ANALYSIS OF HYDRATES

Transcription

1 1 ANALYSIS OF HYDRATES INTRODUCTION An ionic compound is made of positive and negative ions, called cations and anions, respectively. At room temperature, all ionic compounds are solid. Within a solid crystal, the ions are arranged so that a cation is surrounded on all sides by anions, and, an anion is surrounded on all sides by cations. This pattern of ions is often called a crystal lattice since the pattern looks like a lattice. An ionic crystal lattice A lattice fence The cations and anions have a spherical shape. Their sizes may vary. For example, in NaCl, the Cl ions have a radius nearly twice that of Na +. The radius of Na + is 95 pm (picometers), while that of Cl is 181 pm. When spherical objects are arranged in a pattern, there will be small spaces or holes between them. Imagine organizing 10 basketballs and 10 soccer balls in an alternating pattern inside a big box. There would be space in between the balls. In some ionic compounds, H 2 O molecules can lodge themselves in these holes. O H H A water molecule in a crystal lattice hole Water is an example of a polar compound. A polar compound is sometimes called a dipole. Since the oxygen atom of water has a partial negative charge, it is attracted to a cation (opposites attract). The partially positive hydrogens are attracted to anions. These attractions between the water dipole and the ions are referred to as ion-dipole attractions.

2 2 An ionic compound which has water molecules lodged in the holes between its ions is called a hydrate. Hydrates are solid at room temperature. The ratio of H 2 O molecules to ions tends to be constant for a particular compound. For example, calcium sulfate is regularly found in the form which has two H 2 O for every CaSO 4 formula unit. This is shown in a formula as CaSO 4 2H 2 O. The dot separates the two components of this crystal: the ionic portion and the water. To name this hydrate, the name of the ionic portion before the dot is written as usual (calcium sulfate), then the word "hydrate" is added with the appropriate prefix to indicate the number of water molecules (mono=1, di=2, tri=3, tetra=4, penta=5, hexa=6, hepta=7, octa=8, nona=9, deca=10). Therefore, CaSO 4 2H 2 O is calcium sulfate dihydrate. CoCl 2 6H 2 O is cobalt(ii) chloride hexahydrate, and, Na 2 CO 3 10H 2 O is sodium carbonate decahydrate. The formula of a hydrate also has mole implications. Instead of considering individual ions and molecules, a group such as the mole can be used. In the case of CaSO 4 2H 2 O, this view means that there are 2 moles of H 2 O molecules in the lattice for every 1 mole of CaSO 4 units. Many hydrates can have their water driven out simply by heating. The heat breaks the ion-dipole attraction and the water leaves as vapor. The equation CaSO 4 2H 2 O(s) CaSO 4 (s) + 2 H 2 O(g) represents this process for calcium sulfate dihydrate. The ionic product is called the anhydrous form of the compound to distinguish it from the hydrate. "Anhydrous" literally means without water. In some cases the hydrate form and the anhydrous form have different colors. For example, CoCl 2 6H 2 O is pink and CoCl 2 is blue. Different colors usually result only if a transition metal ion is part of the compound; in this example, Co 2+ is such an ion. PURPOSE In this lab, you will heat a hydrate, AB xh 2 O, to drive off its water. Then you will determine the ratio of water to ionic units. The identity of AB will be given. Your goal is to find the value of x for your hydrate.

3 Procedure 3 Safety Notes You will be working with flame and hot objects in this lab. Long hair and loose clothing should be tied back and secured. When not in use, burners should be turned off. Do not touch any heated or hot object with your bare hands. Hot objects should not be placed in a position where they might be bumped. 1. The hydrate will be heated in a crucible. Before using the crucible, it needs to be fire-cleaned. Set up a ring stand, with an attached iron ring and a clay triangle on top. Using crucible tongs to handle the crucible, carefully place it in the triangle. Heat it with a burner until it glows red hot for about two minutes. (Be sure your flame is blue, not yellow. If it is not blue, ask your instructor for assistance). Let it cool at least 5-7 minutes. 2. Using crucible tongs, place your cooled crucible on a balance and weigh. Use the same balance for all weighings in this experiment. Record the weight. 3. Add about one gram of your assigned hydrate. Record the combined mass of the crucible and hydrate to g. 4. Using crucible tongs, place the crucible and hydrate on the clay triangle. Apply heat, gently at first, then more vigorous after several minutes. Heat for a total of 15 minutes. 5. Transfer the crucible to a desiccator, a container which contains a substance that absorbs moisture from the air. This prevents water vapor in the air from getting back into the crystal lattice of your compound. 6. Allow the crucible and contents to cool at least 5-7 minutes. 7. Place the crucible and contents on the balance, weigh and record. 8. Return the crucible and contents to clay triangle. Heat vigorously for 5-10 minutes. 9. Transfer the crucible to a desiccator, and allow the crucible and contents to cool at least 5-7 minutes. 10. Place the crucible and contents on the balance, weigh and record. Compare this mass with that taken in step 7. If they differ by no more than gram, go on to step 12.

4 4 11. Repeat steps 8-10 as many times as necessary until you get two readings in a row which differ by no more than gram. 12. Empty the contents of your crucible into the designated container for wastes. Clean the crucible as instructed.

5 5 ANALYSIS OF A HYDRATE Data and Calculations Name Partner Data Formula of compound used x H 2 O Mass of empty crucible Mass of crucible + hydrate Mass of crucible + contents after first heating after second heating after third heating* after fourth heating* after fifth heating* *if necessary Calculations Show your work clearly for each of the following on a separate piece of paper. Mass of hydrate form of compound Mass of anhydrous form Mass of water driven off Moles of anhydrous form Moles of water Ratio of water:anhydrous form Formula of hydrate mol mol

6 6 ANALYSIS OF A HYDRATE Pre-lab Assignment Name Lab Section 1. Define hydrate. 2. Name each of the following hydrates. A. CaBr 2 2H 2 O B. FeCl 3 6H 2 O C. MgSO 4 7H 2 O 3. Complete the following equations and balance. A. Na 2 CO 3 10H 2 O B. MgSO 4 7H 2 O 4. Consider the following data from a procedure performed as outlined in this experiment. A hydrate of aluminum chloride, AlCl 3 xh 2 O was used. Empty crucible g Crucible + hydrate g Crucible + contents after first heating g after second heating g A. What was the mass of the anhydrous salt? How many moles of anhydrous salt is this? B. What was the mass of the water driven off? How many moles of water is this? C. Find the mole ratio of water:anhydrous form. D. What is the formula of this hydrate?