Exp 3B - Hydrates and Thermal Decomposition

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Exp 3B - Hydrates and Thermal Decomposition

• Background
• In chemistry, a neutral ionic compound is
  typically referred to as a salt
• Hydrates are inorganic salts that contain a
  specific number of water molecules crystallized
  with the salt (water of crystallization or
  water of hydration)
• Examples are
• CaCl2.4H2O 4 mol H2O/mol CaCl2
• CuSO4.5H2O 5 mol H2O/mol CuSO4 (blue)
• BaCl2.2H2O 2 mol H2O/mol BaCl2
• Na2HPO4.12 H2O 12 mol H2O/mol Na2HPO4
• Most hydrates have simple formulas with a ratio
  of 1 salt xH2O,but it is not always the case
• cadmium sulfate hydrate is best represented as
  (CdSO4)3.8H2O
• When determining the formula of a hydrate you
  must not assume that it is one with a simple
  formula.

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Exp 3B - Hydrates and Thermal DecompositionAdsorp
tion and Absorption

• Salts attract water from air and gain (some)
  weight
• Adsorption of H2O on crystal surface
• Some salts incorporate H2O in crystal structure
• Absorption of H2O
• Definite amount of H2O as part of crystal
  structure
• Hydrates or Hydrate Salts

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Exp 3B Hydrates and Thermal Decomposition

• Hydrates can normally be decomposed into the
  anhydrous (without water) salt and water by
  gentle heating.
• From mass data collected the number of molecules
  of hydrated water can be determined per molecule
  of anhydrous salt.
• Heating removes H2O gradually
• yields anhydrous form
• CoCl2.6H2O 6 mol H2O/mol CoCl2 (red)
  (hexahydrate)
• CoCl2.2H2O 2 mol H2O/mol CoCl2 (violet)
  (dihydrate)
• CoCl2 no H2O, anhydrous (blue)
• CoCl2.6H2O CoCl2.2H2O CoCl2

D
D
violet
-2 H2O
-4 H2O
red
blue
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Exp 3B Hydrates and Thermal Decomposition

• Thermal decomposition of BaCl2.2H2O (barium
  chloride dihydrate)
• BaCl2.2H2O(s) ? BaCl2. H2O(s) H2O(g)
• BaCl2.2H2O(s) ? BaCl2(s) 2 H2O(g)
• BaCl2.2H2O(s) ? BaO(s) H2O(g) 2HCl(g)

D
D
D
D Heat
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Exp 3B Hydrates and Thermal Decomposition

• Part A Preparation
• Heat a crucible lid on a clay triangle in a the
  flame of a lab burner for about 3 minutes (as you
  did for Exp. 3A)
• Let the cruciblelid cool to warm-to-the-touch
  and weigh. Record in 4 decimal points
• Reheat the cruciblelid to make sure the crucible
  is really dry and repeat the weighing as above
• Repeat until two consecutive weightings are
  within 0.001 g of each other
• Report the weights on the lab report sheet and
  determine the average of your weightings

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Exp 3B Hydrates and Thermal Decomposition

• Part A Thermal decomposition product of
  BaCl2.2H2O Mass(g)
• Average mass of cruciblelid _14.3087_
• Weigh about 1.4 1.5 g BaCl2.2H2O on an
  electronic balance
• (exact weight is not that important!!)
• Transfer the barium chloride to the crucible
• Weigh crucible lid BaCl2.2H2O (0.0001 g
  precision) _15.7175_
• Mass of BaCl2.2H2O (crucible lid BaCl2.2H2O
  ) (cruciblelid) _1.4088__
• Heat the crucible gently for 2-3 min
• Continue heating at full flame for 15 min
• Cool crucible to warm to touch and
  weigh _15.5059__
• Repeat heating for 3 min, and cool crucible
  (warm to touch)
• Weigh crucible difference not more than 0.001 g
• Weigh crucible lid residue (0.0001 g
  precision) _15.5050_

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Exp 3B Hydrates and Thermal Decomposition

• Part A Thermal decomposition of BaCl2.2H2O -
  Analysis
• Average mass of cruciblelid _14.3087_ (g)
• Mass of crucible lid barium chloride
  hydrate _15.7175_ (g)
• Mass of BaCl2.2H2O _1.4088 (g)
• Mass of crucible lid anhydrous salt _15.5050_
  (g)
• Mass of anhydrous product _1.1963 (g)
• Ratio of anhydrous product/hydrate _0.84916__
• Share your ratio with other groups/write your
  results on the white board

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Exp 3B Hydrates and Thermal Decomposition
Part A Calculate correct decomposition products
Substance Formula Weight Moles Mass (g) Ratio
BaCl2.2H2O 244.27 0.0057674 1.4088
BaCl2. H2O 226.25 0.0057674 1.3048 0.926
BaCl2 208.24 0.0057674 1.2010 0.852
BaO 153.33 0.0057674 0.8843 0.628
Because the observed mass of the anhydrous
product is _1.1963__ g, ratio 1.1963/1.4088
0.849 ? the product is _BaCl2_.
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Exp 3B Hydrates and Thermal Decomposition

• Part B Identifying hydrates
• Put in 4 test tubes 0.5 g each (small amount,
  pea sized)
• Cobalt(II) chloride hexahydrate, CoCl2.6H2O
• Nickel(II) chloride hexahydrate, NiCl2.6H2O
• Iron(II) sulfate heptahydrate, FeSO4.7H2O
• Copper(II) sulfate pentahydrate, CuSO4.5H2O
• Heat gently and observe H2O vapor on side of tube
• Record color changes of hydrate
• Test condensation with blue litmus paper record
  response
• If the color of the litmus paper changes, an
  acid is formed and released
• Record color and appearance of residue
• Heat 1 min in full flame retest with moistened
  litmus paper
• Record color observations

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Exp 3B Hydrates and Thermal Decomposition

• Part B Identifying hydrates
• Observations
• Example CrCl3.6H2O
• Gentle heating green solid changes to dark
  liquid water vapor (condensation on side of
  tube) and acid (change in color of litmus paper)
  evolve
• Intense heating liquid changes to violet solid
  that increases in size evolution of water and
  acid continues
• Tested Samples
• Cobalt(II) chloride hexahydrate, CoCl2.6H2O
• Copper(II) sulfate pentahydrate, CuSO4.5H2O
• Nickel chloride hexahydrate, NiCl2.6H2O
• Iron(II) sulfate heptahydrate, FeSO4.7H2O

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Exp 3B Hydrates and Thermal Decomposition

• Part B Identifying hydrates
• Reaction equations for
• 2CrCl3.6H2O(s) ? Cr2O3(s) 6HCl(g) 9H2O(g)
• CoCl2.6H2O (s)? ?
• NiSO4.6H2O(s) ? ?
• FeCl3.6H2O (s) ? ?
• Fe(NH4)2(SO4)2.6H2O (s)? ?

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Next week

• Lab report for Exp 3B
• Summary of experiment
• Results sheets (p. 73-74)
• Questions 1a d, 2 (p. 75-76)
• Conclusion Discussion of your results
• Exp 4A Conductivity of Aqueous Solutions
• Prelab assignments 1a-d, 2a-b, 3, 4a-b, 6
• Lab preparations
• Goal of Experiment
• Experimental Procedures
• Physical, chemical and toxicological properties
  of
• hydrochloric acid (HCl)
• potassium hydroxide (KOH)
• Potassium nitrate (KNO3)
• ethanol (C2H5OH)
• acetic acid (C2H4O2)