1 Chemical Reactions Chapter 4 Stoichiometry. 2 Chemical Equations ¦ A chemical reaction...

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Transcript of 1 Chemical Reactions Chapter 4 Stoichiometry. 2 Chemical Equations ¦ A chemical reaction...

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1 Chemical Reactions Chapter 4 Stoichiometry Slide 2 2 Chemical Equations A chemical reaction shows the formulas and relative amounts of reactants and products in a reaction. 4 Al (s) + 3 O 2 (g) 2 Al 2 O 3 (s) 4 Al (s) + 3 O 2 (g) 2 Al 2 O 3 (s) reactantsproduct stoichiometric coefficients physical state s, l, g or aq Slide 3 3 Chemical Equations 4 Al (s) + 3 O 2 (g) 2 Al 2 O 3 (s) 4 Al (s) + 3 O 2 (g) 2 Al 2 O 3 (s) Interpret this equation as 4 atoms solid Al react with 3 molecules gaseous O 2 to form 2 formula units of solid Al 2 O 3 4 moles solid Al react with 3 moles gaseous O 2 to form 2 moles of solid Al 2 O 3 Slide 4 4 Balancing Chemical Equations Matter is conserved in chemical change chemical change Antoine Lavoisier, 1789 An equation must be balanced: It must have the same number of atoms of each kind on both sides Slide 5 5 The rules of the game Write the correct formulas of the reactants and products Do not change the formulas to balance the equation Put a coefficient in front of each formula so that the same number of atoms of each kind appear in both the reactants and products The coefficent multiplies through its formula 2 H 2 O shows 4 H atoms and 2 O atoms Slide 6 6 Combustion Reactions In combustion, a hydrocarbon or CHO fuel combines with O 2 to form CO 2 and H 2 O __ CH 4 + __ O 2 __ CO 2 + __ H 2 O 1 CH 4 + 2 O 2 1 CO 2 + 2 H 2 O Balanced equation shows 1 C, 4 H, and 4 O on each side If N or S are in the formula for the fuel, assume it is oxidized to NO 2 or SO 2 Slide 7 7 Example 4-1 Write a balanced equation for the complete combustion of ethylene, C 2 H 4. Write and balanced equation for the complete combustion of thiophene, C 4 H 4 S Slide 8 8 Example 4-2 Write a balanced equation for the complete combustion of glycerol, C 3 H 8 O 3 Write a balanced equation for the complete combustion of thiosalicylic acid, C 7 H 6 O 2 S Slide 9 9 Stoichiometry Stoichiometry is chemical accounting The heart of stoichiometry is the mole ratio given by the coefficients of the balanced equation Slide 10 10 Stoichiometry Stoichiometry is chemical accounting The heart of stoichiometry is the mole ratio given by the coefficients of the balanced equation moles A moles B mole ratio moles B moles A Slide 11 11 Example 4-3A How many moles of O 2 are produced form the decomposition of 1.76 mol potassium chlorate: 2 KClO 3 (s) 2 KCl (s) + 3 O 2 (g) Slide 12 12 Example 4-3B How many moles of Ag are produced in the decomposition of 1.00 kg of silver (I) oxide: 2 Ag 2 O (s) 4 Ag (s) + O 2 (g) Slide 13 13 Example 4-4A How many grams of magnesium nitride are produced when 3.82 g Mg react with excess N 2 : 3 Mg (s) + N 2 (g) Mg 3 N 2 (s) Slide 14 14 Example 4-4B How many grams of H 2 (g) are needed to produce 1.00 kg of CH 3 OH: CO (g) + 2 H 2 (g) CH 3 OH (l) Slide 15 15 Example 4-5A How many grams of H 2 are consumed per gram of O 2 in the reaction 2 H 2 (g) + O 2 (g) 2 H 2 O (l) Slide 16 16 Example 4-5B How many grams of O 2 are consumed per gram of octane (C 8 H 18 ) in the combustion of octane? Slide 17 17 Example 4-6A The model problem describes an Al-Cu alloy composed of 93.7% Al and 6.3% Cu by mass, with a density of 2.85 g/cm 3. The Al (but not the Cu) reacts with HCl: 2 Al (s) + 6 HCl (aq) 2 AlCl 3 (aq) + 3 H 2 (g) What volume of the Al-Cu alloy must be dissolved in HCl to produce 1.00 g H 2 ? Slide 18 18 Example 4-6B The model problem describes an Al-Cu alloy composed of 93.7% Al and 6.3% Cu by mass, with a density of 2.85 g/cm 3. The Al (but not the Cu) reacts with HCl: 2 Al (s) + 6 HCl (aq) 2 AlCl 3 (aq) + 3 H 2 (g) How many grams of Cu are present in a sample of alloy that yields 1.31 g H 2 when it reacts with HCl? Slide 19 19 Example 4-7A The model problem describes an HCl solution which is 28% HCl by mass and has a density of 1.14 g/mL. It reacts with Al: 2 Al (s) + 6 HCl (aq) 2 AlCl 3 (aq) + 3 H 2 (g) How many mg of H 2 are produced when 1 drop (0.05 mL) of the HCl solution reacts with Al? Slide 20 20 Example 4-7B A vinegar contains 4.0% HC 2 H 3 O 2 by mass and has a density of 1.01 g/mL. It reacts with sodium hydrogen carbonate: HC 2 H 3 O 2 (aq) + NaHCO 3 (s) NaC 2 H 3 O 2 (aq) + H 2 O (l) + CO 2 (g) How many grams of CO 2 are produced by the reaction of 5.00 mL of this vinegar with NaHCO 3 ? Slide 21 21 Chemical Reactions in Solution Most reactions occur in aqueous solution SOLUTE is the substance to be dissolved in solution SOLVENT is the substance (often a liquid) the solute dissolves in The concentration of the solution is Molarity (M) = moles solute L solution Slide 22 22 Example 4-8A If 22.3 g acetone, (CH 3 ) 2 CO, are dissolved in enough water to make 1.25 L of solution, what is the concentration (M) of the solution? Slide 23 23 Example 4-8B 15.0 mL of concentrated acetic acid, HC 2 H 3 O 2 (d = 1.048 g/mL), are dissolved in enough water to produce 500.0 mL of solution. What is the concentration of the solution? Slide 24 24 Example 4-9A At 25 C, an aqueous solution saturated with NaNO 3 is 10.8 M NaNO 3. How many grams of NaNO 3 are present in 125 mL of this solution? Slide 25 25 Example 4-9B How many grams of Na 2 SO 4 10 H 2 O are needed to prepare 355 mL of 0.445 M Na 2 SO 4 ? Slide 26 26 Dilution problems It is common to prepare a solution by diluting a more concentrated solution (the stock solution). The moles of solute taken from the stock solution are given by moles solute = volume x molarity All the solute taken from the stock appears in the diluted solution, so moles solute are constant: V stock M stock = V dilute M dilute Slide 27 27 Example 4-10A 15.00 mL of 0.450 M K 2 CrO 4 solution are diluted to 100.00 mL. What is the concentration of the dilute solution? Slide 28 28 Example 4-10B After being left out in an open beaker, 275 mL of 0.105 M NaCl has evaporated to only 237 mL. What is the concentration of the solution after evaporation? Slide 29 29 Stoichiometry in Solution Stoichiometry in solution is just the same as for mass problems, except the conversion into or out of moles uses molarity instead of molar mass: grams A mL A moles A moles B grams B mL B mole ratio moles B moles A Slide 30 30 Example 4-11A K 2 CrO 4 (aq) + 2 AgNO 3 (aq) Ag 2 CrO 4 (s) + 2 KNO 3 (aq) How many mL of 0.250 M K 2 CrO 4 must react with excess AgNO 3 to produce 1.50 g Ag 2 CrO 4 ? Slide 31 31 Example 4-11B K 2 CrO 4 (aq) + 2 AgNO 3 (aq) Ag 2 CrO 4 (s) + 2 KNO 3 (aq) How many mL of 0.150 M AgNO 3 must react with excess K 2 CrO 4 to produce exactly 1.00 g Ag 2 CrO 4 ? Slide 32 32 Limiting reactant In a given reaction, often there is not enough of one reactant to use up the other reactant completely In a given reaction, often there is not enough of one reactant to use up the other reactant completely The reactant in short supply LIMITS the quantity of product that can be formed The reactant in short supply LIMITS the quantity of product that can be formed Slide 33 33 Slide 34 34 Goldilocks Chemistry Imagine reacting different amounts of Zn with 0.100 mol HCl: Zn (s) + 2 HCl (aq) ZnCl 2 (aq) + H 2 (g) Rxn 1Rxn 2Rxn 3 Mass Zn6.54 g3.27 g1.31 g Moles Zn0.100 mol0.0500 mol0.0200 mol Moles HCl0.100 mol0.100 mol0.100 mol Ratio mol HCl1.002.005.00 mol Zn Slide 35 35 Limiting reactant problems The easiest way to do these is to do two stoichiometry calculations Find the amount of product possible from each reactant The smaller answer is the amount of product you can actually make (you just ran out of one reactant) The reactant on which that answer was based is the limiting reactant Slide 36 36 Example 4-12A How many grams of PCl 3 form when 215 g P 4 react with 725 g Cl 2 : P 4 (s) + 6 Cl 2 (g) 4 PCl 3 (l) Slide 37 37 Example 4-12B How many kg of POCl 3 form if 1.00 kg of each reactant are allowed to react: 6 PCl 3 (l) + 6 Cl 2 (g) + P 4 O 10 (s) 10 POCl 3 (l) Slide 38 38 Example 4-13A When 215 g P 4 react with 725 g Cl 2 P 4 (s) + 6 Cl 2 (g) 4 PCl 3 (l) (example 4-12A) which reactant is in excess and what mass of that reactant remains after the reaction is finished? Slide 39 39 Example 4-13B 12.2 g H 2 and 154 g O 2 are allowed to react. Identify the limiting reactant, which gas remains after the reaction, and what mass of it is left over. 2 H 2 (g) + O 2 (g) 2 H 2 O (l) Slide 40 40 Percent Yield In real experiments we often do not get the amount of product we calculate we should, because the reactants may participate in other reactions (side reactions) that produce other products (by-products) The reaction often does not go to completion. Percent yield tells the ratio of actual to theoretical amount formed. Slide 41 41 Percent Yield Suppose you calculate that a reaction will produce 50.0 g of product. This is the theoretical yield. The reaction actually produces only 45.0 g of product. This is the actual yield. Percent yield = 45.0 g (actual) x 100 = 90.0% 50.0 g (theoretical) Slide 42 42 Example 4-14A If 25.7 g CH 2 O is produced per mole CH 3 OH that reacts, what are the theoretical, actual, and percent yield: CH 3 OH (g) CH 2 O (g) + H 2 (g) Slide 43 43 Example 4-14B What is the percent yield if 25.0 g P 4 reacts with 91.5 g Cl 2 to produce 104 g PCl 3 : P 4 (s) + 6 Cl 2 (g) 4 PCl 3 (l) Slide 44 44 Example 4-15A If the observed percent yield for the formation of urea is 87.5%, what mass of CO 2 must react with excess NH 3 to produce 50.0 g CO(NH 2 ) 2 : 2 NH 3 (g) + CO 2 (g) CO(NH 2 ) 2 (s) + H 2 O (l) Slide 45 45 Example 4-15B What mass of C 6 H 11 OH should you start with to produce 45.0 g C 6 H 10 if the reaction has 86.2% yield and the C 6 H 11 OH is 92.3% pure: C 6 H 11 OH (l) C 6 H 10 + H 2 O (l) Slide 46 46 Exercise 26 Balance these equations by inspection (NH 4 ) 2 Cr 2 O 7 (s) Cr 2 O 3 (s) + N 2 (g) + H 2 O (g) NO 2 (g) + H 2 O (l) HNO 3 (aq) + NO (g) H 2 S (g) + SO 2 (g) S (g) + H 2 O (g) SO 2 Cl 2 + HI H 2 S + H 2 O + HCl + I 2 Slide 47 47 Exercise 30 Write balanced equations for these reactions: Sulfur dioxide gas with