Download 1 Chemical Reactions

What happens to the weight of an iron
bar as it rusts? (Assume the rust
doesn’t fall off the bar.)
Chemical Reactions
Substances change into other
substances
A.
The weight
increases
B.
The weight
decreases
C.
The weight
stays the same
Why?
Chemistry and the “atom”
Chemistry and the “atom”
The Law of Definite Proportions
Examples of Definite Proportions
In chemical reactions,
materials combine in
definite proportions by
weight,
weight which do not
change between instances
of a reaction.
{
29 pounds of cinnabar yields:
z
z
{
4 pounds of sulfur
25 pounds of mercury
To neutralize 4 pounds of lime,
z
z
5 pounds of sulfuric acid or
7 pounds of nitric acid
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Chemistry and the “atom”
Chemistry and the “atom”
Oxygen and atomic weights
Atomic weights and water
{
Oxygen forms compounds with all
the elements discovered before
1894!
z
z
{
Most of these compounds are solids
and easy to work with
Therefore atomic weights were
determined relative to oxygen
{
Water can be made from
z
z
{
1 pound of hydrogen
8 pounds of oxygen
What’s the formula?
z
Dalton: “HO”
{
But… oxygen is not the lightest
element!
z
Oxygen weighs 8 times as much as
hydrogen
Others: “H2O”
{
Oxygen weighs 16 times as much as
hydrogen
Chemistry and the “atom”
Chemistry and the “atom”
Definite Proportions of gases
Atomic weights and water
hydrogen
oxygen
{
{
Decomposition of
water
z
z
2 volumes of
hydrogen
1 volume of oxygen
Water can be made from
z
z
{
1 pound of hydrogen = 2 volumes
8 pounds of oxygen = 1 volume
What’s the formula?
z
z
Avogadro’s hypothesis: equal volumes
of gas contain equal numbers of atoms
Water is therefore H2O
{
Oxygen weighs 16 times as much as
hydrogen
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Chemistry and the “atom”
Atoms in electrolysis of water
Stoichiometry
{
“Atoms on the left must be equal to
atoms on the right”
__Na + __Cl2 → __NaCl
Stoichiometry
{
In the open crystal structure of ice,
what is in the spaces?
“Atoms on the left must be equal to
atoms on the right”
__CH4 + __O2 → __CO2 + __H2O
A.
Air
B.
Water vapor
C.
Nothing
How do we know
from the diagram?
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