Experiment 7 Energy Accompanying Reactions Objectives 1. To observe the techniques involved to remove water from a hydrated salt, copper (II) sulphate pentahydrate, CuSO 4 5H 2 O, quantitatively. 2. To calculate the percentage of water in copper (II) sulphate pentahydrate, CuSO 4 5H 2 O, theoretically and experimentally. 3. To classify reactions as endothermic or exothermic reactions. Apparatus: Part A - Instructor Demonstration: 1. Porcelain crucible with lid 5. Bunsen burner and lighter 2. Tongs 6. Desicooler 3. Triangle 7. Spatula 4. Ring clamp 8. Analytical balance 9. Solid copper (II) sulphate pentahydrate, CuSO 4 5H 2 O Part B Apparatus: 1. 4 test tubes 2. Test tube rack 3. Stir rod 4. Water bottle Solids:
1. Solid anhydrous copper (II) sulphate, CuSO 4 2. Solid ammonium chloride, NH 4 Cl 3. Solid potassium hydroxide, KOH 4. Solid potassium nitrate, KNO 3
Introduction Salts are compounds composed of a metal ion plus a non-metal (or polyatomic) ion, e.g., sodium chloride (NaCl), and sodium phosphate (Na 3 PO 4 ). Hydrated salts (or Hydrates) are salts, which have a definite amount of water chemically combined. Some common hydrates are: CuSO 4 5H 2 O Copper (II) sulphate pentahydrate MgSO 4 7H 2 O Magnesium sulphate heptahydrate CoCl 2 6H 2 O Cobalt (II) chloride hexahydrate SnCl 2 2H 2 O Tin (II) chloride dihydrate The dot indicates an attractive force between the polar water molecules and the positively charged metal ion. On heating, the attractive forces are overcome and the water molecules are released leaving behind the anhydrous salt. The water released on heating is called the water of hydration. Since this is a heat absorbing process, the reaction is endothermic. (In an "exothermic" reaction heat is liberated.) In Part A, your instructor set up the apparatus to carry out the proper techniques to remove the water molecules from CuSO 4 5H 2 O. Gentle heating will be carried out by holding the Bunsen burner in a sweeping motion. Care will be taken to avoid letting the flame of the Bunsen burner rest on one spot of the crucible. CuSO 4 5H 2 O decomposes at temperatures greater than 560 o C. When CuSO 4 5H 2 O decomposes a black solid, CuS, will formed. When this happens, it will introduce experimental error in the calculation of percent water in CuSO 4 5H 2 O. A weighed sample of CuSO 4 5H 2 O will be heated in a porcelain crucible at temperatures slightly above 100 o C until all of its water content will be driven off as steam. After the heating, the mass of the anhydrous CuSO 4, which remains in the crucible, will be determined. According to the chemical equation, we will observe a colour change and a decrease in mass after heating since the molar mass of
anhydrous CuSO 4 is less than the molar mass of CuSO 4 5H 2 O. The loss in mass will be the amount of water that is driven off as steam. Therefore, using the following equation and experimental data, the percentage of water in the CuSO 4 5H 2 O sample can be determined. The percentage of water in CuSO 4 5H 2 O can be determined theoretically using the following equation. In Part B, you will study the dissolving process of several compounds and classifying them as endothermic or exothermic. The dissolving process of anhydrous CuSO 4 is the formation of the hydrated form, CuSO 4 5H 2 O. Notice that the phase of the anhydrous CuSO 4 is a solid, (s). The phase of the dissolved CuSO 4 in water is aqueous, (aq). The chemical equation of the dissolving process of solids that are not hydrates is simply a change in phase. For example, When the dissolving process is endothermic, the temperature of the solution will decrease as it is accompanied by a net absorption of energy. Heat can be included on the left-hand side of the chemical equation. When the dissolving process is exothermic, the temperature of the solution will increase as it is accompanied by a net release of energy. Heat can be included on the right-hand side of the chemical equation. An application of an endothermic reaction is used to manufacture athletes chemical ice packs, which usually contains water and a packet of ammonium chloride. The cold pack is activated by breaking the barrier which separates the ammonium chloride, and water. Once the barrier is broken, ammonium chloride begins to dissolve in water, and almost immediately, the pack becomes cold as the chemical reaction absorbs heat from its environment.
Procedure: Part A - Removing Water from a Hydrated Salt Instructor Demonstration 1. Obtain a clean crucible and lid. Inspect the crucible for cracks. 2. Use an analytical balance and determine the mass of the empty crucible. Record the mass of the empty crucible to four decimal places on the data sheet. Record the mass of the empty crucible to four decimal places on the data sheet. 3. Use an analytical balance and weigh approximately 3.9 to 4.1 grams of CuSO 4 5H 2 O into the crucible. Record the mass of the crucible and the hydrate to four decimal places on the data sheet. 4. Place the crucible on the clay triangle and heat gently by holding the Bunsen burner in a sweeping motion for about 12 minutes. 5. Allow to cool to room temperature inside the desicooler and weigh the crucible and the content. Record the mass of the crucible and the content (after 1st heating) to four decimal places on the data sheet. 6. After weighing, place the crucible back on the clay triangle and reheat content for about 5 minutes. Allow to cool and reweigh. Record the mass of the crucible and the content (after re-heating) to four decimal places on the data sheet. If you have done a good job in heating, the mass of the crucible and residue after the 1st heating and the 2nd heating should not differ by more than 0.01 gram.
Procedure: Part B - Energy Changes Associated with Substances Dissolving in Water 1. Place 4 large test tubes in a test tube rack. Label the test tube as #1, #2, #3, and #4. 2. Fill each test tube with 5 ml of distilled water. 3. Use a thermometer and record the temperature of the water in each of the the test tubes. 4. Use a spatula to place a heaping scoop of NH 4 Cl in the first test tube. Mix with a glass rod to dissolve it and record the maximum or minimum temperature reached. 5. Repeat step 4 for KOH, KNO 3, and anhydrous CuSO 4 using the other 3 test tubes.
Datasheet: Part A - Removing Water from a Hydrated Salt 1. Mass of empty crucible 2. Mass of crucible and CuSO 4 5H 2 O 3. Mass of crucible and content (after 1st heating) 4. Mass of crucible and content (after re-heating) 5. Mass of CuSO 4 5H 2 O used 6. Mass of anhydrous CuSO 4 which remains after heating 7. Mass of water released
Datasheet: Part B - Changes Associated with Substances Dissolving in Water Observations Test tube #1 NH 4 Cl Test tube #2 KOH Test tube #3 KNO 3 Test tube #4 CuSO 4 Temperature of the water in the test tube Appearance of the solid Appearance of the solution Maximum or minimum temperature the solution reached Classify the dissolution process as endothermic or exothermic
Calculations: Calculation of % water in CuSO 4 5H 2 O (experimental): Calculation of % water in CuSO 4 5H 2 O (theoretical):
Percent Error Calculation Using the results from the two previous calculations, the % error for the experimentally determined percent water in CuSO 4 5H 2 O can be carried out using the following equation: % Error = theoretical % water in CuSO 4 5H 2 O - experimental % water in CuSO 4 5H 2 O * 100 theoretical % water in CuSO 4 5H 2 O Show work here and report your answer with proper number of significant figures.
Questions: 1. Write the balanced chemical equation which occurred in: (a) Part A. For an endothermic reaction, include heat on the left-hand side of the equation. For an exothermic reaction, include heat on the righthand side. (b) Part B. For endothermic reactions, include heat on the left-hand side of the equation. For exothermic reactions, include heat on the right-hand side. (i) NH 4 Cl (ii) KNO 3 (iii) KOH (iv) Anhydrous CuSO 4
2. Calculate the percent water in sodium dichromate dihydrate, Na 2 Cr 2 O 7 2H 2 O.