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Enduring Understanding 3.B.1: Synthesis and Decomposition

  • Chemical reactions are classified depending on what the reactants and products are, and on how they change from one into the other.
  • Examples of chemical reactions include:
  • Synthesis
  • Decomposition
  • Acid-base
  • Oxidation-reduction (redox)

  • Synthesis reactions involve multiple atoms or molecules combining to form a new compound.
  • Examples:
    • 2Mg +O2 → 2MgO
    C2H4 + H2 → C2H6

  • Decomposition reactions are the opposite of synthesis, in which a substance decomposes into simpler components.
  • Decomposition reactions often involve the use of heat.
  • Decomposition reactions are often used in analysis, like when removing oxygen or water of hydration.
  • Examples:
    • 2 HgO + heat → 2Hg + O2
    CuSO4.5H2O + heat → CuSO4 + 5 H2O

  • Sample Question 1: Silver can exist as different oxides. An 11.60 g sample of a silver oxide was heated in a crucible to a constant mass of 10.80 g. What was the empirical formula of the oxide? MW of silver = 107.87 g/mol
  • Assume the reaction is AgOx → Ag + (x/2) O2, as has gone to completion.
  • The mass of silver present is 10.8 g, so the number of moles present is 10.8/107.87 or 0.100 mol.
  • The amount of O atoms lost is the difference in mass, 11.60-10.80 or 0.80 g.
  • This corresponds to 0.80 g / 16.00 g/mol, or 0.050 mol
  • The ratio of Ag to O is 0.10:0.05 or 2:1
  • The formula of the silver oxide must be Ag2O.

  • Sample Question 2: An element, M, can exist as both MCl2 and MCl4. When 17.7 g of MCl2 reacts completely with excess chlorine, 24.8 g of MCl4 is produced. Is M most likely S, Ti, or Pd?
  • Assume the reaction: MCl2 + Cl2 → MCl4
  • The amount of Cl2 added is (24.8 g -17.7 g) or 7.1 g Cl2
  • This corresponds to: 7.1 g/ 70.91 g/mol = 0.10 mol Cl2
  • This means there was 0.10 mol MCl2 in the starting material, so the molar mass of MCl2 is: 17.7 g/ 0.10 mol = 177 g/mol
  • The molar mass of the unknown M in MCl2 is 177 - (2 x 35.45) = 106 g/mol
  • This corresponds best to palladium, Pd, with a molar mass of 106.42 g/mol.



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