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Stoichiometry

Stoichiometry. Courtesy: www.lab-initio.com. 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 .

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Stoichiometry

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  1. Stoichiometry Courtesy: www.lab-initio.com

  2. 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 understandthese types of chemical changes. Dehydration of sugar by sulfuric acid.

  3. Chemical stoichiometry: study of the quantities of materials consumed and produced in chemical reactions. Counting by Weighing • 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.

  4. 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.

  5. 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.

  6. Atomic Masses • 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.

  7. 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.

  8. 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.

  9. For example, when 12C and 13C are analyzed in a mass spectrometer, the ratio of their masses is found to be • 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.

  10. Average Atomic Mass • 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 50% Labs 25% Classwork 15% Homework 10% Which affects your grade the most? Tests or Homework?

  11. 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)

  12. 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 the relative abundances of , , and atoms.

  13. 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

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