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Exp 3B - Hydrates and Thermal Decomposition. Background In chemistry, a neutral ionic compound is typically referred to as a “ salt ” - PowerPoint PPT Presentation
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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.
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 (redred) (hexahydrate)– CoCl2.2H2O 2 mol H2O/mol CoCl2 (violetviolet) (dihydrate)– CoCl2 no H2O, anhydrous (blueblue)
CoCl2.6H2O CoCl2.2H2O CoCl2 -4 H2O -2 H2Ored
violetblue
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)
“”= “Heat”
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 crucible+lid cool to warm-to-the-touch and weigh. Record in 4 decimal points
• Reheat the crucible+lid 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
Exp 3B Hydrates and Thermal Decomposition
Part A: Thermal decomposition product of BaCl2.2H2O Mass(g)
1. Average mass of crucible+lid _14.3087_
2. Weigh about 1.4 –1.5 g BaCl2.2H2O on an electronic balance
(exact weight is not that important!!)
3. Transfer the barium chloride to the crucible
4. Weigh crucible + lid + BaCl2.2H2O (0.0001 g precision) _15.7175_
5. Mass of BaCl2.2H2O = (crucible + lid + BaCl2.2H2O ) – (crucible+lid)
_1.4088__
6. Heat the crucible gently for 2-3 min
6. Continue heating at full flame for 15 min
7. Cool crucible to “warm to touch” and weigh _15.5059__
8. Repeat heating for 3 min, and cool crucible (“warm to touch”)
9. Weigh crucible: difference not more than 0.001 g
10. Weigh crucible + lid + residue (0.0001 g precision) _15.5050_
Exp 3B Hydrates and Thermal Decomposition
• Part A: Thermal decomposition of BaCl2.2H2O - Analysis
– Average mass of crucible+lid _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
Exp 3B Hydrates and Thermal Decomposition
Part A: Calculate correct decomposition products
SubstanceFormula Weight
Moles Mass (g) Ratio
BaCl2.2H2O 244.27 0.0057674 1.4088
BaCl2. H2O226.25 0.0057674 1.3048 0.926
BaCl2208.24 0.0057674 1.2010 0.852
BaO153.33 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_.
Exp 3B Hydrates and Thermal Decomposition
Part B: Identifying hydrates1. Put in 4 test tubes ~ 0.5 g each (small amount, pea sized)
a) Cobalt(II) chloride hexahydrate, CoCl2.6H2O
b) Nickel(II) chloride hexahydrate, NiCl2.6H2O
c) Iron(II) sulfate heptahydrate, FeSO4.7H2O
d) Copper(II) sulfate pentahydrate, CuSO4.5H2O
1. Heat gently and observe H2O vapor on side of tube2. Record color changes of hydrate3. Test condensation with blue litmus paper; record response
If the color of the litmus paper changes, an acid is formed and released5. Record color and appearance of residue6. Heat 1 min in full flame; retest with moistened litmus paper7. Record color observations
Exp 3B Hydrates and Thermal Decomposition
Part B: Identifying hydrates
Observations
Example: CrCl3.6H2O
a) Gentle heating: green solid changes to dark liquid; water vapor (condensation on side of tube) and acid (change in color of litmus paper) evolve
b) Intense heating: liquid changes to violet solid that increases in size; evolution of water and acid continues
Tested Samplesa) Cobalt(II) chloride hexahydrate, CoCl2.6H2O
b) Copper(II) sulfate pentahydrate, CuSO4.5H2O
c) Nickel chloride hexahydrate, NiCl2.6H2O
d) Iron(II) sulfate heptahydrate, FeSO4.7H2O
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) ?
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)