Chemical Bonding in Solids

Description

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The crystals studied are of two different types. KBr is an ionic crystal. In this type of crystal, ions are located at lattice sites, and the binding between them is mainly electrostatic. Electrostatic bonding is essentially nondirectional. When the crystal forms, the ions arrange themselves so as to maximize attractions and minimize repulsions. Because the forces are strong, ionic crystals have large lattice energies. They are not easily deformed (hard to the touch), and they have high melting points.

p-Dichlorobenzene is a covalent compound that forms a molecular crystal. In molecular crystals, molecules (sometimes individual atoms) occupy lattice sites. The attractive forces between the molecules are due to either dispersion forces or hydrogen bonds. These forces are much weaker than the covalent bonds that exist within the molecules. Because of this, molecular crystals tend to have small lattice energies and are easily deformed (that is, they are soft to the touch). Also, since relatively small amounts of thermal energy are required to overcome these forces of attraction due to temporary dipoles that form instantaneously and in a random fashion, molecular solids have low melting points.

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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.

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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 set out for student use, any conflagration can be avoided, and any fire can be smothered with a wet towel.
• Review the toxicity problems with some of the chemicals used, and remind students to wash their hands before leaving the laboratory. Wash spills of cyclohexane immediately using soap and water.

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1. Place a spatulaful or spoonful of each of the substances on a watch glass. Label each appropriately.
2. Use a fanning motion of the hand to carefully smell samples of p-dichlorobenzene and potassium bromide. (Prolonged exposure to p-dichlorobenzene is a serious hazard. The TLV is 75 ppm, while the room temperature vapor pressure is 1200 ppm.) Note any distinguishing odors.
3. Test the hardness of the two substances by rubbing or crushing small samples against the watch glass with the end of a plastic pen. Record your results.
4. Perform the next four experiments under a hood or with excellent ventilation. Place a sample of p-dichlorobenzene in a test tube. Heat the sample of p-dichlorobenzene in a water bath until the solid melts. Note the approximate time it takes to melt the substance.
5. Place approximately the same amount of KBr in a crucible and heat one minute.
6. Place a spatula-tip full of sample of KBr in a test tube containing 10 mL of water.
7. Place a similar sample of p-dichlorobenzene into another test tube with 10 mL of water. Shake or stir both tubes. Observe if the substances dissolve. Compare the solubility of the two substances.
8. Place a spatula-tip full of sample of KBr in a test tube containing 10 mL of cyclohexane.
9. Place a similar sample of p-dichlorobenzene into another test tube with 10 mL of cyclohexane. Shake or stir both. Observe if the substances dissolve.
10. Compare the solubility of the two substances.

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Name _____________________________ Class _______

Teacher______________________________

DoChem 036 Chemical Bonding in Solids

1. Explain the data related to odor in terms of types of bonds. Do you think that a substance can be relatively volatile and not have an odor? Give an example. Describe the effect which the sensitivity of the nose has on the above observations.
2. Explain the comparative melting points of the two substances in terms of type of bonds.
3. Are the relative strengths of covalent and ionic bonds compared when melting points are compared? Explain.
4. Are the intermolecular or intramolecular forces greatest for p-dichlorobenzene? Explain your answer.
5. Use the periodic table or chart of electronegativities to predict whether or not the following substances are largely ionic or largely covalent: LiF, ICl, Na₂O, SO₂, and BaI₂.

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Name _____________________________ Class _______

Teacher______________________________

DoChem 036 Chemical Bonding in Solids

Watch the movie. Use the data collected below to answer the questions.

A diamond is held together by covalent bonds. Predict the outcome of these same experiments performed on diamonds. What is the distinction between bonding in diamonds and bonding in p-dichlorobenzene?

1. Explain the data related to odor in terms of types of bonds. Do you think that a substance can be relatively volatile and not have an odor? Give an example. Describe the effect which the sensitivity of the nose has on the above observations.
2. Explain the comparative melting points of the two substances in terms of type of bonds.
3. Are the relative strengths of covalent and ionic bonds compared when melting points are compared? Explain.
4. Are the intermolecular or intramolecular forces greatest for p-dichlorobenzene? Explain your answer.
5. Use the periodic table or chart of electronegativities to predict whether or not the following substances are largely ionic or largely covalent: LiF, ICl, Na₂O, SO₂, and BaI₂.

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Purpose

To compare the properties of a molecular solid with those of an ionic solid.

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• 10 8 cm watch glass
• 5 spoons or microspatula
• 5 plastic pen, or similar object
• 25 test tube, 18 x 120 mm
• 20 test tube cork
• 5 small porcelain crucible
• 250-mL beaker
• Heating apparatus: burner, support stand, ring, gauze, triangle, tongs, hot pad
• 100 mL cyclohexane, C₆H_¹2
• 30 g of potassium bromide, KBr(s)
• 30 g of p-dichlorobenzene, C₆H₄Cl₂(s)
• distilled water
• working hood
• gloves

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Use small samples of para-dichlorobenzene in order to minimize odors in the room.

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Teacher preparation: 30 minutes

Class Time: 40-50 minutes

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• Save the cyclohexane for disposal with organic solvents. Other material may be disposed at the sink or with routine solid wastes.
• p-Dichlorobenzene should be stored for solid waste disposal.

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Presentation Question:

• A diamond is held together by covalent bonds. Predict the outcome of these same experiments performed on diamonds. What is the distinction between bonding in diamonds and bonding in p-dichlorobenzene?
  • Diamonds will not dissolve in these solvents, or melt under these conditions. In diamonds, the covalent forces exist throughout the solid. This is called a network covalent solid. In p-dichlorobenzene, weak intermolecular forces hold the molecules at their lattice sites within the solid structure, while strong covalent forces hold the atoms together in a p-dichlorobenzene molecule.

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There are very marked differences in the properties of covalent and ionic compounds. Their differences are due to the type of intermolecular forces that bond particles in the solid substances.

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Closure Questions:

1. Explain the data related to odor in terms of types of bonds. Do you think that a substance can be relatively volatile and not have an odor? Give an example. Describe the effect which the sensitivity of the nose has on the above observations.
2. Explain the comparative melting points of the two substances in terms of type of bonds.
3. Are the relative strengths of covalent and ionic bonds compared when melting points are compared? Explain.
4. Are the intermolecular or intramolecular forces greatest for p-dichlorobenzene? Explain your answer.
5. Use the periodic table or chart of electronegativities to predict whether or not the following substances are largely ionic or largely covalent: LiF, ICl, Na₂O, SO₂, and BaI₂.

Answers to Closure Questions:

1. Van der Waals bonds are weak forces. The molecules connected by these forces can easily escape. For this reason, we can smell the p-dichlorobenzene and it deforms easily. Because of this force it has a low boiling and a low melting point.

   The smaller the amount of material that our noses can detect, the less of it needs to be present to be detected. Even a substance with very strong bonds holding it in a condensed state will give an odor if our noses are very sensitive to it.

2. Van der Waals forces are weak so the molecules do not have a strong attraction for each other. Thus the p-dichlorobenzene boils easily and freezes easily.
3. No. With p-dichlorobenzene the separation of one molecule from another within the lattice is taking place. There is no breaking apart of molecules. Van der Waals forces are being overcome. With KBr, ionic attractions must be overcome for melting to occur.
4. Intramolecular forces are greater in p-dichlorobenzene. The intramolecular forces are covalent, and hold the carbon, hydrogen, and chlorine atoms together tightly in molecules of this substance. The intermolecular forces in the p-dichlorobenzene crystals that hold the molecules in place within crystals are weak; the crystals are easily disrupted.
5. LiF, ionic, metal and nonmetal; ICl, covalent, both nonmetals; Na₂O, ionic, metal and nonmetal; SO2, covalent, both nonmetals; BaI₂, ionic, metal and nonmetal.

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• crystal
• crystalline solid
• ion
• ionic solid
• electrostatic bonding
• ionic bond
• covalent solid
• molecular crystal
• covalent bond
• dispersion forces
• hydrogen bond
• lattice energy
• crystal lattice
• melting point
• freezing point
• solubility
• intermolecular bonding
• intramolecular bonding
• Van der Waals bond

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