1. 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.

2. 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
-4 H2O
-2 H2O
red
blue

3. 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”

4. 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 g of each other
Report the weights on the lab report sheet and determine the average of your weightings

5. Exp 3B Hydrates and Thermal Decomposition
Part A: Thermal decomposition product of BaCl2.2H2O Mass(g)
Average mass of crucible+lid _ _
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 ( g precision) _ _
Mass of BaCl2.2H2O = (crucible + lid + BaCl2.2H2O ) – (crucible+lid) _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 _ __
Repeat heating for 3 min, and cool crucible (“warm to touch”)
Weigh crucible: difference not more than g
Weigh crucible + lid + residue ( g precision) _ _

6. Exp 3B Hydrates and Thermal Decomposition
Part A: Thermal decomposition of BaCl2.2H2O - Analysis
Average mass of crucible+lid _ _ (g)
Mass of crucible + lid + barium chloride hydrate _ _ (g)
Mass of BaCl2.2H2O _ (g)
Mass of crucible + lid + anhydrous salt _ _ (g)
Mass of anhydrous product _ (g)
Ratio of anhydrous product/hydrate _ __
Share your ratio with other groups/write your results on the white board

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

8. 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

9. 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

10. 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) ?

11. Next week Lab report for Exp 3B
Summary of experiment
Results sheets (p )
Questions 1a – d, 2 (p )
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)