Fundamental Chemical Laws and Dalton’s Atomic Theory and Cannizzaro’s Contribution

1. Lecture #4 Fundamental Chemical Lawsand Dalton’s Atomic Theory andCannizzaro’s Contribution Chemistry 142 A James B. Callis, Instructor Winter Quarter, 2006

2. Some Definitions • Matter- The ‘stuff’ of the universe. That which occupies space. • Substance - A material that cannot be separated by physical means into two or more materials with different properties. • Element - A substance that that cannot be decomposed into simpler substances by chemical means. • Compound – A substance containing two or more chemical elements.

3. Three Laws that Led to the Atomic Theory • Law of Mass Conservation: The total mass of substances does not change during a chemical reaction (Lavoisier). • Law of Definite (or Constant) Composition: No matter what its source, a particular chemical compound is composed of the same elements in the same parts (fractions) by mass (Proust). • The Law of Multiple Proportions:When two elements form a series of compounds, the masses of one element that combine with a fixed mass of the other element are in the ratio of small integers to each other (Dalton).

4. Conservation of Mass • Matter is neither created nor destroyed in a chemical reaction. • In every chemical operation an equal quantity of matter exists before and after the operation. • Mass is conserved in a chemical reaction. • Moreover, in chemical change, the mass of the elements is conserved, element by element. • Development of this law was made possible by the analytical balance.

6. Problem 4-1: Definite Composition Chemical analysis of a 9.07 g sample of calcium phosphate shows that it contains 3.52 g of Ca. How much Ca could be obtained from a 1.000 kg sample? Mass fraction Ca = Mass Ca in 1.000 kg =

7. Problem4-2: Law of Multiple Proportions If elements A and B react to form two compounds, the different masses of B that combine with a fixed mass of A can be expressed as a ratio of small whole numbers. Example: Nitrogen Oxides I & II

8. Dalton’s Atomic Theory • Matter is composed of indivisible atoms. • All atoms of a given chemical element are identical in mass and in all other properties. • Different chemical elements are composed of different atoms of different masses. • Atoms are indestructible. They retain their identities in a chemical reaction. • A compound forms from its elements through the combination of atoms of unlike elements in small whole number ratios.

9. Compounds and Molecules The composition of a compound is shown by its chemical formula. The symbol "CO2" for the substance carbon dioxide indicates that it contains two atoms of oxygen for every atom of carbon. In most cases, the chemical formula also defines a molecule. Here, the atoms in the formula are linked together by attractive forces strong enough to keep the group together for a reasonable period of time.

10. The Theory’s First Challenge: The Relative Masses of Atoms • Dalton incorrectly assigned a mass of 8 to elemental oxygen because he thought that elemental hydrogen was monatomic (mass =1). He concluded that the formula of water was HO. • Gay-Lussac found that 2 volumes of hydrogen and 1 volume of oxygen combined to form 2 volumes of water. This implied that the formula of water was H2O.

11. However, another of Gay-Lussac’s results was confusing: 2 L of water vapor were produced for every 2 L of hydrogen gas that reacted. Here are the expected and actual results: Expected Results: If water is HO: 1 L hydrogen gas + 1 L oxygen gas -> 1 L water vapor If water is H2O: 2 L hydrogen gas + 1 L oxygen gas -> 1 L water vapor Actual Result: 2 L hydrogen gas + 1 L oxygen gas -> 2 L water vapor Implication: If is assumed that oxygen is monatomic, the oxygen must split in half!!?

12. Gay – Lussac’s Law of Combining Volumes The volumes of two reacting gases (at the same temperature and pressure) are in the ratio of simple integers. Moreover, the ratio of the volume of each product gas to the volume of either reacting gas is the ratio of simple integers. Avogadro’s Hypothesis Equal volumes of different gases (at the same temperature and pressure contain equal numbers of particles.

13. Cannizzaro’s Contribution • Cannizzaro analyzed many gaseous compounds and showed that their chemical formulas could be established with a consistent scheme that used Avogadro’s hypothesis but avoided any extra assumptions about molecular formulas. • Many gaseous elements (hydrogen, oxygen, nitrogen, fluorine, chlorine, etc.) did indeed consist of diatomic molecules under ordinary conditions.

14. Stoichiometry • The study of mass relationships in chemical compounds and chemical reactions. • The mass balance makes possible the elucidation of stoichiometry.

15. According to the atomic theory, the mass of one molecule of a compound, mcpd, is related to the number and masses of its constituent atoms as follows: The Atomic Theory Explains Stoichiometry It is assumed that one molecule of the compound is comprised of N types of atoms, identified by the subscript i. The number of atoms of type i in the molecule is given the symbol niatom and the mass of each type of atom is given the symbol miatom. According to the fifth statement of the atomic theory, the niatoms are integers. Note also that the above equation is equivalent to a statement of the conservation of mass.

16. The Catch The masses of individual atoms and the molecules formed from them are vanishingly small, on the order of 10-23 - 10-21 grams. The laboratory balance can measure masses only as small as milligrams. This forces the chemist to work with gram amounts of compounds. How can such measurements relate to and be used to confirm the atomic theory?

17. The Solution According to atomic theory, the mass of a sample of a compound, mcpdsam is simply the mass of one molecule of the compound, mcpd, multiplied by the number of molecules in the sample, ncpdsam:

18. In Context F = ma Newton: E = mc2 Einstein: Dalton:

19. Working in Mass Per Cent Eliminates the Need to Know ncpdsam The percent by mass of the ith atom in a compound is given by the ratio:

20. The Law of Multiple Proportions • When two elements form a series of compounds, the masses of one element that combine with a fixed mass of the other element are in the ratio of small integers to each other. • Now we can make a table of calculations using the symbolic notation developed above.

21. How Dalton Predicts the Law of Multiple Proportions These are ratios of integers.

22. Problem 4-3: Multiple Proportions A chemist obtains the following composition data for four compounds of nitrogen and oxygen. Compound a contains 63.65 % nitrogen and 36.35 % oxygen. Compound b contains 46.68 % nitrogen and 53.32 % oxygen. Compound c contains 30.45 % nitrogen and 69.55 % oxygen. Compound d contains 25.94 % nitrogen and 74.06 % oxygen. (1) calculate the mass of oxygen per gram of nitrogen in each compound. (2) Calculate the ratio of ratios using the smallist ratio. How do the ratio of ratios support Dalton’s atomic theory?

23. Multiple Proportions Problem

24. Caution! There are limitations to the information in Dalton’s interpretation of the multiple proportions table. • You can use Dalton’s equation to go from known composition to predicted ratios. • But you cannot go from the experimental ratios to prediction of molecular composition (inverse direction). • Examination of the multiple proportions table shows that for N (N>1) compounds, it provides N-1 equations in 2N unknowns. For a table of two compounds there is one equation in four unknowns. For three compounds there are two equations in six unknowns, etc.

25. Answers to Problems in Lecture #4 • 388 g Ca • 2/1 • 1:2:4:5