9/10/2013
1
Chapter 5A
- Atoms and
Elements
Chemical Formulas
Molecular formulas
Formula units
Classifying Elements and Compounds
Elements as Atomic or Molecular
Compounds-Ionic vs Molecular
Thinking about Chemistry
How should I think about chemical reactions?
Macroscopically
Submicroscopically
Symbolically
•
Models/pictures
•
Mathematical & chemical equations
•
Words, names, & chemical symbols (Ch. 5)
Central Theme in Chemistry:
Macroscopic properties
and behaviors are the results submicroscopic
properties and behaviors.
Proust’s Macroscopic Observation
Law of constant composition-
all samples of a given compound
have the same proportion of their constituent elements.
Example:
9.0 g of water can be separated into 1.0 g of
hydrogen and 8.0 g of oxygen.
(Note: mass conserved).
Two ways to think about this:
As a fraction (or %):
Oxygen=
8
/
9
(89%)
Hydrogen=
1
/
9
(11%)
If we had an 18 g sample
Oxygen=
8
/
9
*18g=16 g
Hydrogen=
1
/
9
*18g=2 g
As a proportion (O:H ratio):
If a second sample had
2 g of hydrogen:
X= 16 g of oxygen
8
1g
8g
H
O
=
=
2g
Xg
1g
8g
H
O
=
=
Macroscopic Connected with Sub-microscopic
Since water is a pure compound (1) water is chemically the same
even if the sample is very small (2) and law of constant
composition should still apply to a small sample.
***
***
Dalton’s theory says that this is true until we get to the atomic
level. At some point we can’t get a smaller size and still be water.
Sub-microscopic masses too
Macroscopic
mass
1g
H
8 g Oxygen
1
8
If we know the atomic mass of
H (1 amu) and O (16 amu), we
can find out the ratio of the
atoms of each type.
(recall: 1 amu = 1.6x10
-24
g)
H’s=1g/1.6x10
-24
g=6x10
23
O’s=8g/(16*1.6x10
-24
g)=3x10
23
**Atom ratio: 2 H per 1 O!
(**This is the relative number of H’s and O’s, more about this in Ch. 6)
Left
most
To
Right
most
Chemical Formula
We write a Chemical Formula to summarize the elements present in
the compound, e.g. H
2
O.
use elemental symbol for each atom in compound
use subscript to indicate number of atoms of each type.
( “1” is implied if nothing written, and use parentheses when needed.)
Elements are written in this order: (metal
left
, non-metal
right
)
http://old.iupac.org/publications/books/principles/principles_of_nomenclature.pdf
Examples: NaCl not ClNa
CO
2
not O
2
C
CHCl
3
not HCl
3
C or Cl
3
CH, etc. (exception, OH
-
)
F
O
Application: Law of Constant Composition
Two Samples of Iron oxide are found, each decomposed to
Fe and O
Are they the same Iron Oxide?
Look at the mass ratios:
Analysis: based on 4 sig figs, ratios are different.
Conclusion:
Not the same Iron Oxide!
(FYI: Hematite, Fe
2
O
3
vs Magnetite, Fe
3
O
4
, notice how chemical
formulas are often a more meaningful summary than masses).
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- Fall '13
- RusselLarsen
- Chemistry, Charge, Ion, Anion
-
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