CH131_1v2

CH131_1v2 - Atomic theory of Matter: provides an historical...

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Atomic theory of Matter: provides an historical and logical basis for understanding chemical activity 1. Law of conservation of mass (Lavoisier) “No detectable change in mass observed in ordinary (non-relativistic) chemical reaction - mass neither created or destroyed in chemical change.” => implies matter just rearranged in chemical reaction 1. Law of Definite Proportions (Joseph Proust) “A given compound always contains the same proportion of elements by weight.” => true for all samples regardless of origin or method of preparation. By the end of the 18th century 2 basic phenomenological laws of chemistry determined:
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Early in 19th century (1808) John Dalton proposed a theory, later known as the Atomic theory , to explain these laws. Dalton’s ideas are summarized by the following 5 postulates : 1. Matter consists of atoms (Greek idea orig.) minute particles; shown later ~10 -8 cm (~angstrom Å) 2. All atoms of a given element are alike; identical in mass and properties. 3. Conversely, atoms of diff. elements => diff. properties. In particular, masses of diff. atoms are diff. 4. Atoms are indestructible & retain identity in chem. reactions, i.e. chem. reaction is just a rearrangement of atoms. 5. Atoms of diff. elements combine in simple numerical ratios to form compounds: 1:1 =>AB NaCl, 2:1=> A 2 B H 2 O #4 explains law of conservation of mass #2, 3, 5 rationalize law of definite proportions (a given compound always has same proportion of elements by weight.)
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From Dalton’s postulate #5, The law of Multiple Proportions is subsequently deduced! (good theory makes predictions) “If 2 elements, say A and B , form more than one compound, the masses of A that combine with a fixed a mass of B are in the ratio of small whole numbers.” This prediction was shown to be true! For example:… Dalton’s Atomic Theory provides a basis for relating macroscopic (chemical and physical) properties to a microscopic picture! (conceptual breakthrough of this theory)
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Law of multiple proportions example, cont. carbon (C) and oxygen (O) form 2 compounds by weight : 1. 42.9% C 57.1% O 1. 27.3% C 72.7%O For compound 1 , we find 1.00 g C in some amt. of comp. 1. How much O in compound 1? (let x = # grams O) % O % C = wt . O wt . C thus 57.1% O 42.9% C = x O 1.00 g C x =1.00 g 57.1 42.9 =1.33 g O For compound 2 , we find 1.00 g C in some amt. of comp. 2. How much O in compound 2? (let x = # grams O) % O % C = wt . O wt . C thus 72.7% O 27.3% C = x O 1.00 g C x g 72.7 27.3 = 2.66 g O Thus the weights of O that combine w/ 1.00 g C are in a 2:1 ratio (Illustration of the law of multiple proportions) explanation….
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Compound 1 is (molecule) CO (1atom C - 1 atom O) Compound 2 is (molecule) CO 2 (1 atom C - 2 atom O) Dalton could not have known this (how many atoms per molecule) - only the ratio of weights determined. Furthermore, in 1808, he could not distinguish between CO/CO 2 or C 2 O 2 /C 2 O 4 - only relative amount of O in the two compounds (1:2) could be found.
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At about the same time another observational “law” was determined (by Gay-Lussac): 3. Law of combining volumes: “Volumes of 2 reacting gases stand in the ratio of simple integers. The ratio of the volume of each product gas to reacting gas volume is also in ratio of simple whole numbers.”
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This note was uploaded on 04/17/2008 for the course CAS CH131 taught by Professor Zigler during the Spring '08 term at BU.

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CH131_1v2 - Atomic theory of Matter: provides an historical...

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