Stoichiometry Chemical Equations Reactants are written on the left side of the arrow and products are written on the right side of the arrow. The Law of Conservation of Mass tells us that the number of atoms of a given element on the reactant side of the arrow must equal the number of atoms of that same element on the product side of the arrow. The coefficients of a balanced chemical equation tell us how many of each species are involved in the reaction. When balancing a chemical equation, you must not change the identities (the subscripts) of any of the species; you may only change how many (the coefficients) of the species participate. Ex. 1 Balance the following chemical equations: NaBr + H 3 PO 4 HBr + Na 3 PO 4 C 3 H 5 N 3 O 9 CO 2 + N 2 + H 2 O + O 2 Ca(OH) 2 + SO 2 + O 2 H 2 O + CaSO 4 1
Tips for Balancing Chemical Equations: We have been learning how to balance chemical equations. The steps for balancing equations are outlined in your textbook. There are, however, a few tricks which you can apply to balancing chemical equations. The first involves a quick inspection to determine if you have an element that appears in only one compound on both sides of the equation and has subscripts like 2 and 3, 4 and 5, etc. If so, then try making the subscripts the coefficients, but switch sides of the equation. For example: Fe + Cl 2 FeCl 3 In this reaction, Cl appears only once on the left side of the reaction and has a subscript of 2. It appears only once on the right side of the reaction and has a subscript of 3. A good first step for balancing an equation like this would be to make 3 (the subscript for Cl on the right side of the equation) the coefficient in front of Cl 2 on the left side of the equation, and make 2 the coefficient in front of FeCl 3 on the right side of the equation. You then have: Fe + 3 Cl 2 2 FeCl 3 All that s left is to balance the Fe: 2 Fe + 3 Cl 2 2 FeCl 3 Another trick that will help you in counting is to count polyatomic ions as units if they appear on both sides of the equation: KCl + HNO 3 + O 2 KNO 3 + Cl 2 + H 2 O Here you can count the NO 3 as a unit instead of counting out the individual N s and O s. Set HNO 3 to have a coefficient of 1 : KCl + 1 HNO 3 + O 2 KNO 3 + Cl 2 + H 2 O Now, you have 1 NO 3 group on the left side of the equation and 1 NO 3 group on the right side of the equation: KCl + 1 HNO 3 + O 2 1 KNO 3 + Cl 2 + H 2 O Balance the rest as usual: 4 KCl + 4 HNO 3 + O 2 4 KNO 3 + 2 Cl 2 + 2 H 2 O For more information on balancing chemical equations see http://www.chem.uic.edu/chem112/fall02/bal_eqn.pdf 2
Patterns of Chemical Reactivity Using Periodic Trends Combustion in Air Ex. 2 Write a balanced chemical reaction for the combustion of octane, C 8 H 18(l), in air. Combination and Decomposition 3
Ex. 3 Write the balanced chemical equation for the combination of the metallic element calcium with the nonmetallic element oxygen, O 2. Atomic and Molecular Weights Atomic Mass Molecular Weight Ex. 4 Calculate the molecular weight of (a) N 2 O 5 (b) Ca(C 2 H 3 O 2 ) 2 Percentage Composition 4
Ex. 5 Calculate the percentage by mass of oxygen in 4a and 4b above. The Mole Ex. 6 Calculate the number of O atoms in 0.470 mol of C 6 H 12 O 6. Molar Mass Ex. 7 What is the mass in grams of 1 mol of glucose, C 6 H 12 O 6? 5
How to interconvert between grams, moles and number of molecules Ex. 8 How many moles of chloride ions are in 0.0750 g of magnesium chloride? Ex. 9 What is the mass, in grams, of 1.75 10 20 molecules of caffeine, C 8 H 10 N 4 O 2? Ex. 10 What is the molar mass of cholesterol if 0.00105 mol has a mass of 0.406 g? Empirical Formulas from Experiment 6
Ex. 11 Determine the empirical formula of a compound containing 0.104 mol K, 0.052 mol C, and 0.156 mol O. Molecular Formula from Empirical Formula Ex. 12 What is the molecular formula of a compound that has an empirical formula, CH 2, and a molar mass of 84 g mol 1? 7
Combustion Analysis Ex. 13 Combustion analysis of toluene, a common organic solvent, gives 5.86 mg of CO 2 and 1.37 mg of H 2 O. If the compound contains only carbon and hydrogen, what is its empirical formula? Stoichiometry of Balanced Equations grams moles moles grams reactant reactant product product 8
Ex. 14 Propane, C 3 H 8, is a common fuel used for cooking and home heating. What mass of O 2 is consumed in the combustion of 1.00 g of propane? Limiting Reactants The reactant that is completely consumed, or used up, in a chemical reaction is called the limiting reactant. Ex. 15 When hydrogen sulfide gas is bubbled into a solution of sodium hydroxide, the reaction forms sodium sulfide and water. How many grams of sodium sulfide are formed if 2.50 g of hydrogen sulfide is bubbled into a solution containing 1.85 g of sodium hydroxide, assuming that the limiting reactant is completely consumed? 9
Theoretical and Percentage Yield Ex. 16 When ethane, C 2 H 6, reacts with chlorine, Cl 2, the main product is C 2 H 5 Cl, but other products are also obtained in small quantities. The formation of these other products reduces the yield of C 2 H 5 Cl. Calculate the percent yield of C 2 H 5 Cl if the reaction of 125 g of C 2 H 6 with 255 g of Cl 2 produced 206 g of C 2 H 5 Cl. 10