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Stoichiometry

Learn about stoichiometry and how to predict the amounts of reagents needed or the amounts of products made in a chemical reaction. Topics include composition stoichiometry, mass relationships, balanced equations, and more.

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Stoichiometry

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  1. Stoichiometry Predicting amounts of reagents needed or amounts of products made

  2. Stoichiometry • Composition stoichiometry • Mass relationships of elements in compounds • Reaction stoichiometry • Mass relationships between participants in a chemical reaction • Must begin all stoichiometry problems with a BALANCED chemical equation.

  3. Conservation of Charge • Balanced chemical equation must be balanced for both mass and charge. • Total charge on reactant side must equal total charge on product side. • So far, most of the equations you have seen are neutral on each side, but this is not always the case.

  4. Coefficients in Balanced Equations • Give relative number of particles in rxn. • Give relative number of moles in rxn. • Coefficients give relationship between moles of each component in equation. • You use these relationships in MOLE-MOLE problems to convert from moles of 1 substance to moles of another substance.

  5. Stoichiometry Problems • Come in several flavors. • Start with the simplest: mole-mole • Given: amount of one substance in moles • Unknown: amount of some other substance in moles

  6. Mole-mole stoichiometry problem • Stoichiometry problems are factor-label problems or conversion problems. • Use the balanced chemical equation to generate the conversion factors.

  7. 2 C2H6 + 7 O2 4 CO2 + 6 H2O • How much CO2 will be produced if 4 moles of C2H6 are consumed? Identify a stoichiometry problem: 1) You always get a balanced chemical equation. 2) The problem will ask how many moles or how many liters of one species given a certain amount of another species. 3) The equation has a  arrow.

  8. 2C2H6 + 7 O2 4CO2 + 6 H2O • How much CO2 will be produced if 4 moles of C2H6 are consumed? • Identify given: 4 moles of C2H6. • Identify conversion factor from equation: 4 moles CO2 2 moles C2H6

  9. Conversion 4 moles C2H6 4 moles CO2 = 2 moles C2H6 4 X 4 moles CO2 = 8 moles CO2 2

  10. 2C2H6 + 7 O2 4CO2 + 6 H2O • How much H2O will be produced in the combustion of 5 moles of C2H6? • Given: 5 mol C2H6 • Conversion factor: 6 mol H2O 2 mol C2H6

  11. Conversion 5 mol C2H6 6 mol H2O = 2 mol C2H6 5 X 6 mol H2O = 15 mol H2O 2

  12. 2C2H6 + 7 O2 4CO2 + 6 H2O • How much oxygen will react with 5 moles of C2H6? • Given: 5 moles C2H6 • Conversion factor: 7 mol O2 2 mol C2H6

  13. Conversion 5 moles C2H6 7 mol O2 = 2 mol C2H6 5 X 7 mol O2 = 17.5 mol O2 2

  14. Other flavors of problems • Other types of stoichiometry problems include: • Volume-Volume (for gas phase only) • Mole-mass or mass-mole • Mass-Mass

  15. Gas-Phase Equations • 1 mole of any gas occupies the same volume as any other gas. At STP, 22.4 L. • So coefficients in equations represent the ratio of the volumes of gases involved in the reaction. • For rxns where all reactants & products are gases,  volume-volume problems. • Unit doesn’t matter as long as it’s constant throughout.

  16. N2(g) + 3 H2(g)  2 NH3(g) • How much hydrogen gas will react with 15 liters of nitrogen gas? • Given: 15 L of N2(g) • Conversion factor: 3 L H2 1 L N2

  17. Conversion 3 L H2 15 L N2(g) = 1 L N2 15 X 3 L H2 = 45 L H2 1

  18. Other types of stoich problems • General strategy: • Convert given into moles • Perform stoichiometric calculation using mole ratios from balanced equation • Convert to desired unit

  19. A word about … Double Replacement Reactions

  20. Reactions in aqueous solution • Many reactions, esp. many double replacement reactions, occur in water. • What happens when substances dissolve in water? • Depends on if they are ionic or covalent.

  21. Dissolving • Covalent substance – sugar or C6H12O6 • C6H12O6(s)  C6H12O6(aq) • The sugar molecules are spread out among the water molecules.

  22. Dissolving • Ionic substance – table salt or NaCl • NaCl(s)  Na+(aq) + Cl-(aq) • The ions are spread out among the water molecules.

  23. Double Replacement Reactions • Often occur when you mix 2 solutions of ionic compounds. • 1 product may be water or • 1 product may be a gas or • 1 product may be a solid • We say these reactions “go to completion.”

  24. Reactions producing Solids • Precipitation: the opposite of dissolving! • What do you see in the following clips: • S1043.mov • S1045.mov • S1046.mov • S1050.mov • S1057.mov • S1058.mov and S1060.mov

  25. Reactions producing Solids 2 NaOH(aq) + CuCl2(aq)  2 NaCl(aq) + Cu(OH)2(s) Complete Chemical Equation

  26. Complete Ionic Equations 2Na+(aq) + 2OH-(aq) + Cu2+(aq) + 2Cl-(aq)  2Na+(aq) + 2Cl-(aq) + Cu(OH)2(s) Substances that are ions in solution are written as ions in solution.

  27. Net Ionic Equations 2Na+(aq) + 2OH-(aq) + Cu2+(aq) + 2Cl-(aq)  2Na+(aq) + 2Cl-(aq) + Cu(OH)2(s) Notice that some ions do not participate in the reaction. They are spectator ions. Cross out all the spectator ions & you get the net ionic equation. 2OH-(aq) + Cu2+(aq)  Cu(OH)2(s)

  28. Reactions that form Water HBr(aq) + NaOH(aq)  H2O(l) + NaBr(aq) H+(aq) + Br-(aq) + Na+(aq) + OH-(aq)  H2O(l) + Na+(aq) + Br-(aq) H+(aq) + OH-(aq)  H2O(l)

  29. Reactions that form Gases HCl(aq) + NaHCO3(aq)  H2CO3(aq) + NaCl(aq) H2CO3(aq)  H2O(l) + CO2(g) ______________________________________ HCl(aq) + NaHCO3(aq)  H2O(l) + CO2(g) + NaCl(aq)

  30. Reactions that form Gases HCl(aq) + NaHCO3(aq)  H2O(l) + CO2(g) + NaCl(aq) H+(aq) + Cl-(aq) + Na+(aq) + HCO3-(aq)  H2O(l) + CO2(g) + Na+(aq) + Cl-(aq) H+(aq) + HCO3-(aq)  H2O(l) + CO2(g)

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