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 (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

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”

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) 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_

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 Substance Formula Weight Moles Mass (g) Ratio BaCl2.2H2O 244.27 0.0057674 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 = 1.1963/1.4088 = 0.849  the product is _BaCl2_.

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

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

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)