + 1 CO2(g)
Rules for Balancing Chemical
Equations
• We will work through a second example
The rusting of iron (III)
Word Equation
Skeleton Equation
Balanced Chemical Equation
More Examples
Mg
+
HNO3 
H2 +
Na
+
H2O

Mg(NO3)2
NaOH
+
H2
Homework
• Read pg. 160-167
• Define: reactant, product, chemical reaction,
chemical equation, coefficient
• Pg. 164 #1-4
• Pg. 165 #1-2