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Conserving Matter Keeping Track of Atoms Law of Conservation of Matter • In a chemical reaction, substances are changed to new forms, not created or destroyed. • The Law of Conservation of Matter has been determined by careful observation: In a chemical reaction, matter is neither created nor destroyed. Balanced Chemical Equations • Remember that we learned to take ATOM INVENTORIES of both the reactants and the products in a chemical reaction. • Because of the Law of Conservation of Matter, we learned that the number of atoms of an element in the reactants is equal to the number of atoms of the same element in the products. • In a balanced chemical reaction, there are EQUAL numbers of each kind of atoms on BOTH sides of the equation. Balancing Equations • Is this equation balanced? C3H8 + O2 → CO2 + H2O Take an atom inventory: Reactant side Product side ___ carbon atoms ___ hydrogen atoms ___ oxygen atoms ___ carbon atoms ___ hydrogen atoms ___ oxygen atoms • Is this equation balanced? 4Ag + 4 H2S + O2 → 2 Ag2S + 4H2O Take an atom inventory: Reactant side Product side ___ silver atoms ___ hydrogen atoms ___ sulfur atoms ___ oxygen atoms ___ silver atoms ___ hydrogen atoms ___ sulfur atoms ___ oxygen atoms What is a mole? • Well, yes, but we’re not discussing biology or dermatology now. • We want the CHEMIST’S mole. So far we have discussed chemical equations in terms of atoms and molecules. 2 CuO(s) + C(s) → 2 Cu(s) + CO2(g) But for a metal-processing plant making copper metal, they need to know how much carbon is needed to react with a large amount of copper (II) oxide ore. The Mole So chemists devised a counting unit called the MOLE. You use counting units all the time—dozen, pair—to mean a specific number. A mole is 6.02 x 1023 units of a substance. That’s 602 000 000 000 000 000 000 000 water molecules or 602 000 000 000 000 000 000 000 atoms of copper. This number is commonly known as Avogadro’s number. Molar Mass • The MOLAR MASS of a substance is the mass of one mole (6.02 x 1023 units) of that substance. • The mass in grams of one mole of a substance is equal to the atomic weight of that substance. What is the molar mass of carbon? What is the atomic weight of carbon? So, the molar mass is 12.01 grams. 1 mol C = 12.01 g C Molar Masses of Compounds • What is the molar mass of O2? What is the atomic weight of oxygen? 16.00 g. The subscript tells us we have two moles of oxygen, so the molar mass is 2 mol O x 16.00 g O = 32.00 g O 1 mol O so 1 mol O2 = 32.00 g O2 What is the molar mass of carbon dioxide? 1 mol C = 12.01 g C 1 mol O = 16.00 g O 1 mol C x 12.01 g C = 12.01 g C 1 mol C 2 mol O x 16.00 g O = 32.00 g O 1 mol O Molar mass of CO2 = (12.01 g + 32.00g) = 44.01 g CO2