Transcript Chapter 3 Sections 3.1-3.2 Powerpoint

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Dehydration of sugar by
sulfuric acid.
•Do chemical reactions play a part in our
everyday lives?
•Food is converted to energy in the
human body.
•Nitrogen and hydrogen combine to
form ammonia, which is used as a
fertilizer.
•Fuels and plastics are produced from
petroleum.
•Human insulin is produced in
laboratories by bacteria.
•These are all examples of chemical
reactions.
•The main goal of chemistry is to
understand these types of chemical
changes.
•Chemical stoichiometry: study of the quantities of
materials consumed and produced in chemical reactions.
•Suppose you need 1000 jelly beans for a
customer.
•Would it be easier to count 1000 jelly
beans or have a system in place that
counts jelly beans by weighing them?
•All jelly beans are not identical so you
would use the average mass of the jelly
beans.
•Objects do not need to have identical masses to be counted
by weighing; all that is needed is the average mass.
•For purposes of counting, the objects behave as if they were
all identical.
•As seen below it is much easier to weigh out 600 hex nuts
than count them one by one.
•Atoms are counted by using an
average mass.
•Atoms are very small and we deal
with samples of matter that
contain huge numbers of atoms.
•Even if we could see atoms it
would not be possible to count
them directly.
•To determine the number of
atoms in a given sample we can
use the mass of the sample.
•The modern system of atomic masses, instituted in 1961,
is based on 12C (“carbon twelve”) as the standard.
•12C (“carbon twelve”) is assigned a mass of exactly 12
atomic mass units (amu) and the masses of all other atoms
are given relative to the standard.
•A mass spectrometer is used to determine the masses of
atoms.
•Stage 1: Ionization
•An atom is ionized by knocking off one or more electrons to
give a positive ion.
•Stage 2: Acceleration
•The ions are accelerated so they all have the same kinetic
energy.
•Stage 3: Deflection
•The ions are then deflected by a magnetic field according
to their masses. The lighter they are the more they are
deflected.
•Stage 4: Detection
•The beam of ions passing through the machine is detected
electrically.
•For example, when 12C and 13C are analyzed in a mass
spectrometer, the ratio of their masses is found to be
Mass
Mass
13
C
 1.0836129
12
C
•Since the atomic mass unit is defined such that the mass of
12C is exactly 12 atomic mass units, then on this same scale,
Mass of 13C = (1.0836129)(12 amu) = 13.003355 amu
Exact number
by definition
•The masses of other atoms can be determined in the
same way.
•Average atomic masses for the elements are located
below the symbol on the periodic table.
•The average atomic mass of each element is an average
of the masses of the naturally occurring isotopes for that
element.
•This is a weighted average that reflects the abundance of
each isotope (e.g., the more abundant the isotope the
more it contributes to the average atomic mass).
•Your grade in most classes is a weighted average.
Tests
Labs
Classwork
Homework
50%
25%
15%
10%
Which affects your grade the most?
Tests or Homework?
•Example: Naturally occurring chlorine is a mixture of two
isotopes. In every sample of this element 75.77% of the
atoms are 35Cl and 24.23% are atoms of 37Cl. The accurately
measured mass of 35Cl is 34.9689 amu and that of 37Cl is
36.9659 amu. From this data, calculate the average atomic
mass of chlorine.
•75.77% of the mass is contributed by 35Cl and 24.23% of
the mass is from 37Cl.
•Change the percentages to a decimal and multiply by the
masses.
0.7577 x 34.9689 amu = 26.50 amu (for 35Cl)
0.2423 x 36.9659 amu = 8.957 amu (for 37Cl)
______________________________________
Total mass of average atom = 35.46 amu
(see periodic table for average atomic mass of chlorine)
•The mass spectrometer is used to determine the isotopic
composition of a natural element.
•For example, when a sample of natural neon is injected,
the mass spectrum below is obtained.
•The areas of the “peaks” or the heights of the bars indicate
21
22
Ne
Ne
the relative abundances of 20
,
,
and
10
10
10 Ne atoms.
Example: When a sample of natural copper is vaporized
and injected into a mass spectrometer, the results shown
the figure below are obtained. Use these data to compute
the average mass of natural copper. (63Cu = 62.93 amu and
65Cu = 64.93 amu)
•Of every 100 atoms of natural copper,
69.09 are 63Cu and 30.91 are 65Cu.
(69.09)(62.93 amu) + (30.91)(64.93 amu) =
6355 amu
The average mass of a copper atom is
6355 amu
 63.55 amu / atom
100 atoms