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Exp 3B - Hydrates and Thermal Decomposition

Exp 3B - Hydrates and Thermal Decomposition. Background In chemistry, a neutral ionic compound is typically referred to as a “ salt ”

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Exp 3B - Hydrates and Thermal Decomposition

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  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 -2 H2O -4 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 decimalpoints • 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

  5. Exp 3B Hydrates and Thermal Decomposition Part A: Thermal decomposition product of BaCl2.2H2O Mass(g) • Average mass of crucible+lid_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 ) – (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 _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_

  6. 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

  7. Exp 3B Hydrates and Thermal Decomposition Part A: Calculate correct decomposition products Because the observed mass of the anhydrous product is _1.1963__ g, ratio = 1.1963/1.4088 = 0.849 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. 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)

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