CHEM-1111 – Exp 3B – Thermal Decomposition of Hydrates
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
• CuSO4.5H2O
• BaCl2.3H2O
• Na2HPO4.12 H2O
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
4 mol H2O/mol CaCl2
5 mol H2O/mol CuSO4 (blue)
3 mol H2O/mol BaCl2
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.(H2O)8
• 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
– Yields anhydrous form
– CoCl2.6H2O
– CoCl2.2H2O
– CoCl2
CoCl2.6H2O
red
6 mol H2O/mol CoCl2 (red) (hexahydrate)
2 mol H2O/mol CoCl2 (violet) (dihydrate)
no H2O, anhydrous (blue)
-4 H2O
CoCl2.2H2O
violet
-2 H2O
CoCl2
blue
Exp 3B Hydrates and Thermal Decomposition
Part A: Preparation
• Thermal decomposition of BaCl2.2H2O
– 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 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, 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
1
CHEM-1111 – Exp 3B – Thermal Decomposition of Hydrates
Exp 3B Hydrates and Thermal Decomposition
Part A: Thermal decomposition product of BaCl2.2H2O
1.
Average mass of crucible+lid
2.
Weigh about 1.4 –1.5 g BaCl2.2H2O on an electronic balance
Mass(g)
_14.3087_
Exp 3B Hydrates and Thermal Decomposition
• Part A: Thermal decomposition of BaCl2.2H2O - Analysis
(exact weight is not that important!!)
3.
Transfer the barium chloride to the crucible
4.
Weigh crucible + lid + BaCl2.2H2O (0.0001 g precision)
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
8.
Repeat heating, cooling and weighing for 3 min
9.
Cool crucible to “warm to touch” and weigh
_15.7175_
BaCl2. H2O
BaCl2
BaO
244.27
0.0057674
226.25
0.0057674
208.24
0.0057674
153.33
0.0057674
– Mass of crucible + lid + anhydrous salt
_15.5050_ (g)
– Mass of anhydrous product
_1.1963 (g)
– Ratio of anhydrous product/hydrate
_0.84916__
Exp 3B Hydrates and Thermal Decomposition
1.
BaCl2.2H2O
_1.4088 (g)
Part B: Identifying hydrates
Part A: Calculate correct decomposition products
Moles
_15.7175_ (g)
– Mass of BaCl2.2H2O
_15.5050_
Exp 3B Hydrates and Thermal Decomposition
Formula
Weight
_14.3087_ (g)
– Mass of crucible + lid + barium chloride hydrate
Share your ratio with other groups/write your results on the white board
10. Weigh crucible + lid + residue (0.0001 g precision)
Substance
– Average mass of crucible+lid
Put in 4 test tubes ~ 0.5 g each (small amount, pea sized)
a)Cobalt(II) chloride hexahydrate, CoCl2 .6H2O
Mass (g)
b)Nickel sulfate hexahydrate, NiSO4.6H2O
c)Iron(III) chloride hexahydrate, FeCl3.6H2O
1.4088
d)Iron(II)ammonium sulfate hexahydrate, Fe(NH4)2(SO4)2.6H2O (Mohr’s salt)
Because the observed mass of the anhydrous product is ______ g,
the product is _______.
1.
2.
3.
5.
6.
7.
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
2
CHEM-1111 – Exp 3B – Thermal Decomposition of Hydrates
Exp 3B Hydrates and Thermal Decomposition
Part B: Identifying hydrates
Observations
Example: CrCl3.6H2O
a)
b)
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
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) ?
Tested Samples
a)
Cobalt(II) chloride hexahydrate, CoCl2.6H2O
b)
Nickel sulfate hexahydrate, NiSO4.6H2O
c)
Iron(III) chloride hexahydrate, FeCl3.6H2O
d)
Iron(II)ammonium sulfate hexahydrate, Fe(NH4)2(SO4)2.6H2O (Mohr’s salt)
Next week
Lab report for Exp 3B
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•
•
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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
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Prelab assignments: 1a-d, 2a-b, 3, 4a-b, 6
Lab preparations
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–
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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)
3