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Transcript
Introduction to Chemical Reactions and Equations 1 What are the clues that a chemical reaction has occurred? 2 A gas bubbles off. 3 A new solid material is formed (a precipitate.) 4 Color change. 5 Heat and/or light are emitted. 6 The key to chemical reactions! The Law of Conservation of Mass In a chemical reaction, no atoms are created or destroyed. The atoms in the reacting molecules simply re-arrange to form new molecules. Since atoms aren’t created or destroyed, no mass is created or destroyed either. Mass is “conserved.” 7 Count the atoms, before & after 2H2 + O2 2H2O 8 In the reaction N2 + 3H2 2NH3 • If 14 grams of N2 reacted with 3 grams of H2, how many grams of ammonia would be produced? In the reaction 2NaCl 2Na + Cl2 • If 58.5 grams of NaCl is decomposed, and 23 grams of Na is formed, how many grams of Cl2 must also be formed? 9 What else is conserved? • In N2 + 3H2 2NH3 besides mass (grams), what else is being conserved? o Atoms? o Molecules? o Moles? 10 Coefficients • In a chemical reaction, if more than 1 molecule of a substance reacts or is produced, the number of molecules is shown with a coefficient. • 6CO2 + 6H2O C6H12O6 + 6O2 • Coefficients multiply subscripts through the whole molecule that follows, so 6CO2 shows that there are 6 Carbon atoms and 12 Oxygen atoms. 11 How many? • How many of each atom are shown by 4Al2(SO4)3 • How many of each atom are shown by 3(NH4)3PO4 12 Terminology • In a chemical reaction, the materials to the left of the arrow are the reactants. Reactants are consumed in the reaction. • The arrow is like an equal sign in math. It can be read “react to produce” or “produces” or “forms”. • The materials to the right of the arrow are the products. They form as the reaction proceeds. • H2O + SO3 H2SO4 13 Phase Notation • A chemical equation can also show the physical state of the materials in the reaction. This is done with phase notations: • (s) solid (l) liquid (g) gas • (aq) aqueous or dissolved in water • (ppt) or ( ) means “a precipitate forms” • ( ) means “a gas bubbles off” • means “is heated” • Phase notations are written to the lower right of each formula. Clues to phase notations are often given in equations written in words. 14 An example • When pellets of sodium hydroxide are dropped into an aqueous solution of iron(III) chloride, iron(III) hydroxide precipitates and sodium chloride stays dissolved. • 3NaOH(s) + FeCl3(aq) Fe(OH)3 (ppt) + 3NaCl(aq) 15 Now onward to “balancing chemical equations” 16
