Physical Properties and Chemical Bonding in Solids

Description

Solubility and conductivity experiments are performed with sodium chloride, p-dichlorobenzene, silicon dioxide, and iron.

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• p-Dichlorobenzene is an irritant to the eyes, skin, and respiratory tract, and is toxic by ingestion. Cyclohexane is flammable and toxic.
• The conductivity apparatus is a potential source of electrical shock. Students should only battery powered LED style tester.

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• Work in a hood. Wear gloves. Test breathing air to ensure that threshold limit values (TLVs) are not exceeded. Caution the students to heat the p-dichlorobenzene gently. It will melt quickly.
• Caution--do NOT use flames within 3 meters of cyclohexane. Also, p-dichlorobenzene is somewhat flammable.

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1. Place one gram samples of NaCl, p-dichlorobenzene, SiO₂, and iron filings in separate evaporating dishes. Test the hardness of each solid by rubbing with the bottom end of a plastic pen, or similar device.
2. By fanning with your hand, smell each substance cautiously. An odor implies some degree of volatility.
3. Place an evaporating dish on a triangle, and heat with a Bunsen burner. As soon as a solid melts remove the flame.
4. Repeat this procedure for every substance except p-dichlorobenzene; do not heat any substance for more than 5 minutes. Heat p-dichlorobenzene using a boiling water bath set under a hood.
5. Arrange a test tube rack with four pairs of test tubes. Add 0.5 g of the ionic solid (NaCl) to each test tube in the first pair. Repeat the same procedure for the remaining three pairs, using each of the other three solids. Add 5 mL of water to the first member of each pair. Add 5 mL of cyclohexane to the second member of each pair.
6. Cork and shake the test tubes, noting any evidence for the dissolving of the solid.
7. Carefully note differences between compounds.
8. Obtain an unknown solid and attempt to identify its bond type by repeating the tests suggested above.
9. Instructor Demonstration only unless LED conductivity apparatus is available for students.
10. The electrical conductivity of a substance or solution may be tested with an inexpensive light bulb tester. Plug the testing device into a ground fault interrupter for safety.
11. The light bulb will light if the substance contains mobile charge carriers -- either electrons in metals or ions in solutions of ionic substances.
12. Place a sample of one of the solids in an evaporating dish, and touch the solid to the electrodes of the tester. Test the conductivity of: NaCl, C₆H₄Cl₂, SiO₂, and iron filings.
13. Test the conductivity of distilled water and cyclohexane. Test the conductivity of water solutions, cyclohexane solutions, and any solvent/solid mixtures of NaCl, C₆H₄Cl₂, SiO₂, and iron filings. Place the solution or mixture in a 50-mL beaker, and immerse the electrodes. Rinse the electrodes with the solvent before and after each test.
14. Note whether the light is glowing.

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Name _____________________________ Class _______

Teacher______________________________

DoChem 037 Physical Properties and Chemical Bonding in Solids

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Name _____________________________ Class _______

Teacher______________________________

DoChem 037 Physical Properties and Chemical Bonding in Solids

Record as many observations as possible from the movie. Predict the properties that are missing from the type of bonding, and record them in ( ). Those tests completely missing from the movies are in ( ).

Predict the outcome of testing molten NaCl and molten p-dichlorobenzene for electrical conductivity.

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Purpose

To investigate some of the physical properties of solids containing ionic, covalent, molecular or metallic bonds.

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(for 10 students working in pairs)

• 25 evaporating dish
• 40 test tube, 16 x 125 mm; 40 cork stopper
• 5 test tube rack
• 5 support stand, iron ring, clay triangle, wire gauze, burner
• 5 crucible tongs
• 5 conductivity tester (with ground fault interrupters)
• 20 g NaCl
• 20 g SiO₂ (silica gel)
• 20 g p-dichlorobenzene, (C₆H₄Cl₂)
• 20 g iron filings
• 150 g cyclohexane
• 50 50-mL beaker
• 5 10-mL graduated cylinder
• distilled water
• 5 spatula
• 5 plastic pen (or other device for crushing solids)
• 5 250-mL beaker
• 10 gloves
• hood

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• The differences between the different classes of solids are investigated using a typical member from each class. Remind students when doing an unknown that there are exceptions to these rules.
• Warn students to stop heating a substance once it has melted. If students have difficulty in melting NaCl in an evaporating dish, suggest they heat a small amount in a small test tube over the hottest part of the flame.
• Each laboratory group does not need individual test samples of the solids and solutions for the conductivity tests. Prepare labeled beakers of the solids and the cyclohexane solutions and have the students share these. This will speed up the procedure, and reduce the amount of equipment needed.
• You may also use an LED conductivity apparatus which is battery power for students to test the solutions. See Expt. 062 for details about building this apparatus.
• Dip the loop of the wire into the solution and stir with the loop to make even contact.
• Note the flashing near the bottom of the LED in weak electrolytes.
• Note the flashing is more obvious in a darker room and in strong electrolytes.
• Wash the loop after each measurement.

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Teacher preparation: 40 minutes

Class Time: 45-50 minutes

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• p-Dichlorobenzene is an irritant to the eyes, skin, and respiratory tract, and is toxic by ingestion. Cyclohexane is flammable and toxic.
• The conductivity apparatus is a potential source of electrical shock.

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• Work in a hood. Wear gloves. Test breathing air to ensure that threshold limit values (TLVs) are not exceeded. Caution the students to heat the p-dichlorobenzene gently. It will melt quickly.
• Caution the students not to use flames within 3 meters of cyclohexane. Also, p-dichlorobenzene is somewhat flammable.
• Have a fire extinguisher nearby, and review the procedures in case of fire. By limiting amounts of chemicals set out for student use, any conflagration can be avoided, and any fire smothered with a wet towel.
• Review the toxicity problems with some of the chemicals used, and remind students to wash their hands before leaving laboratory. Wash spills of cyclohexane immediately using soap and water.
• Use a ground fault interrupter with the conductivity apparatus.

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Save cyclohexane for disposal with organic solvents. Save p-dichlorobenzene for disposal with toxic solids. SiO₂ and NaCl may be discarded with ordinary solids.

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Presentation Question:

Predict the outcome of testing molten NaCl and molten p-dichlorobenzene for electrical conductivity.

Only the molten ionic solid, sodium chloride(l), is expected to conduct electricity.

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• Once the data have been collected, a systematic discussion of the types of bonding in the various solids is appropriate.
• To a large degree, the physical properties of a solid substance are determined by the type of bonding which holds the molecules, atoms or ions together in a solid. Depending on the type of bonding, solids may be described as ionic, molecular, metallic or covalent network solids. Each type of bonding affects the physical properties of a solid in different ways. The type of bonding in a solid can be determined by studying its physical properties.
• Ionic solids contain ions held together by ionic bonds. These solids are typically hard and have high melting points. They are often soluble in water, but insoluble in organic solvents. They are not observed to conduct electricity in the solid state, but do so both in aqueous solutions and in pure molten form. (Use a solid like calcium sulfate as an ionic solid not soluble in water.)
• Molecular solids are made up of molecules held together by relatively weak forces. They tend to be soft, easily melted and volatile. Molecular solids are likely to be insoluble in water, but soluble in organic solvents. Neither the solid nor the molten state conducts electricity, nor do any of the solutions. (Use solid sucrose as a molecular solid that does dissolve in water.)
• Covalent network solids contain atoms held together by a network of covalent bonds that link every atom in the solid to every other atom. The molecules are gigantic; each particle of the crystal is essentially one molecule. This type of solid is hard, nonvolatile, very high melting and insoluble in both water and inorganic solvents. They do not conduct electricity.
• Metallic solids contain atoms bonded together by metallic bonds. These bonds are strong but not localized. Since the electrons in the metallic bonds are relatively mobile, metals tend to have high melting points and be hard, malleable, nonvolatile and shiny. Metals are soluble neither in water nor organic solvents. Some metals, such as sodium, dissolve by reacting with water. Metals sometimes dissolve in liquid metallic mercury.

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Some of the observations are not clear in the movies; consequently, some latitude must be allowed. SiO₂ is so translucent that it is difficult to impossible to see in both solvent particularly the hexane. The hardness is very difficult to see. See Sample Data for expected laboratory results.

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• solubility
• conductivity
• ionic bond
• covalent bond
• molecular bond
• metallic bond
• covalent network bond
• ionic bonding
• covalent bonding
• molecular bonding
• metallic bonding
• covalent network bonding
• ionic solid
• covalent solid
• molecular solid
• metallic solid
• covalent network solid
• ionic crystal
• covalent crystal
• molecular crystal
• metallic crystal
• covalent network crystal
• hardness
• volatile
• volatility
• Van der Waals forces

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