Rexn Stoich - Reaction Stoichiometry Mass and Mole...

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1 Mass is neither created nor destroyed during the course of a normal chemical reaction. 2 A useful generalization is that compounds containing only nonmetals are molecular in character, while compounds containing both metals and nonmetals are ionic. The examples chosen in this discussion happen to be molecular and the of the products generated in the experimental section are ionic in nature. The stoichiometric principles applied, however, are equivalent. 71 Reaction Stoichiometry Mass and Mole Relationships within Chemical Reactions DISCUSSION Rexn-Stoich.wpd During a chemical reaction, reactants are transformed into new substances called products. Since atoms are neither created nor destroyed during the course of a normal chemical change, the total number of atoms of each element within the reactants must be equal to the total number of atoms of each element within the products. One simple perspective is that the reaction has produced a rearrangement of atoms from within the reactant compounds to form new combinations of atoms within the product compounds. Therefore reactant compound formulas yield the new, different compound formulas of the products. This concept nicely explains the Law of Conservation of Mass. 1 Consider the reaction below at the molecular level: 2 C 2 H 2 + 5 O 2 4 CO 2 + 2 H 2 O 4 C atoms 4 C atoms 4 H atoms 4 H atoms 10 O atoms 10 O atoms Two molecules of acetylene, C 2 H 2 , react with five molecules of oxygen to yield four molecules of carbon dioxide and two molecules of water. 2 Since the number of atoms of each element is the same for both the reactants and products, it follows that the total mass of the reactants must equal the total mass of the products. The conservation of mass is demonstrated when the sum of all the molecular masses of the reactants is seen to be equal to the sum of all the molecular masses of the products: 2 C 2 H 2 + 5 O 2 4 CO 2 + 2 H 2 O 2 molecules + 5 molecules 4 molecules + 2 molecules 2 (26.038 amu) + 5 (31.9988 amu) 4 (44.010 amu) + 2(18.0152 amu) 212.070 amu 212.070 amu 2 mole + 5 moles 4 moles + 2 moles 2(26.038 g) + 5(31.9988 g) 4(44.010 g) + 2(18.0152 g) 212.070 g 212.070 g The Mole Concept. It is impractical to manipulate only a few molecules at a time in the laboratory; therefore it is necessary to "scale up" the reaction using larger, tangible quantities of reactant and product masses for routine chemical syntheses. A mole is defined as the formula mass of a compound expressed in grams (instead of amu's), but note that the numerical value of the molar mass is the same as the
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3 Avogadro's number or one mole of molecules 72 molecular mass . For example, the molecular mass of CO 2 is 44.010 amu while the molar mass is 44.010 g. The molecular mass gives the mass of one molecule while the molar mass happens to give the total mass of 6.02 x 10 23 molecules. 3
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This note was uploaded on 11/16/2011 for the course CHM 1025 taught by Professor J during the Summer '09 term at Santa Fe College.

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Rexn Stoich - Reaction Stoichiometry Mass and Mole...

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