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Chemical Reactions

Chemical Reactions. Chapter 4 Stoichiometry. Balancing Chemical Equations. “Matter is conserved in chemical change” Antoine Lavoisier, 1789 An equation must be balanced: It must have the same number of atoms of each kind on both sides. Combustion Reactions.

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Chemical Reactions

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  1. ChemicalReactions Chapter 4 Stoichiometry

  2. Balancing Chemical Equations “Matter is conserved in chemical change” Antoine Lavoisier, 1789 An equation must be balanced: It must have the same number of atoms of each kind on both sides

  3. Combustion Reactions • In combustion, a hydrocarbon or C–H–O fuel combines with O2 to form CO2 and H2O __ CH4 + __ O2 __ CO2 + __ H2O 1 CH4 + 2 O2 1 CO2 + 2 H2O 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 NO2 or SO2

  4. Example 4-2 • Write a balanced equation for the complete combustion of glycerol, C3H8O3 • Write a balanced equation for the complete combustion of thiosalicylic acid, C7H6O2S

  5. Stoichiometry • Stoichiometry is chemical accounting • The heart of stoichiometry is the mole ratio given by the coefficients of the balanced equation

  6. Stoichiometry • Stoichiometry is chemical accounting • The heart of stoichiometry is the mole ratio given by the coefficients of the balanced equation mole ratio moles B moles A moles A moles B

  7. Example 4-3B • How many moles of Ag are produced in the decomposition of 1.00 kg of silver (I) oxide: 2 Ag2O (s)  4 Ag (s) + O2 (g)

  8. 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/cm3.The Al (but not the Cu) reacts with HCl: 2 Al (s) + 6 HCl (aq)  2 AlCl3 (aq) + 3 H2 (g) • How many grams of Cu are present in a sample of alloy that yields 1.31 g H2 when it reacts with HCl?

  9. Example 4-7B • A vinegar contains 4.0% HC2H3O2 by mass and has a density of 1.01 g/mL. It reacts with sodium hydrogen carbonate: HC2H3O2 (aq) + NaHCO3 (s)  NaC2H3O2 (aq) + H2O (l) + CO2 (g) How many grams of CO2 are produced by the reaction of 5.00 mL of this vinegar with NaHCO3?

  10. 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

  11. Example 4-8B • 15.0 mL of concentrated acetic acid, HC2H3O2 (d = 1.048 g/mL), are dissolved in enough water to produce 500.0 mL of solution. What is the concentration of the solution?

  12. Example 4-9B • How many grams of Na2SO4 • 10 H2O are needed to prepare 355 mL of 0.445 M Na2SO4?

  13. 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: VstockMstock = VdiluteMdilute

  14. Example 4-10A • 15.00 mL of 0.450 M K2CrO4 solution are diluted to 100.00 mL. What is the concentration of the dilute solution?

  15. 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?

  16. grams A grams B mole ratio moles B moles A moles A moles B mL A mL B 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:

  17. Example 4-11B K2CrO4 (aq) + 2 AgNO3 (aq)  Ag2CrO4 (s) + 2 KNO3 (aq) • How many mL of 0.150 M AgNO3 must react with excess K2CrO4 to produce exactly 1.00 g Ag2CrO4?

  18. Limiting reactant • 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

  19. Goldilocks Chemistry • Imagine reacting different amounts of Zn with 0.100 mol HCl: Zn (s) + 2 HCl (aq)  ZnCl2 (aq) + H2 (g) Rxn 1Rxn 2Rxn 3 Mass Zn 6.54 g3.27 g1.31 g Moles Zn 0.100 mol0.0500 mol0.0200 mol Moles HCl 0.100 mol0.100 mol0.100 mol Ratio mol HCl1.002.005.00 mol Zn

  20. 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

  21. Example 4-13A • When 215 g P4 react with 725 g Cl2 P4 (s) + 6 Cl2 (g)  4 PCl3 (l) (example 4-12A) which reactant is in excess and what mass of that reactant remains after the reaction is finished?

  22. Example 4-13B • 12.2 g H2 and 154 g O2 are allowed to react. Identify the limiting reactant, which gas remains after the reaction, and what mass of it is left over. 2 H2 (g) + O2 (g)  2 H2O (l)

  23. 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.

  24. 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)

  25. Example 4-14B • What is the percent yield if 25.0 g P4 reacts with 91.5 g Cl2 to produce 104 g PCl3: P4 (s) + 6 Cl2 (g)  4 PCl3 (l)

  26. Example 4-15B • What mass of C6H11OH should you start with to produce 45.0 g C6H10 if the reaction has 86.2% yield and the C6H11OH is 92.3% pure: C6H11OH (l)  C6H10 + H2O (l)

  27. Exercise 26 • Balance these equations by inspection • (NH4)2Cr2O7 (s)  Cr2O3 (s) + N2 (g) + H2O (g) • NO2 (g) + H2O (l)  HNO3 (aq) + NO (g) • H2S (g) + SO2 (g)  S (g) + H2O (g) • SO2Cl2 + HI  H2S + H2O + HCl + I2

  28. Exercise 30 • Write balanced equations for these reactions: • Sulfur dioxide gas with oxygen gas to produce sulfur trioxide gas • Solid calcium carbonate with water and dissolved carbon dioxide to produce aqueous calcium hydrogen carbonate • Ammonia gas and nitrogen monoxide gas to produce nitrogen gas and water vapor

  29. Exercise 32 • 3 Fe (s) + 4 H2O (g)  Fe3O4 (s) + H2 (g) • How many moles of H2 can be produced from 42.7 g Fe and excess steam? • How many grams of H2O are consumed in the conversion of 63.5 g Fe to Fe3O4? • If 7.36 mol H2 are produced, how many grams of Fe3O4 must also be produced?

  30. Exercise 36 • Silver oxide decomposes above 300 °C to yield metallic silver and oxygen gas. 3.13 g impure silver oxide yields 0.187 g O2. Assuming there is no other source of O2, what is the % Ag2O by mass in the original sample?

  31. Exercise 42 • How many grams of CO2 are produced in the complete combustion of 406 g of a bottled gas that consists of 72.7% C3H8 (propane) and 27.3% C4H10 (butane), by mass?

  32. Exercise 45 • What are the molarities of these solutes? • 150.0 g sucrose (C12H22O11) in 250.0 mL aqueous solution • 98.3 mg of 97.9% pure urea, CO(NH2)2, in 5.00 mL aqueous solution • 12.5.0 mL methanol (CH3OH, density = 0.792 g/mL) in 15.0 L aqueous solution

  33. Exercise 52 • After 25.0 mL of aqueous HCl solution is diluted to 500.0 mL, the concentration of the diluted solution is found to be 0.085 M HCl. What was the concentration of the original HCl solution?

  34. Exercise 56 • Ca(OH)2 (s) + 2 HCl (aq)  CaCl2 (aq) + 2 H2O (l) • How many grams of Ca(OH)2 will react completely with 415 mL of 0.477 M HCl? • How many kilograms of Ca(OH)2 will react with 324 L of an HCl solution that is 24.28% HCl by mass, density = 1.12 g/mL?

  35. Exercise 63 • 0.3126 g oxalic acid, H2C2O4, is exactly neutralized by 26.21 mL of a NaOH solution. What is the concentration of the NaOH solution? H2C2O4 + 2 NaOH  Na2C2O4 + 2 H2O

  36. Exercise 70 • Chlorine can be generated by heating calcium hypochlorite and hydrochloric acid to form chlorine gas, calcium chloride, and water. If 50.0 g Ca(OCl)2 and 275 mL 6.00 M HCl react, how many grams of Cl2 gas form? Which reactant is left over, and how much (in grams)?

  37. Exercise 72 2 C6H5NO2 + 4 C6H14O4 (C6H5N)2 + 4 C6H12O4 + 4 H2O nitrobenzene triethylene azobenzene glycol • If 0.10 L nitrobenzene (d = 1.20 g/mL) react with 0.30 L triethylene glycol (d = 1.12 g/mL) to form 55 g azobenzene, find • Theoretical yield • Actual yield • Percent yield

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