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Unformatted text preview: Chapter 3 Stoichiometry The area of study that deals with the examination of quantities of substances consumed and produced in a chemical reaction is called stoichiometry. The word comes from the Greek which means element measure. Stoichiometry is built upon the understanding of atomic masses, chemical formulas and the Law of Conservation of Mass. One of the premises upon which stoichiometry is based is upon the concept that atoms are neither created nor destroyed during any chemical reaction. The atoms are merely rearranged in the chemical process. Chemical Equations Chemical reactions are represented in a concise way by chemical equations. The reactants are on the left side of the equation, while the products are on the right side of the equation. The plus sign, +, indicates reacts with, while the , arrow sign, indicates produces, or makes. The subscripts from the chemical compounds do not change because they indicate the number of atoms of each type in the compound. Whereas, in a chemical reaction, the coefficients, the number in front of each compound or element, represents how many molecules of each compound or element there are. For example, 2H 2 + O 2 2H 2 O This reaction means the following: 2 molecules of H 2 reacts with 1 molecule of O 2 to produce 2 molecules of H 2 O. On the reactant side, there are 4 H atoms total and 2 O atoms total. The same applies to the product side, thus, we have a balanced equation. Look at the following reaction. Balance the following: Al(s) + HCl(aq) AlCl 3 (aq) + H 2 (g) At first look, only the Al is in balance on both side because there is 1 Al on each side. We must therefore attempt to balance the rest. When I line it out, the result is such. Al(s) + HCl(aq) AlCl 3 (aq) + H 2 (g) Al 1 Al 1 H 1 H 2 Cl 1 Cl 3 I begin by balancing the Cl. I put 3 in front of the HCl. This allows for the H to balance on the left side with that of the AlCl 3 on the right side. Al(s) + 3HCl(aq) AlCl 3 + H 2 (aq) There are now 3 Hs on the left side and 2 Hs on the right side. In order to balance the H, we multiply the left side by 2 and the right side, we multiply by 3 on the right side. This will unbalance the HCl, because now we have 3 X 2 Cls or 6 Cl. We rebalance the Hydrogen as follows. Al(s) + 6HCl 2AlCl 3 (aq) + 3H 2 (g) Now we finish by balancing the equation with 2 mol of Al on the reactant side. 2Al(s) + 6HCl(aq) 2AlCl 3 (aq) + 3H 2 (g) There are 2 Als on each side of the reaction, 6 Hs and 6 Cls on each side of the reaction. It is balanced. Simple Patterns of Chemical Reactivity Many simple reactions follow the same pattern in terms of their reactivity. There are 3 different type of chemical reaction that are fairly simple....
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