Chapter 5 Chemistry Notes - Memorize polyatomic ions in...

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).