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Chapter 8 (8.1-8.3)

Chapter 8 (8.1-8.3). Chemical Reactions. Chemical Reactions and Equations. What is a chemical reaction? A chemical equation uses symbols to represent what happens at the atomic level. What happens when you boil water? What happens during the electrolysis of water?

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Chapter 8 (8.1-8.3)

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  1. Chapter 8 (8.1-8.3) Chemical Reactions

  2. Chemical Reactions and Equations • What is a chemical reaction? • A chemical equation uses symbols to represent what happens at the atomic level. • What happens when you boil water? What happens during the electrolysis of water? • Chemical equations follow the Law of Conservation of Mass • Conservation of atoms, not conservation of molecules (molecular structures can change) Reaction Equation 2

  3. Balancing Equations • Adjust coefficients, not subscripts, to balance an equation • Coefficients are whole-number integers in the lowest ratio • Reactants  Products • H2 + O2  H2O 3

  4. Balancing Equations Coefficients vs. Subscripts

  5. Balancing Equations • Draw structures for the following substances: • H2, Br2, HBr, CO, CO2, Cl2, AlCl3, N2, H2, NH3 • Use your drawings to balance the equations below. Add more substances as needed by adding coefficients. • H2 (g) + Br2 (l)  HBr (g) • CO (g) + O2 (g)  CO2 (g) • Al (s) + Cl2 (g)  AlCl3 (g) • N2 (g) + H2 (g)  NH3 (g)

  6. Guidelines to Balancing Equations • Write correct formulas for reactants and products. • Begin balancing with the most complex formula. • Balance polyatomic ions as a single unit. • Check each reactant and product to verify the coefficients. • Mg(NO3)2 + Cr2(SO4)3 MgSO4 + Cr(NO3)3 6

  7. Practice Balancing Equations Gummy Bear • Balance the following equations: • Mg + O2 MgO • H2O2 H2O + O2 • NaN3Na + N2 • CaCO3 CaO + CO2 • Mg3(PO4)2 + NaOH  Mg(OH)2 + Na3PO4 • KClO3  KCl + O2 • Na + H2O  NaOH + H2 • AgNO3 + MgCl2 AgCl + Mg(NO3)2 • CH3OH + O2 CO2 + H2O Na + H2O Alkali metals in H2O Burn methanol 7

  8. Practice Balancing Equations • Balanced equations: • 2Mg + O22 MgO • 2 H2O22 H2O + O2 • 2 NaN32Na + 3 N2 • CaCO3 CaO + CO2 • Mg3(PO4)2 + 6 NaOH  3 Mg(OH)2 + 2 Na3PO4 • 2 KClO3  2 KCl + 3 O2 • 2 Na + 2 H2O  2 NaOH + H2 • 2 AgNO3 + MgCl2 2 AgCl + Mg(NO3)2 • 2 CH3OH + 3 O22 CO2 + 4 H2O 8

  9. Group Quiz #17 • Balance the following equations: • __Fe2(SO4)3 + __K3PO4  __K2SO4 + __FePO4 • __Hg(NO3)2 + __ KI __HgI2 + __ KNO3 • __Li2O(s) + __ H2O  __LiOH • __HBr + __Ba(OH)2 __BaBr2 + __H2O • __K2PtCl4 + __NH3 __Pt(NH3)2Cl2 + __KCl

  10. Chapter 2 • Section 2.6

  11. Amounts of Reactants and Products • In lab, we want to predict amount of product we can expect from a reaction • Have to use stoichiometry: quantitative study of reactants and products in a reaction (mole to mole ratio) • Use the mole to relate one substance in a reaction to another (equation coefficients, see Table 3.1) • 2H2 (g) + O2 (g)  2H2O (g)

  12. Mole Ratios • We use coefficients in a balanced equation to determine the mole ratio in which substances need to react. • MgCl2(aq) +2AgNO3(aq) 2AgCl(s) + Mg(NO3)2(aq) • 1 molecule of MgCl2 reacts with 2 molecules of AgNO3 to produce 2 molecules of AgCl and 1 molecule of Mg(NO3)2 • Molecules are not practical! Scale up to moles!

  13. 2 ways to calculate moles of substance • Use molar mass: • Mass ÷ molar mass = moles • Concentration: amount of solute present in a given amount of solution • Molarity: moles of solute in 1 L of solution • moles solute / liters solution; M = mol / L • If I have 1.50 mL of a 0.100 M solution of NaCl, how many moles are in the sample? • M * V = moles 13

  14. Mole Diagram Mass Mole Use coefficients from equation molar mass Mole 1 mole = 6.022 x 1023 particles (atoms or molecules) volume Molarity Particles Courtesy: www.unit5.org/christjs/Stoichiometry/Mole%20Island%20Diagram.ppt

  15. Stoichiometry Calculations • Mole to mole conversions • MgCl2(aq) +2AgNO3(aq) 2AgCl(s) + Mg(NO3)2(aq) 1.00 mol xs ? • How many moles of solid product do you predict to form? • Cannot compare grams of one to grams of another!! 15

  16. Mole Ratios • Predict products (with phases) and balance each equation. • Calculate the moles of precipitate (ppt) that can be made in the following reactions. • NaCl (aq) + Hg(NO3)2 (aq)  • 0.500 mol NaCl • FeCl3 (aq) + NaOH (aq)  • 2.35 mol NaOH • Li2SO4 (aq) + AgCH3COO (aq)  • 0.1098 mol AgCH3COO

  17. Avogadro’s Number and the Mole • We need to be able to determine amounts of chemicals to use so they react in the exact ratio needed. • For example, we need 2 molecules of H2 and 1 molecule of O2 to make 2 molecules of H2O. • Do you want to count individual atoms and molecules? Size of atoms

  18. Avogadro’s Number • Who wants to count: Peppers? Water Molecules? Grains of rice?

  19. The Mole • We can measure the mass of a pure sample but how do we know how many atoms or molecules are in that sample? • Mole: amount of a substance that contains a specific number of particles. • A mole (mol) is a quantity that tells us two things: • the number of particles in a substance • the mass of that substance (containing a specific number of particles)

  20. Avogadro’s Number and the Mole • 1 12C atom weighs 12 amu, 1 mole of 12C atoms weighs 12 grams. • 1 dozen = 12 objects; 1 mole = 6.022 x 1023 objects. This is Avogadro’s number. • The moral: 1 mole of carbon-12 weighs 12 grams and contains 6.022 x 1023 atoms.

  21. Avogadro’s Number and the Mole • How big IS Avogadro’s number? • An Avogadro's number of standard soft drink cans would cover the surface of the earth to a depth of over 200 miles. • If you had Avogadro's number of unpopped popcorn kernels, and spread them across the US, the country would be covered in popcorn to a depth of over 9 miles. • If we were able to count atoms at the rate of 10 million per second, it would take about 2 billion years to count the atoms in one mole.

  22. Avogadro’s Number and the Mole • What do these substances have in common? • Atoms measured in amu, moles measured in grams • amu and g are the same value, different units • 1 atom 12C = 12 amu; 1 mol 12C atoms = 12 g • 1 atom 4He = 4 amu; 1 mol 4He atoms = 4 g 22

  23. Avogadro’s Number and the Mole • 1 mole contains 6.0221415 x 1023 particles • Particles can be atoms, ions, molecules, or formula units • Determine what type of particle Avogadro’s number will describe for the following: • Kr • C6H12O6 • MgCO3 • SF6 • CaBr2 • P

  24. Molar Masses • Atomic, molecular, and formula masses are not practical in lab. We can scale up to calculate the mass of a mole of substances. • Molar mass is the same value as atomic mass, just with different units. Molar mass (g/mol: (mass per 1 mole of substance) • What is molar mass of C6H6? • What is molar mass of Mg(OH)2? • Molar masses have 6.022 x 1023 particles in the substance.

  25. Calculate Molar Masses of the following: • Al • CO2 • Cl2 • SO2 • CaCl2 • H2SO4 • AlCl3 • Mg3(PO4)2 • Na2CO3

  26. Avogadro’s Number and the Mole • Conversions: • If we know the mass of a substance, how can we calculate number of moles? • mass  moles (use molar mass) • If we know how many moles of a substance, how can we calculate number of particles? • moles  number of particles (use Avogadro’s number)

  27. Mole Diagram Mass molar mass Mole 1 mole = 6.022 x 1023 particles (atoms or molecules) Particles Courtesy: www.unit5.org/christjs/Stoichiometry/Mole%20Island%20Diagram.ppt

  28. Practice – Group Work • How many moles are in a 2.67 g sample of Ag? • How many grams are in a 0.45 mol sample of Mg3(PO4)2? • How many atoms are in a 16.3 g sample of S? • How many formula units are in 0.81 g of MgCl2? • How many ions are in 0.81 g of MgCl2? (Hint: How many ions are in 1 formula unit of MgCl2?) • How many moles of ions are in 0.81 g of MgCl2? • How many Catoms are in 3.64 g of C6H12O6? • How many O atoms are in 0.0765 g of CO2 • Answers: 0.0248 mol Ag, 1.2 x 102 g Mg3(PO4)2, 3.06 x 1023 S atoms, 5.12 x 1021 f. un., 1.54 x 1022 ions, 0.026 mol ions, 7.30 x 1022 C atoms, 2.09 x 1021 O atoms

  29. Practice – Group Work • Calculate the number of atoms that are in a 6.00 g sample of carbon. • Calculate the number of moles in 12.04 x 1023 atoms of uranium. • Calculate the mass of 12.04 x 1023 atoms of uranium. • If you have 1 g samples of NaCl, CO2, and H2O, which sample will contain the fewest particles? • Answers: 4.34 x 1025 C atoms, 2 mole Uranium, 476.1 g Uranium, NaCl (fewest moles because largest MM)

  30. Group Quiz #18 • How many atoms are in 12.987 g of iron? • How many H atoms are in a 3.4215 gram sample of H2O?

  31. The End • Atom/molecule/mole ratios in C6H12O6

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