© 2014 Pearson Education, Inc. Chapter 10 Properties of Solids and Liquids (Molecular Structures)...

© 2014 Pearson Education, Inc.

Chapter 10 Properties of Solids and Liquids

(Molecular Structures)

Learning Goal •Draw Lewis structures for molecular compounds or polyatomic ions.•Draw resonance structures for certain polyatomic ions.•Predict molecular structures of molecule and ions.•Use electronegativity to determine the polarity of a bond or a molecule.•Describe the attractive forces among molecules.•Describe the changes of state between solids, liquids, and gases; calculate the energy involved.


10.1 Electron-Dot Symbols

Electron-dot symbols show single dots or pairs of dots on the sides of an atomic symbol


Electron-Dot Formulas (Lewis Structures)

Electron-dot formulas show• The sequence of bonded atoms in a molecule or polyatomic ion

• The bonding pairs of electrons shared between atoms

• The nonbonding or unshared (lone pairs) of electrons

• The central atom bonded to other atoms

The electron-dot formula for H2O contains:• 8 valence electrons (6 from O plus 1 from each hydrogen atom)• Two bonding pairs (between the H and O atoms) and two lone

pairs (on the O atom)


The Octet Rule and the Bonding Trend in the Periodic Table

Octet Rule: each atom would like to attain 8 electrons in its valence shell. (noble gas configuration)


Bonding Trends in C, N, and O

C N O O-

C

C

C

C

N

N

N

O

O

O

• In a single bond, one pair of electrons is shared.

• In a double bond, two pairs of electrons are shared.

• In a triple bond, three pairs of electrons are shared.


Exceptions to the Octet Rule

Exceptions to the octet rule include:• Hydrogen, H, only gets a single bond• Boron, B, has only 3 electrons around it to form

bonds

BF3

• Compounds of P, S, Cl, Br, and I can have expanded octets with 10, 12, or even14 valence electrons around them

SF4, SF6, PF5, ClO3-, ClO4

-


Learning Check

Draw the Lewis Structure for:

a. NF3

b. H2CO

F N F

F..: :

: :.... ..

.. ..


Draw the electron-dot formula for PCl3.

P forms 3 bonds and Cl forms one bond.


Multiple Bonds, CO2

In carbon dioxide, CO2, octets are achieved by sharing two pairs of electrons between atoms; this is called a double bond.


Multiple Bonds, N2

In nitrogen, N2, octets are achieved by sharing three pairs of electrons between atoms; this is called a triple bond.


Multiple Bonds, O2

• In oxygen, O2, octets are achieved by sharing two pairs of electrons between atoms; Draw Lewis structure.


Learning Check

Draw Lewis structures for SO2 and SO3. Complete each structure by adding the missing valence electrons.

O S O

OS

O

O


Resonance Structures • Occurs in molecules or polyatomic ions with multiple bonds• Are written by changing the location of a double bond

2−


10.2 Shapes of Molecules and Ions (VSEPR Theory)

In the valence shell electron repulsion theory (VSEPR), the three-dimensional shape of a molecule is determined by

1. Number of electron groups surrounding the central atom

2. Number of atoms bonded to the central atom

• Draw the electron-dot formula• Counting the number of electron groups (one or more electron pairs) around the central atom• Placing the electron groups as far apart as possible around the central atom


Two Electron Groups

Central atoms with two electron groups (two double bonds) such as CO2 have a •linear electron-group geometry•linear shape with a bond angle of 180° to minimize repulsion


Three Electron Groups

Central atoms with three electron groups (two single bonds, one double bond) such as H2CO have a •Trigonal planar electron-group geometry•Trigonal planar shape with a bond angle of 120° to minimize repulsion

H


Three Electron Groups

Central atoms with three electron groups (a single bond, double bond, lone pair) such asSO2 have a •Trigonal planar electron-group geometry•Bent shape with a bond angle of 120° to minimize repulsion


Four Electron Groups

Central atoms with four electron groups (four single bonds) such as CH4 have a •Tetrahedral electron-group geometry •Tetrahedral shape with a bond angle of 109° to minimize repulsion

The carbon-hydrogen dash indicates the bond is behind the plane of the paper,and the wedge indicates the carbon-hydrogen bond is in front of the paper plane.



Central atoms with four electron groups (three single bonds, one lone pair) such as NH3 have a •Tetrahedral electron-group geometry•Trigonal pyramidal shape with a bond angle of 109° to minimize repulsion

The wedge-dash notation represents the three-dimensional shape of the molecule.



Central atoms with four electron groups (two single bonds, two lone pairs) such as H2O have a •Tetrahedral electron-group geometry •Bent shape with a bond angle of 109° to minimize repulsion

The wedge-dash notation represents the three-dimensional shape of the molecule.


Molecular Shapes


Learning Check

State the number of electron groups, lone pairs, and bonded atoms, and use VSEPR theory to determine the shape of the following molecules:(1) Tetrahedral (2) Trigonal pyramidal (3) Bent

A. PF3 B. H2S

C. CCl4 D. PO43–


Solution

State the number of electron groups, lone pairs, and bonded atoms, and use VSEPR theory to determine the shape of the following molecules:(1)tetrahedral (2) trigonal pyramidal (3) bent

A.PF3 4 electron groups, 3 bonded atoms, 1 lone pair, trigonal pyramidal (2)

B. H2S 4 electron groups, 2 bonded atoms, 2 lone pairs, bent (3)


Solution

State the number of electron groups, lone pairs, and bonded atoms, and use VSEPR theory to determine the shape of the following molecules:(1)tetrahedral (2) trigonal pyramidal (3) bent

C.CCl4 4 electron groups, 4 bonded atoms, 0 lone pairs, tetrahedral (1)

D. PO43– 4 electron groups, 4 bonded atoms,

0 lone pairs, tetrahedral (1)


10.3 Electronegativity and Polarity

We can learn more about the chemistry of compounds by understanding how electrons are shared in bonds.•Bonds formed by identical atoms share the bonding electrons equally.•Bonds formed between different atoms share the bonding electrons unequally.

Electronegativity•is the relative ability of atoms to attract shared electrons •is higher for nonmetals; fluorine has the highest with a value of 4.0 •is lower for metals; cesium and francium have the lowest value of 0.7


Electronegativity

Figure 10.1 The electronegativity values of the representative elements in Group 1A (1) to Group 7A (17), which indicate the ability of atoms to attract shared electrons, increase going across a period from left to right and decrease going down a group.


Learning Check

Using the periodic table, predict the order of increasing electronegativity for the elements O, P, and S.

O > S > P


Polarity of Covalent BondsTwo types of covalent bonds occur in molecules:•Polar covalent bonds; bonding electrons are shared unequally•Nonpolar covalent bonds; bonding electrons are shared equally

Example: Assign the polarity of the bonds:C-O N-O Cl-F


Learning Check

Complete the following table for each of the bonds indicated.


Polarity of Molecules—Nonpolar

In a nonpolar molecule, all the bonds are nonpolar, or the polar bonds (dipoles) cancel each other out.

Molecules such as H2, Cl2 and CH4 are nonpolar because they contain only nonpolar bonds.


Polarity of Molecules—Nonpolar

A nonpolar molecule also occurs when polar bonds (dipoles) cancel each other because of a symmetrical arrangement.

Molecules such as CO2 and CCl4 contain polar bonds with dipoles that cancel each other out.


Polarity of Molecules—Polar

A polar molecule occurs when the dipoles from individual bonds do not cancel each other out.For molecules with two or more electron groups, the shape (such as bent or trigonal pyrimidal) determines whether or not the dipoles cancel.



Examples of polar molecules include HCl, H2O, and NH3.•HCl is linear and contains a polar bond.

•H2O is bent and contains two polar bonds as well as two lone pairs on oxygen.



• NH3 is trigonal pyrimidal, and contains three polar bonds and a lone pair on nitrogen.


Learning Check

Determine if the molecule OF2 is polar or nonpolar.


10.4 Attractive Forces in Compounds

Attractive forces between molecules and ions hold them close together in liquids and solids. Solids melt, and liquids boil when the attractive forces between molecules are broken. Attractive forces between molecules can be•dipole−dipole attractions•hydrogen bonding •dispersion forces


Dipole-Dipole Attractions

Polar molecules are attracted to each other by dipole-dipole attractions•when the positive end of one dipole is attracted to the negative end of a second dipole•such as the attractive forces between two molecules of H—Cl


Hydrogen Bonds

Hydrogen bonds, especially strong dipole-dipole attractions, occur between•polar molecules containing hydrogen atoms bonded to very electronegative atoms such as fluorine (F), nitrogen (N), and oxygen (O) •a hydrogen atom with a partial positive charge attached to N, O, or F and a partial negative charge on N, O, or F


Dispersion Forces

Dispersion forces, very weak attractive forces that occur between nonpolar molecules,•occur when movement induces a temporary distortion of the electrons in a molecule, creating a temporary dipole•make it possible for nonpolar molecules to exist as liquids and solids


Bonding and Attractive Forces


Learning Check

Indicate the major type of molecular interaction such as dipole-dipole attractions, hydrogen bonds, or dispersion forces expected between molecules of:

A.NF3

dipole-dipole attractions

B.Cl2

dispersion forcesC. HF

hydrogen bonds


Size, Mass, and Boiling Points

As the size and mass of similar types of molecules increase, •The attractive forces between the molecules also increase •There are more electrons available to produce stronger temporary dipolesThese increased attractive forces increase the boiling points of these molecules.


Molar Mass and Boiling Points, Alkanes


Attractive Forces and Melting Points

The melting point of a substance is related to the strength of the attractive forces between its particles. Molecules with•Weaker dispersion forces have lower melting points; it takes less energy to break the dispersion forces•Stronger dipole-dipole forces, or hydrogen bonds, require more energy to break the forces between them


Highest Melting Points

The highest melting points occur in ionic compounds that have very strong attractive forces between positive and negative ions.Ionic compounds require large amounts of energy to break these forces and melt the substance.


Melting Points of Selected Substances


Chemistry Link to Health

Biological molecules such as proteins have many different functions. They are needed for•Structural components such as cartilage, muscles, hair, and nails•The formation of enzymes to regulate biological reactions•Transport of oxygen in blood and muscles



Proteins are composed of building blocks called amino acids. Amino acids have a central carbon atom bonded to•An –NH3

+ from an amine and a –COO− from a carboxylic acid•An H atom and a side chain called an R group



Different amino acids have different R groups. The R group on alanine is a –CH3 group.

(a) The ionized form of alanine contains —NH3+, —COO–, H, and

a —CH3 group. (b) The structure of alanine represented as a ball-and-stick model.



Examples of other amino acids with their side chains include:



In the primary structure of proteins, amino acids are linked by peptide bonds between the amine and the carboxylic acid groups.



In a higher level structure ofproteins, hydrogen bonds areformed to fold the proteinsinto different shapes, such asan alpha helix.



Hydrogen bonding inproteins also occursbetween the polarside chains on aminoacids on the outersurface of the protein,and the –OH and –Hof polar watermolecules.


Learning Check

Identify the compound in each pair that has the higher melting point. Explain.

A. NCl3 or NH3

B. HBr or Br2

C. KCl or HCl


Solution


A. NCl3 or NH3

NH3 has a higher melting point than NCl3 because the hydrogen bonds in NH3 are stronger than the dipole-dipole attractions in NCl3.


Solution


B. HBr or Br2

HBr would have a higher melting point than Br2 because the dipole-dipole attractions in HBr are stronger than the dispersion forces in Br2.


Solution

Identify the compound in each pair that has the higher melting point. Explain.C. KCl or HCl

KCl would have a higher melting point than HCl because the ionic bonds in KCl are stronger than the dipole-dipole attractions in HCl.


10.5 Change of State

Matter undergoes a change of state when it is converted from one state to another at a constant temperature.

Figure 10.3 Changes of state include melting and freezing, boiling and condensation, sublimation and deposition.


Melting and Freezing

A substance • changes from a solid to a liquid at its melting

point (mp)• changes from a liquid to a solid at its freezing

point (fp)Water has a freezing (melting) point of 0 °C.


Evaporation, Boiling, and Condensation

Evaporation occurswhen water moleculesgain sufficient energyto escape the liquidsurface and enter thegas phase.



At the boiling point, themolecules have enoughenergy to overcometheir attractive forcesand become a gas.



In condensation water vapor is converted to a liquid as the water molecules lose kinetic energy and slow down.Condensation occurs at the same temperature as boiling, but heat is removed.


Heating and Cooling Curves

All the changes of state during the heating of a solid can be illustrated using a heating curve. Heating curves are a graph, with temperature on the vertical axis and addition of heat on the horizontal axis.Heating curves illustrate a change•of state, using a horizontal line•in temperature of a substance as energy is added or released by a sloped line


Heating Curve, H2O

Figure 10. 6 A heating curve diagrams the temperature increases and changes of state as heat is added.


Cooling Curve, H2O

The cooling curve is a diagram of the cooling process and the reverse of a heating curve.

A cooling curve for water illustrates the change in temperature and changes of state as heat is removed.


Learning Check

A plateau (horizontal line) on a heating curve represents

A. a temperature changeB. a constant temperatureC. a change of state

A sloped line on a heating curve representsA. a temperature changeB. a constant temperatureC. a change of state


Solution

A plateau (horizontal line) on a heating curve represents

C. a change of state

A sloped line on a heating curve representsA. a temperature change


Learning Check

Use the cooling curve for H2O to answer each.A. Water condenses at a temperature of(1) 0 °C (2) 50 °C (3) 100 °C B. At a temperature of 0 °C, liquid water(1) freezes (2) melts (3) changes to a gasC. When water freezes, heat is(1) removed (2) added


Solution

Use the cooling curve for H2O to answer each.A. Water condenses at a temperature of(3) 100 °CB. At a temperature of 0 °C, liquid water(1) freezesC. When water freezes, heat is(1) removed

© 2014 Pearson Education, Inc. Chapter 10 Properties of Solids and Liquids (Molecular Structures)...

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Transcript of © 2014 Pearson Education, Inc. Chapter 10 Properties of Solids and Liquids (Molecular Structures)...