Conservation of Mass
and Chemical Equations
section 4.3
Law of Conservation of Mass
In a chemical reaction, the atoms that make up
the starting material(s) undergo a
rearrangement to form the product(s).
Since no atoms are lost of gained in a chemical
reaction, mass is conserved.
LCM: In a chemical reaction, the total mass of
the reactant(s) = total mass of the product(s)
Demonstration: Law of conservation of Mass (LCM)
NaHCO3(s) + HC2H3O2(aq) 
NaC2H3O2(aq) + H2O(l) + CO2(g)
1. Carry out rxn in open flask:
Does NOT demonstrate LCM
CO2(g) escapes
2. Collect CO2(g) in a balloon over the flask
Does NOT demonstrate LCM
Loss of mass = mass of room air “pushed” out of
the way by expanding balloon . . .
3. Carry out rxn in a sealed, plastic 2-L pop
bottle.
LCM proven—bottle does not expand as the
CO2(g) is produced
Moral of the story:
Just because you think you have a good
experiment (eg #2 above) doesn’t mean that
the experiment is good.
Historical Note
LCM Proposed by Antione Laviosier, the father
of modern chemistry
He also:
• devised systematic chemical
nomenclature
• recognized and named oxygen
• helped construct the metric
system
• wrote the first extensive list
of elements
• worked closely with his wife,
who translated English scientific
papers into French; she made
drawings of his experiments
• lost his head in
the French Revolution
NOTE: He married at the age of 28;
Anne-Marie was 13.
Writing Chemical Equations
Here’s one way of doing things:
1.
equations
2.
3.
equations
- ed chemical equations
Notations for Physical State
Physical state
Notation
solid
(s)
liquid
(l)
gas
(g)
aqueous
(aq)
Aside: English Lesson
what does homonuclear mean?
the same atoms
what does diatomic mean?
atoms
what does homonuclear diatomic mean?
two of the same atoms—bonded
to each other
Need to know:
the Homonuclear Diatomics
H2(g)
N2(g)
O2(g)
F2(g)
Cl2(g)
Br2(l)
I2(s)
halogens
Balancing Chemical Equations
As we said:
word eq’n
skeleton eq’n
balanced chemical eq’n
(include physical states)
eg. Water is formed from the reaction of
molecular hydrogen with molecular oxygen.
Write the word, skeleton and balanced eq’ns.
hydrogen + oxygen  water
H2 + O2  H2O
2 H2(g) + 1 O2(g)  2 H2O(l)
(The 1 and 2 are called coefficients.)
check your answer—atom count; physical state
eg. Gaseous ammonia (NH3) is prepared by the
reaction of molecular nitrogen with molecular
hydrogen.
nitrogen + hydrogen  ammonia
N2 + H2  NH3
N2(g) + 3 H2(g)  2 NH3(g)
eg. Gaseous methane (CH4) reacts with oxygen
to produce gaseous carbon dioxide and water
vapour.
methane + oxygen  carbon dioxide + water
CH4 + O2  CO2 + H2O
CH4(g) + 2 O2(g)  1 CO2(g) + 2 H2O(g)
eg. Gaseous butane (C4H10), the fuel in a
disposable lighter, reacts with oxygen to
produce carbon dioxide and water.
butane + oxygen  carbon dioxide + water
C4H10 + O2  CO2 + H2O
C4H10 + ____ O2  4 CO2 + 5 H2O
13/2*
--or-2 C4H10 + 13 O2  8 CO2 + 10 H2O
* fractions okay  think: mol
Practice
Complete section 3.1 in the text. Do at least
some of the
Learning Check
Practice Problems
Review Questions
Also do the worksheet on
Balancing chemical eqns.