example exercise 13.1 intermolecular attraction · example exercise 13.1 intermolecular attraction....

© 2011 Pearson Education, Inc.Introductory Chemistry: Concepts and Critical Thinking, 6th EditionCharles H. Corwin

Example Exercise 13.1 Intermolecular Attraction

In a liquid composed of polar molecules, the intermolecular attraction is the result of both permanent and temporary dipoles. Molecules that contain either H—O or H—N bonds have the strongest type of intermolecular attraction, that is, hydrogen bonds. Water, H2O, and ammonia, NH3, are examples of liquids with hydrogen bonds.

Solution

In a liquid with polar molecules, what is the strongest type of intermolecular attraction?

In a liquid with nonpolar molecules, what is the strongest type of intermolecular attraction?

Answer: The intermolecular attraction between nonpolar molecules is a dispersion force resulting from temporary dipoles.

Practice Exercise

Which of the following liquids has the stronger intermolecular attraction between molecules: CH3–CH2–OH or CH3–O–CH3?

Answer: See Appendix G.

Concept Exercise


Example Exercise 13.2 Boiling Point Predictions

The normal boiling point of a liquid is the temperature at which the vapor pressure equals standard atmospheric pressure, that is, 760 mm Hg. From the graph, we find that the vapor pressure of ether is 760 mm Hg at about 35 °C. Thus, the normal boiling point of ether is about 35 °C. The actual observed value for the boiling point of ether is 36 °C.

Solution

Refer to Figure 13.5 and determine the approximate boiling point of ether.

Refer to Figure 13.5 and determine the approximate boiling point of ethanol.

Answer: ~80 °C (The actual boiling point of ethanol is 78 °C.)

Practice Exercise

Which of the following liquids has the higher boiling point: CH3–CH2–OH or CH3–O–CH3?


Concept Exercise Figure 13.5 Vapor Pressure versus Temperature The vapor pressure of a liquid increases as the temperature increases. The vapor pressure of water equals 760 mm Hg at 100 °C. Thus, at 100 °C the vapor pressure of water equals the standard atmospheric pressure (760 mm Hg), and water begins to boil.


Example Exercise 13.3 Physical Property Predictions

For a liquid having a strong attraction between molecules, properties (b), (c), and (d) are generally high; property (a) is low.(a) Molecular attraction slows vaporization. Therefore, vapor pressure is low for liquids with a strong intermolecular attraction.(b) Attraction between molecules inhibits boiling. Thus, the boiling point is high for liquids with a strong intermolecular attraction.(c) Molecular attraction increases the resistance of a liquid to flow. Viscosity is high for liquids with a strong intermolecular attraction.(d) Attraction between molecules causes a drop of liquid to form a sphere. Surface tension is high for liquids with a strong intermolecular attraction.

Solution

Consider the following properties of liquids. State whether the value for each property is high or low for a liquid with a strong intermolecular attraction:(a) vapor pressure (b) boiling point(c) viscosity (d) surface tension

The intermolecular attraction is greater in isopropyl alcohol, C3H7OH, than in pentane, C5H12. Predict which liquid has the higher value for each of the following:(a) vapor pressure (b) boiling point(c) viscosity (d) surface tension

Practice Exercise


Example Exercise 13.3 Physical Property Predictions

Answers:(a) C5H12 (b) C3H7OH(c) C3H7OH (d) C3H7OH

Practice Exercise

Explain why the paper clip can float on water even though its density is much greater than the density of water.


Concept Exercise

Continued


Example Exercise 13.4 Classifying Crystalline Solids

The type of crystalline solid is dictated by the type of particle in the solid.(a) Nickel is a metal composed of atoms; thus, Ni is a metallic solid.(b) Nickel(II) oxide contains ions and is therefore an ionic solid.

Solution

Classify each of the following crystalline solids as ionic, molecular, or metallic:(a) nickel, Ni (b) nickel(II) oxide, NiO

Classify each of the following crystalline solids as ionic, molecular, or metallic:(a) iodine, I2 (b) silver iodide, AgI

Answers: (a) molecular; (b) ionic

Practice Exercise

Which type of crystalline solid has a high melting point and is a nonconductor of electricity in the solid state, but a good conductor of electricity when melted?


Concept Exercise


Example Exercise 13.5 Change in Heat Problem

In this problem we have to consider (1) the specific heat of water, (2) the heat of solidification, and (3) the specific heat of ice.

Solution

Calculate the amount of heat released when 15.5 g of liquid water at 22.5 °C cools to ice at –10.0 °C.

1. To calculate the amount of heat released when cooling the water, consider the mass, the temperature change (22.5 °C to 0.0 °C), and the specific heat of water, 1.00 cal/(g × °C).

We can summarize the solution visually as follows:

2. The heat of solidification, found in Table 13.5, is 80.0 cal/g. The heat released when water solidifies to ice is

3. The specific heat of ice is 0.50 cal/(g × °C). The heat released as the ice cools to –10.0 °C is found as follows:

The total heat energy released when the water cools to ice at –10.0 °C equals the sum of the values obtained in Steps 1–3.

349 cal + 1240 cal + 78 cal = 1670 calThus, the heat released when the water cools is 1670 cal, or 1.67 kcal.


Example Exercise 13.5 Change in Heat Problem

Calculate the amount of heat required to convert 50.0 g of steam at 100.0 °C to ice at 0.0 °C.

Answer: 3.60 × 104 cal (36.0 kcal)

Practice Exercise

Which of the following changes involves more energy: (a) heating 1 g of ice at 0 °C to water at 0 °C, or (b) heating 1 g of water at 100 °C to steam at 100 °C?


Concept Exercise

Continued


Example Exercise 13.6 Naming Hydrates

First, name the anhydrous compound and then indicate the water of hydration by a Greek prefix. (Refer to Table 7.4 if necessary.)

Solution

Supply a systematic name for each of the following hydrate compounds:(a) Co(CN)3 ● 3H2O (b) FeSO4 ● H2O

(a) Co(CN)3 is named cobalt(III) cyanide according to the Stock system, and cobaltic cyanide according to the Latin system. The Greek prefix for 3 is tri-, so the hydrate is named cobalt(III) cyanide trihydrate, or cobaltic cyanide trihydrate.

(b) FeSO4 is named iron(II) sulfate according to the Stock system, and ferrous sulfate according to the Latin system. The Greek prefix for 1 is mono-, so the hydrate is named iron(II) sulfate monohydrate, or ferrous sulfate monohydrate.


Example Exercise 13.6 Naming Hydrates

Provide the chemical formula for each of the following hydrate compounds:(a) zinc sulfate heptahydrate (b) sodium chromate tetrahydrate

Answers: (a) ZnSO4 ● 7H2O; (b) Na2CrO4 ● 4H2O

Practice Exercise

What is the systematic name for the following hydrates: CaCl2 ● H2O, CaCl2 ● 2H2O, and CaCl2 ● 6H2O?


Concept Exercise

Continued


Example Exercise 13.7 Percentage of Water in Hydrates

In each example, first obtain the molar mass of the anhydrous compound from the periodic table. The molar mass of water is 18.02 g.

Solution

Calculate the percentage of water in each of the following hydrates:(a) CuSO4 ● 5H2O (b) Na2B4O7 ● 10H2O

Practice Exercise

(a) Using the periodic table, we calculate that the molar mass of CuSO4 is 159.62 g. Since the water of hydration for the hydrate is 5, we have

(b) Using the periodic table, we calculate that the molar mass of Na2B4O7 is 201.22 g. Since the water of hydration for the hydrate is 10, we have

Calculate the percentage of water in each of the following hydrates:(a) NaC2H3O2 ● 3H2O (b) Na2S2O3 ● 5H2O

Answers: (a) 39.72%; (b) 36.30%


Example Exercise 13.7 Percentage of Water in Hydrates

Which of the following hydrates has the lowest percentage of water: CaCl2 ● H2O, CaCl2 ● 2H2O, or CaCl2 ● 6H2O


Concept Exercise

Continued


Example Exercise 13.8 Water of Hydration for a Hydrate

We begin by writing an equation for the decomposition.

Since the hydrate contains 34.0% water, the percentage of MgI2 is 66.0 % (100.0 – 34.0 = 66.0 % ). Assume that we have a 100.0 g sample of hydrate. Therefore, we have 34.0 g of water and 66.0 g of anhydrous compound. The moles of water are

Using the periodic table, we calculate the molar mass of MgI2 as 278.11 g. We find the moles of the anhydrous compound as follows:

We can write the mole ratio of the hydrate as MgI2 ● (1.89/0.237)H2O. The ratio 1.89/0.237 reduces to 7.97 and rounds off to the whole number 8. The water of hydration is 8 and the chemical formula is MgI2 ● 8H2O. The name of the hydrate is magnesium iodide octahydrate.

Solution

Determine the water of hydration for a hydrate of magnesium iodide. In an experiment, MgI2 ● XH2O was found to contain 34.0% water.


Example Exercise 13.8 Water of Hydration for a Hydrate

Determine the water of hydration for the hydrate of copper(II) fluoride. In an experiment, CuF2 ● XH2O was found to contain 26.2% water.

Answer: CuF2 ● 2H2O

Practice Exercise

Which of the following hydrates has the highest water of hydration: CaCl2 ● H2O, CaCl2 ● 2H2O, or CaCl2 ● 6H2O?


Concept Exercise

Continued

example exercise 13.1 intermolecular attraction · example exercise 13.1 intermolecular attraction....

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