Slide 1 / 31 Slide 2 / 31 New Jersey Center for Teaching and Learning AP Chemistry Progressive Science Initiative This material is made freely available at www.njctl.org and is intended for the non-commercial use of students and teachers. These materials may not be used for any commercial purpose without the written permission of the owners. NJCTL maintains its website for the convenience of teachers who wish to Unit 3: Presentation B make their work available to other teachers, participate in a virtual professional learning Chemical Reactions and community, and/or provide access to course materials to parents, students and others. Stoichiometry Click to go to website: www.njctl.org www.njctl.org Slide 3 / 31 Slide 4 / 31 Chemical Reactions Chemical Reactions and Stoichiometry The world is full of chemical reactions. They power our batteries, To review, all chemical reactions must obey the law of conservation of allow us to see, and make nice red paint for sports cars! mass - they must be "balanced". light 3H 2 (g) + N 2 (g) --> 2NH 3 (g) Reactants Products 6 H atoms 6 H atoms cis-retinal trans-retinal 2 N atoms 2 N atoms The conversion of cis-retinal to trans-retinal is the first step of many that allows us to see. This supports the Bohr notion that atoms are simply rearranged in a chemical reaction, not created or destroyed. Nuclear reactions do "destroy" atoms as we shall see but still obey the law of conservation of mass Slide 5 / 31 Slide 6 / 31 Chemical Reactions Chemical Reactions When balancing a reaction, only the coefficients may be changed as Make sure to write the correct formulas of reactants and products changing the subscripts changes the nature of the material itself. NH 4 CO 3 --> NH 3 + CO 2 + H 2 O H 2 (g) + O 2 (g) --> H 2 O(g) Incorrect formula makes balancing impossible One cannot balance the O atoms by making O 2 suddenly O! Singular O is not reacting here, O 2 gas is! (NH 4 ) 2 CO 3 --> 2NH 3 + CO 2 + H 2 O Instead, one must change the number of oxygen or water Correct formula makes things easy!! molecules that react. H 2 (g) + 1/2 O 2 (g) --> H 2 O(g)
Slide 7 / 31 Slide 8 / 31 Chemical Reactions Reaction Stoichiometry Fractions may be used as coefficients which can then be Coefficients represent the relative number of molecules, multiplied through by a number to get whole number coefficients. elements, or compounds involved in the reaction. 4 NaCl(s) + 2 H 2 O(g) + 2 SO 2 (g) + O 2 (g) --> 4 HCl(g) + 2 Na 2 SO 4 2 NaCl(s) + H 2 O(g) + SO 2 (g) + 1/2 O 2 (g) --> 2 HCl(g) + Na 2 SO 4 4 moles of NaCl(s) produce 2 moles of Na 2 SO 4 To get whole number coefficients, multiply all 0.1 moles of NaCl(s) produce ______ moles Na 2 SO 4 coefficients by 2! 4.5 x 10 23 formula units NaCl(s) produce ___________ formula units Na 2 SO 4 4 NaCl(s) + 2 H 2 O(g) + 2 SO 2 (g) + O 2 (g) --> 4 HCl(g) + 2 Na 2 SO 4 *Note, the coefficients DO NOT represent the mass ratios - ie. 4 grams of NaCl will NOT produce 4 grams of HCl. Slide 9 / 31 Slide 10 / 31 Reaction Stoichiometry 1 What is the proper coefficient in front of the oxygen molecule after the following equation is balanced? Coefficients can be used to determine the relative amounts of substances involved in a reaction. NH 3 (g) + O 2 (g) --> NO(g) + H 2 O(l) 4 NaCl(s) + 2 H 2 O(g) + 2 SO 2 (g) + O 2 (g) --> 4 HCl(g) + 2 Na 2 SO 4 How many grams of oxygen would be needed to produce 3 moles of HCl(g)? 3 moles of HCl x 1 mol O 2 x 32 g O 2 = 24 g O 2 4 mol HCl 1 mol O 2 molar ratio from balanced equation Slide 11 / 31 Slide 12 / 31 2 The fermentation of sugar produces ethyl alcohol and 3 What is the sum total of coefficients when the equation carbon dioxide gas. How many moles of carbon dioxide below is balanced and all coefficients are simplified to the would be produced from the fermentation of 0.4 mol of lowest whole number ratio? glucose? C 6 H 12 O 6 (s) --> C 2 H 5 OH + CO 2 C 2 H 5 NH 2 + O 2 --> CO 2 + N 2 + H 2 O
Slide 13 / 31 Slide 14 / 31 4 Given the UNBALANCED reaction below, determine how 5 Phosphorus pentachloride decomposes into phosphorus many L of nitrogen gas would be produced when 110 trichloride gas and chlorine gas. How many total moles of grams of NaN 3 decompose completely @STP? gas will be produced after 50% of a 414 gram sample of phosphorus pentachloride decomposes? NaN 3 (s) ---> Na(s) + N 2 (g) A 22.4 L A 0.5 moles B 44.8 L B 1.0 moles C 33.1 L C 1.5 moles D 57.2 L D 2.0 moles E 11.2 L E 3.0 moles Slide 15 / 31 Slide 16 / 31 Reaction Stoichiometry 6 If a 20 mL solution of 0.3 M Ca(NO 3 ) 2 were mixed with a solution of 3 M NaOH, how many mL of the 3 M NaOH solution would be needed to react with all of the calcium The theoretical yield is the anticipated amount of product that ions? should be made based on the amounts of reactants used and reaction conditions. Ca 2+ (aq) + 2OH - (aq) --> Ca(OH) 2 (s) CaCO 3 (s) --> CO 2 (g) + CaO(s) What is the theoretical yield of CaO (in moles) if 200 grams of calcium carbonate completely decompose? 200 g CaCO 3 x 1 mol CaCO 3 x 1 mol CaO = 2 mol CaO 100 g CaCO 3 1 mol CaCO 3 Slide 17 / 31 Slide 18 / 31 Reaction Stoichiometry Reaction Stoichiometry Steps When two or more reactants are present, the reactant that is STEP 1: Convert known quantities to moles used up first (the limiting reactant) will determine the theoretical yield of product. N 2 (g) + 3H 2 (g) --> 2NH 3 (g) N 2 (g) + 3H 2 (g) --> 2NH 3 (g) If 44.8 L of nitrogen gas reacts with 44.8 L of hydrogen gas @STP, what is the theoretical yield of ammonia? If 44.8 L of nitrogen gas reacts with 44.8 L of hydrogen gas @STP, what is the theoretical yield of ammonia? 44.8 L N 2 (g) = 2 mol N 2 (g) 44.8 L H 2 (g) = 2 mol H 2 (g) This type of problem can be easily solved via a series of steps. One might think that since both reactants are found in the same quantities, we would run out of both at the same time. Why is this not true for this reaction?
Slide 19 / 31 Slide 20 / 31 Reaction Stoichiometry Steps Reaction Stoichiometry Steps STEP 3: Use limiting reactant amount to determine the STEP 2: Choose a reactant and determine how much of the other would be needed and compare to how much of that reactant is theoretical yield of product or amount of excess reactant that available. reacted. N 2 (g) + 3H 2 (g) --> 2NH 3 (g) N 2 (g) + 3H 2 (g) --> 2NH 3 (g) If 44.8 L of nitrogen gas reacts with 44.8 L of hydrogen gas @STP, If 44.8 L of nitrogen gas reacts with 44.8 L of hydrogen gas @STP, what is the theoretical yield of ammonia? what is the theoretical yield of ammonia? Moles Moles of other reactant Reactant Finding Theoretical Yield available needed 2 mol H 2 x 2 mol NH 3 = 1.33 mol NH 3 N 2 (g) 2 moles 2 mol N 2 x 3/1 = 6 mol H 2 3 mol H 2 2 mol H 2 x 1/3 = 0.66 mol H 2 (g) 2 moles N 2 Finding Amount of Excess Reactant that Reacted Since 6 mol of H 2 are required and only 2 moles are available, 2 mol H 2 x 1 mol N 2 = 0.66 mol N 2 reacted H 2 is the limiting reactant. 3 mol H 2 Slide 21 / 31 Slide 22 / 31 7 How many grams of excess reactant will remain if 4 8 If 16 grams of methane react with 32 grams of oxygen grams of hydrogen gas react with 16 grams of oxygen gas by the reaction below, what would be the theoretical gas by the reaction below? yield of carbon dioxide ( in grams)? 2H 2 (g) + O 2 (g) --> 2H 2 O(g) CH 4 (g) + 2O 2 (g) --> CO 2 (g) + 2H 2 O(g) A 2 grams B 11 grams C 22 grams D 44 grams E 0.5 grams Slide 23 / 31 Slide 24 / 31 9 How many total liters of gas can be produced @STP if 10 If 8 molecules of propane (C 3 H 8 ) react with 10 molecules 4.8 grams of magnesium metal react with 100 mL of 0.5 of oxygen gas, which would be true of the reaction M HCl? mixture after the reaction had gone to completion? C 3 H 8 (g) + 5O 2 (g) --> 3CO 2 (g) + 4H 2 O(g) Mg(s) + 2H + (aq) --> Mg 2+ (aq) + H 2 (g) C 3 H 8 O 2 CO 2 H 2 O Answer A 0 3 2 2 6 0 6 8 B 2 5 3 4 C
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