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KIMIA DASAR(FPU 617107)

Jurusan Teknologi Hasil Pertanian Fakultas Pertanian Universitas Lampung

BAGIAN 4

1


IKATAN KIMIA• Teori Elektron Lewis

➡ Elektron yang berasal dari kulit elektron terluar (valensi), penting dalam ikatan kimia.

➡ Elektron dipindahkan dari satu atom ke atom lainnya. Ion positif dan negatif terbentuk dan saling menarik melalui kekuatan elektrostatik yang disebut Ikatan Ionik.

➡ Satu atau lebih pasang elektron dibagi di antara atom. Suatu ikatan yang dibentuk oleh pembagian elektron di antara atom disebut Ikatan Kovalen.

➡ Elektron ditransfer atau dibagi sedemikian rupa sehingga membentuk konfigurasi elektron yang sangat stabil (konfigurasi gas mulia), yaitu delapan elektron terluar (oktet).

2


• Simbol Titik Lewis➡ Adalah simbol yang ditulis dalam bentuk titik-titik

pada atom dan satu titik untuk setiap elektron valensi dalam sebuah atom.

• Struktur Lewis➡ kombinasi dari simbol Lewis yang menunjukan

transfer atau pembagian elektron dalam ikatan kimia

Gilbert Newton Lewis

(1875 1946)

Lewis s contribution to thestudy of chemical bonding isevident throughout this text.Equally important, however,was his pioneering introductionof thermodynamics intochemistry.

396 Chapter 10 Chemical Bonding I: Basic Concepts

Si

these theories to what is known about molecular structures from experimentalmeasurements. In Chapter 11 we will examine the subject of chemical bondingin greater depth, and in Chapter 12 we will describe intermolecular forcesforces between molecules and explore further the relationship between mole-cular shape and the properties of substances.

10-1 Lewis Theory: An Overview

In the period from 1916 to 1919, two Americans, G. N. Lewis and IrvingLangmuir, and a German, Walther Kossel, advanced an important proposalabout chemical bonding: Something unique in the electron configurations ofnoble gas atoms accounts for their inertness, and atoms of other elementscombine with one another to acquire electron configurations like those ofnoble gas atoms. The theory that grew out of this model has been most closelyassociated with G. N. Lewis and is called the Lewis theory. Some fundamentalideas associated with Lewis s theory follow:

1. Electrons, especially those of the outermost (valence) electronic shell, playa fundamental role in chemical bonding.

2. In some cases, electrons are transferred from one atom to another. Positiveand negative ions are formed and attract each other through electrostaticforces called ionic bonds.

3. In other cases, one or more pairs of electrons are shared between atoms. A bond formed by the sharing of electrons between atoms is called acovalent bond.

4. Electrons are transferred or shared in such a way that each atom acquiresan especially stable electron configuration. Usually this is a noble gas con-figuration, one with eight outer-shell electrons, or an octet.

Lewis Symbols and Lewis Structures

Lewis developed a special set of symbols for his theory. A Lewis symbol con-sists of a chemical symbol to represent the nucleus and core (inner-shell)electrons of an atom, together with dots placed around the symbol to representthe valence (outer-shell) electrons. Thus, the Lewis symbol for silicon, which hasthe electron configuration , is3Ne43s23p2

Since 1962, a number ofcompounds of Xe and Krhave been synthesized. As we will see in this chapter, afocus on noble-gas electronconfigurations can still beuseful, even if the idea that they confer completeinertness is invalid.

The term covalent wasintroduced by IrvingLangmuir.

Electron spin had not yet been proposed when Lewis framed his theory,and so he did not show that two of the valence electrons are paired and13s22two are unpaired. We will write Lewis symbols in the way Lewis did.We will place single dots on the sides of the symbol, up to a maximum of four.Then we will pair up dots until we reach an octet. Lewis symbols are com-monly written for main-group elements but much less often for transitionelements. Lewis symbols for several main-group elements are written inExample 10-1.

13p22

A Lewis structure is a combination of Lewis symbols that represents eitherthe transfer or the sharing of electrons in a chemical bond.

(10.1)

(10.2)

ClNa* + [Na]+[*Cl ],Ionic bonding

(transfer of

electrons):Lewis symbols Lewis structure

ClH* + H*ClCovalent bonding

(sharing of

electrons):Lewis symbols Lewis structure

M10_PETR4521_10_SE_C10.QXD 1/16/10 3:11 AM Page 396

y

y

3


4


IKATAN ION➡ Ikatan ion adalah kekuatan elektrostatik yang menahan

ion bersama dalam senyawa ionik.➡ Untuk senyawa ionik, (1) simbol Lewis dari ion logam

tidak memiliki titik jika semua elektron valensi hilang, dan (2) muatan ion dari kedua kation dan anion ditunjukkan


Solve(a) Ba loses two electrons, and O gains two.

(b) A Cl atom can accept only one electron because it already has seven valence electrons. One more willgive it a complete octet. Conversely, a Mg atom must lose two electrons to have the electron configura-tion of the noble gas neon. So two Cl atoms are required for each Mg atom.

(c) The formula of aluminum oxide follows directly from the Lewis structure. The combination of oneAl atom, which loses three electrons, and one O atom, which gains two, leaves an excess of one lostelectron. To match the numbers of electrons lost and gained, the formula unit must be based on twoAl atoms and three O atoms.

AssessWe almost never write Lewis structures for ionic compounds, except when we want to emphasize the ratio inwhich the ions combine. The structures of ionic compounds are much more complicated than is suggested bythe Lewis structure. See, for example, the structure of NaCl shown in Figure 10-1.

PRACTICE EXAMPLE A: Write plausible Lewis structures for (a) and (b)

PRACTICE EXAMPLE B: Write plausible Lewis structures for (a) calcium iodide; (b) barium sulfide; (c) lithium oxide.

Mg3N2.Na2S

Al

* 2[Al]3*3[ O ]2+

O

O

Lewis structureAl O

Mg * [Mg]2*2[ Cl ]+

Cl

ClLewis structure

OBa * [Ba]2*[ O ]2+

Lewis structure

FIGURE 10-1Portion of an ionic crystalThis structure of alternating

and ions extends inall directions and involvescountless numbers of ions.

Cl-Na+

The compounds described in Example 10-2 are binary ionic compounds con-sisting of monatomic cations and monatomic anions. Commonly encounteredternary ionic compounds consist of monatomic and polyatomic ions. Bondingbetween atoms within the polyatomic ions is covalent. Some ternary ioniccompounds are considered later in the chapter.

With the exception of ion pairs, such as that may be found in thegaseous state, formula units of solid ionic compounds do not exist as separateentities. Instead, each cation is surrounded by anions and each anion bycations. These very large numbers of ions are arranged in an orderly networkcalled an ionic crystal (Fig. 10-1). Ionic crystal structures and the energy changesaccompanying the formation of ionic crystals are described in Chapter 12.

1Na+Cl-2,

10-1 CONCEPT ASSESSMENT

How many valence electrons do the Lewis symbols for the elements in group 16have? Which of the following are correct Lewis symbols for sulfur?

SS SS









Mg3N2.Na2S

Al

* 2[Al]3*3[ O ]2+

O

O

Lewis structureAl O

Mg * [Mg]2*2[ Cl ]+

Cl

ClLewis structure

OBa * [Ba]2*[ O ]2+

Lewis structure



Cl-Na+



1Na+Cl-2,



SS SS









Mg3N2.Na2S

Al

* 2[Al]3*3[ O ]2+

O

O

Lewis structureAl O

Mg * [Mg]2*2[ Cl ]+

Cl

ClLewis structure

OBa * [Ba]2*[ O ]2+

Lewis structure



Cl-Na+



1Na+Cl-2,



SS SS


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IKATAN KOVALEN➡ Ikatan kovalen adalah ikatan di mana dua elektron dibagi

oleh dua atom.➡ Senyawa kovalen adalah senyawa yang hanya

mengandung ikatan kovalen.➡ Struktur Lewis adalah representasi ikatan kovalen di

mana pasangan elektron bersama ditunjukkan sebagai garis atau sebagai pasang titik antara dua atom, dan pasangan tunggal ditunjukkan sebagai pasang titik pada atom individu.

➡ Pembentukan molekul ini menggambarkan aturan oktet, yitu atom selain hidrogen cenderung membentuk ikatan sampai dikelilingi oleh delapan elektron valensi.

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10-2 Covalent Bonding: An Introduction 399

10-2 Covalent Bonding: An Introduction

A chlorine atom shows a tendency to gain an electron, as indicated by its elec-tron affinity From which atom, sodium or hydrogen, can theelectron most readily be extracted? Neither atom gives up an electron freely,but the energy required to extract an electron from isNa 1I1 = 496 kJ>mol2

1-349 kJ>mol2.

much smaller than that for In Chapter 9 we learned thatthe lower its ionization energy, the more metallic an element is; sodium is muchmore metallic than hydrogen (recall Figure 9-12). In fact, hydrogen is consid-ered to be a nonmetal. A hydrogen atom in the gaseous state does not give upan electron to another nonmetal atom. Bonding between a hydrogen atom anda chlorine atom involves the sharing of electrons, which leads to a covalent bond.

To emphasize the sharing of electrons, let us think of the Lewis structure ofHCl in this manner.

H 1I1 = 1312 kJ>mol2.

H Cl

The broken circles represent the outermost electron shells of the bonded atoms.The number of dots lying on or within each circle represents the effective num-ber of electrons in each valence shell. The H atom has two dots, as in the electronconfiguration of He. The Cl atom has eight dots, corresponding to the outer-shell configuration of Ar. Note that we counted the two electrons between Hand Cl twice. These two electrons are shared by the H and Cl atoms. Thisshared pair of electrons constitutes the covalent bond. Written below are twoadditional Lewis structures of simple molecules.

(*)

* * *H andH HHO O Cl Cl ClCl*O O

Water Dichlorine monoxide

As was the case for Cl in HCl, the O atom in the Lewis structure of andin is surrounded by eight electrons (when the bond-pair electrons aredouble counted). In attaining these eight electrons, the O atom conforms to theoctet rule a requirement of eight valence-shell electrons for the atoms in aLewis structure. Note, however, that the H atom is an exception to this rule.The H atom can accommodate only two valence-shell electrons.

Lewis theory helps us to understand why elemental hydrogen and chlorineexist as diatomic molecules, and In each case, a pair of electrons isshared between the two atoms. The sharing of a single pair of electronsbetween bonded atoms produces a single covalent bond. To underscore theimportance of electron pairs in the Lewis theory the term bond pair applies toa pair of electrons in a covalent bond, while lone pair applies to electron pairsthat are not involved in bonding. Also, in writing Lewis structures it is cus-tomary to replace bond pairs with lines ( ). These features are shown in thefollowing Lewis structures.

(10.3)

(10.4)

Cl2.H2

Cl2OH2O

The Lewis structures forand suggest that

these molecules have alinear shape. They do not.Lewis theory by itself doesnot address the questionof molecular shape (see Section 10-7).

Cl2OH2O

*

*

*

Bond pair

Lone pairs

or

Bond pair

H H H HH

or

H

Cl Cl ClCl ClCl

Coordinate Covalent Bonds

The Lewis theory of bonding describes a covalent bond as the sharing of a pairof electrons, but this does not necessarily mean that each atom contributes anelectron to the bond. A covalent bond in which a single atom contributes bothof the electrons to a shared pair is called a coordinate covalent bond.

M10_PETR4521_10_SE_C10.QXD 1/16/10 3:11 AM 99


10-2 Covalent Bonding: An Introduction

A chlorine atom shows a tendency to gain an electron, as indicated by its elec-tron affinity From which atom, sodium or hydrogen, can theelectron most readily be extracted? Neither atom gives up an electron freely,but the energy required to extract an electron from isNa 1I1 = 496 kJ>mol2

1-349 kJ>mol2.

much smaller than that for In Chapter 9 we learned thatthe lower its ionization energy, the more metallic an element is; sodium is muchmore metallic than hydrogen (recall Figure 9-12). In fact, hydrogen is consid-ered to be a nonmetal. A hydrogen atom in the gaseous state does not give upan electron to another nonmetal atom. Bonding between a hydrogen atom anda chlorine atom involves the sharing of electrons, which leads to a covalent bond.

To emphasize the sharing of electrons, let us think of the Lewis structure ofHCl in this manner.

H 1I1 = 1312 kJ>mol2.

H Cl

The broken circles represent the outermost electron shells of the bonded atoms.The number of dots lying on or within each circle represents the effective num-ber of electrons in each valence shell. The H atom has two dots, as in the electronconfiguration of He. The Cl atom has eight dots, corresponding to the outer-shell configuration of Ar. Note that we counted the two electrons between Hand Cl twice. These two electrons are shared by the H and Cl atoms. Thisshared pair of electrons constitutes the covalent bond. Written below are twoadditional Lewis structures of simple molecules.

(*)

* * *H andH HHO O Cl Cl ClCl*O O

Water Dichlorine monoxide

As was the case for Cl in HCl, the O atom in the Lewis structure of andin is surrounded by eight electrons (when the bond-pair electrons aredouble counted). In attaining these eight electrons, the O atom conforms to theoctet rule a requirement of eight valence-shell electrons for the atoms in aLewis structure. Note, however, that the H atom is an exception to this rule.The H atom can accommodate only two valence-shell electrons.

Lewis theory helps us to understand why elemental hydrogen and chlorineexist as diatomic molecules, and In each case, a pair of electrons isshared between the two atoms. The sharing of a single pair of electronsbetween bonded atoms produces a single covalent bond. To underscore theimportance of electron pairs in the Lewis theory the term bond pair applies toa pair of electrons in a covalent bond, while lone pair applies to electron pairsthat are not involved in bonding. Also, in writing Lewis structures it is cus-tomary to replace bond pairs with lines ( ). These features are shown in thefollowing Lewis structures.

(10.3)

(10.4)

Cl2.H2

Cl2OH2O

The Lewis structures forand suggest that

these molecules have alinear shape. They do not.Lewis theory by itself doesnot address the questionof molecular shape (see Section 10-7).

Cl2OH2O

*

*

*

Bond pair

Lone pairs

or

Bond pair

H H H HH

or

H

Cl Cl ClCl ClCl

Coordinate Covalent Bonds

The Lewis theory of bonding describes a covalent bond as the sharing of a pairof electrons, but this does not necessarily mean that each atom contributes anelectron to the bond. A covalent bond in which a single atom contributes bothof the electrons to a shared pair is called a coordinate covalent bond.



Multiple Covalent BondsIn the preceding description of the Lewis model for covalent chemical bond-ing, we have used a single pair of electrons between two atoms to describe asingle covalent bond. Often, however, more than one pair of electrons mustbe shared if an atom is to attain an octet (noble gas electron configuration).

and are two molecules in which atoms share more than one pair ofelectrons.

First, let s apply the ideas about Lewis structures to From the Lewissymbols, we see that the C atom can share a valence electron with each Oatom, thus forming two carbon-to-oxygen single bonds.

But this leaves the C atom and both O atoms still shy of an octet. The problemis solved by shifting the unpaired electrons into the region of the bond, asindicated by the red arrows.

(10.7)

In Lewis structure (10.7), the bonded atoms are seen to share two pairs of elec-trons (a total of four electrons) between them a double covalent bond

Now let s try our hand at writing a Lewis structure for the molecule.Our first attempt might again involve a single covalent bond and the incorrectstructure shown below.

Each N atom appears to have only six outer-shell electrons, not the expectedeight. The situation can be corrected by bringing the four unpaired electronsinto the region between the N atoms and using them for additional bond pairs.In all, we now show the sharing of three pairs of electrons between the N atoms.The bond between the N atoms in is a triple covalent bond Doubleand triple covalent bonds are known as multiple covalent bonds.

(10.8)

The triple covalent bond in is a very strong bond that is difficult to breakin a chemical reaction. The unusual strength of this bond makes quite inert. As a result, coexists with in the atmosphere and formsoxides of nitrogen only in trace amounts at high temperatures. The lack ofreactivity of with is an essential condition for life on Earth. The inert-ness of also makes it difficult to synthesize nitrogen compounds.

Another molecule whose Lewis structure features a multiple bond is which has a double bond.

(10.9)

The blue question mark suggests that there is some doubt about the validity ofstructure (10.9), and the source of the doubt is illustrated in Figure 10-3. Thestructure fails to account for the paramagnetism of oxygen the moleculemust have unpaired electrons. Unfortunately, no completely satisfactory Lewisstructure is possible for but in Chapter 11, bonding in the molecule isdescribed in a way that accounts for both the double bond and the observedparamagnetism.

We could continue applying ideas introduced in this section, but our abilityto write plausible Lewis structures will be greatly aided by a couple of newideas that we introduce in Section 10-3.

O2O2,

O2

*O O O ?O O O

O2,N21g2

O2N2

O21g2N21g2N21g2

N2

N N N N

(+ ).N2

N NN N (Incorrect)*

N2

(* ).

O O CO OCO OC

O OC O OC

CO2.

N2CO2

FIGURE 10-3Paramagnetism of oxygenLiquid oxygen is attracted intothe magnetic field of a largemagnet.

KEEP IN MIND

that merely being able to writea plausible Lewis structuredoes not prove that it is thecorrect electronic structure.Proof can come only throughconfirming experimentalevidence.







(10.7)




(10.8)



(10.9)



O2O2,

O2

*O O O ?O O O

O2,N21g2

O2N2

O21g2N21g2N21g2

N2

N N N N

(+ ).N2

N NN N (Incorrect)*

N2

(* ).

O O CO OCO OC

O OC O OC

CO2.

N2CO2


KEEP IN MIND








(10.7)




(10.8)



(10.9)



O2O2,

O2

*O O O ?O O O

O2,N21g2

O2N2

O21g2N21g2N21g2

N2

N N N N

(+ ).N2

N NN N (Incorrect)*

N2

(* ).

O O CO OCO OC

O OC O OC

CO2.

N2CO2


KEEP IN MIND



7


Lengths of Covalent Bonds

Bond LengthsTriple bond < Double Bond < Single Bond

8


ELECTRONEGATIVITY➡ Elektronegativitas adalah kemampuan atom untuk

menarik elektron ke arah dirinya sendiri dalam ikatan kimia.

10-3 Polar Covalent Bonds and Electrostatic Potential Maps 405

relative to an atom with a low ionization energy and a small electron affinity,such as sodium.

There are several methods for converting qualitative comparisons toactual numerical values of the electronegativities of the elements. Onewidely used electronegativity scale, with values given in Figure 10-6, is thatdevised by Linus Pauling (1901 1994). Pauling s EN values range fromabout 0.7 to 4.0. In general, the lower its EN, the more metallic the elementis, and the higher the EN, the more nonmetallic it is. From Figure 10-6 wealso see that electronegativity decreases from top to bottom in a group andincreases from left to right in a period of the periodic table. These are theexpected trends when we interpret electronegativity in terms of the quantity

That is, as the ionization energy (I) increases across the period weexpect the electronegativity to increase. The distinction between electronaffinity and electronegativity is clearly seen when we consider the electronaffinities of F and chlorine Although theelectron affinity of Cl is somewhat more negative than that ofF the EN of Cl (3.0) is significantly lower than that of F (4.0)because of the decreased ionization energy of chlorine relativeto fluorine 11681 kJ>mol2.

11251 kJ>mol21-328 kJ>mol2,

1-349 kJ>mol21-349 kJ mol-12:1-328 kJ mol-12

1I - EA2.


With the aid of only a periodic table, decide which is the most electronegativeatom of each of the following sets of elements: (a) As, Se, Br, I; (b) Li, Be, Rb, Sr;(c) Ge, As, P, Sn.

Electronegativity values allow an insight into the amount of polar characterin a covalent bond based on electronegativity difference, the absolutevalue of the difference in EN values of the bonded atoms. If for two¢EN

EN

Li

Na

1.5

H

1.2

K1.0

Rb

Cs

Sr

Ba

Fr

0.9

1.3 1.5

1.2 1.4

1.3

AcLr

V1.6

1.6

1.5

1.8

2.4

1.5 1.8

1.9

1.9

2.2

2.2

1.8 1.8

2.2

2.2

2.2

2.2

1.9Zn1.6

1.9

2.4

1.7

1.9

Ga1.6

Ge1.8

1.7

1.8

1.8

1.8

As

2.0Se2.4

1.9

1.9

2.1

2.0

Br2.8

2.5

2.2

B2.0

C2.5

Al1.5

Si1.8

N3.0

O3.5

P2.1

S2.5

F4.0

Cl3.0

1

2

3 4 5 6 7 98 10 11 12

13 14 15 16 17

* Lanthanides: 1.1 1.3Actinides: 1.3 1.5

below 1.0

1.0 1.4

1.5 1.9

2.0 2.4

2.5 2.9

3.0 4.0

2.1

1.0

0.9

0.8

0.8

0.8

0.7

Be

Mg

Ca

1.0

0.9

Ra

LaLu*

Y

Sc Ti1.6

Zr

Hf Ta

Nb

Cr Mn Fe Co Ni Cu

Mo Tc Ru Rh Pd Ag

W Re Os Ir Pt Au Hg Tl Pb Bi Po At

Cd In Sn Sb Te I

FIGURE 10-6

Electronegativities of the elementsAs a general rule, electronegativities decrease from top to bottom in a group and

increase from left to right in a period of elements. The values are from L. Pauling, The

Nature of the Chemical Bond, 3rd ed., Cornell University, Ithaca, NY, 1960, page 93.

Values may be somewhat different when based on other electronegativity scales.



9


Jurusan Teknologi Hasil Pertanian Fakultas Pertanian Universitas LampungJurusan Teknologi Hasil Pertanian

Fakultas Pertanian Universitas Lampung

H F

FH

e-riche-poor

δ+ δ-

10


0 - 0.4Nonpolar CovalentShare e- equally

0.5 - 1.9Polar Covalent

partial transfer of e-

≥ 2 Ionic

Transfer e-

Increasing difference in electronegativity

Classification of bonds by difference in electronegativity406 Chapter 10 Chemical Bonding I: Basic Concepts

Although Figure 10-7suggests that the bondbetween two identical metalatoms should be covalent [asit is in for example],in solid metals, where bondingextends throughout a networkof many, many atoms, thebonding is of a type calledmetallic (explored in the next chapter).

Li21g2,

*

EXAMPLE 10-4 Assessing Electronegativity Differences and the Polarity of Bonds

(a) Which bond is more polar, or (b) What is the percent ionic character of each of these bonds?

AnalyzeTo decide which bond is more polar, look up EN values for H, Cl, and O in Figure 10-6, and then compute elec-tronegativity differences, for and bonds. The greater the electronegativity difference, themore polar the bond. To determine the percentage ionic character, we use the curve in Figure 10-7.

Solve(a) For the bond, For the

bond, Because its is somewhat greater, we expect the bond to bethe more polar bond.

(b) Determine the percent ionic character from Figure 10-7.

AssessIn this example, we used EN values to decide which of two bonds is more polar. EN values can also be usedto decide which end of a given bond will be slightly negative. For example, because is greater than

we conclude that the Cl end of the bond will be slightly negative; thus, the H end will beslightly positive.

PRACTICE EXAMPLE A: Which of the following bonds are the most polar, that is, have the greatest ioniccharacter:

PRACTICE EXAMPLE B: Which is the most polar bond: or O¬F?P¬O,C¬P,C¬S,

P¬Cl?N¬O,N¬H,H¬Br,

H¬ClENH,ENCl

H¬O bond: ¢EN = 1.4 L35% ionic

H¬Cl bond: ¢EN = 0.9 L20% ionic

H¬O¢EN¢EN = 3.5 - 2.1 = 1.4.H¬O¢EN = 3.0 - 2.1 = 0.9.H¬ClENO = 3.5.ENCl = 3.0;ENH = 2.1;

H¬OH¬Cl¢EN,

H¬O?H¬Cl

Per

cen

t io

nic

ch

arac

ter

100

75

50

25

010 2

IBr HBr

HCl

HF

LiBr

KICsIKBr

KCl

CsCl

NaCl

LiF

KF

CsFLiCl

LiI

H IICl

3

Electronegativity difference

FIGURE 10-7Percent ionic character of a chemicalbond as a function of electronegativitydifference

*

atoms is very small, the bond between them is essentially covalent. If islarge, the bond is essentially ionic. For intermediate values of the bondis described as polar covalent. A useful rough relationship between andpercent ionic character of a bond is presented in Figure 10-7.

Large EN differences are found between the more metallic and the morenonmetallic elements. Combinations of these elements are expected to pro-duce bonds that are essentially ionic. Small EN differences are expected fortwo nonmetal atoms, and the bond between them should be essentially cova-lent. Thus, even without a compilation of EN values at hand, you should beable to predict the essential character of a bond between two atoms. Simplyassess the metallic/nonmetallic characters of the bonded elements from theperiodic table (recall Figure 9-12).

¢EN¢EN,

¢EN

M10_PETR4521_10_SE_C10.QXD 1/16/10 3:11 AM 06

11


ELECTRON AFFINITY

Lanthanides

Actanides

12


LARUTAN➡ Larutan adalah campuran homogen dari 2 zat atau lebih.➡ Zat terlarut adalah zat yang ada dalam jumlah yang

lebih sedikit.➡ Pelarut adalah zat yang ada dalam jumlah yang lebih

banyak.➡ Larutan jenuh mengandung jumlah maksimum zat

terlarut yang akan larut dalam pelarut yang diberikan pada suhu tertentu.

➡ Larutan Lewat jenuh mengandung lebih banyak zat terlarut daripada yang ada dalam larutan jenuh pada suhu tertentu.

➡ Kristalisasi adalah proses di mana larutan terlarut keluar dari larutan dan membentuk kristal

13


Interaksi dalam Larutan➡ Daya tarik antarmolekul yang menyatukan molekul

(solute dan solvent) bersama memainkan peran sentral dalam pembentukan solusi.

➡ Ketika satu zat (zat terlarut) larut dalam yang lain (pelarut), partikel zat terlarut akan menyebar ke seluruh pelarut. Partikel zat terlarut menempati posisi yang biasanya diambil oleh molekul pelarut.

➡ Kemudahan dimana partikel zat terlarut menggantikan molekul pelarut tergantung pada kekuatan relatif dari tiga jenis interaksi:• interaksi pelarut-pelarut

• interaksi zat terlarut

• interaksi pelarut-zat terlarut

14


ΔHsoln = ΔH1 + ΔH2 + ΔH3

15


➡ Jika daya tarik pelarut zat terlarut lebih kuat daripada daya tarik pelarut-pelarut dan daya tarik zat terlarut, • Maka proses eksotermik (ΔHsoln. < 0), mudah larut.

➡ Jika interaksi pelarut zat terlarut lebih lemah daripada interaksi pelarut dan zat terlarut, • Maka proses endotermik (ΔHsoln. > 0), sulit larut.

➡ Kelarutan adalah ukuran seberapa banyak zat terlarut dapat larut dalam pelarut pada suhu tertentu.

➡ Dua zat dengan gaya antarmolekul serupa cenderung larut satu sama lain.• Molekul non-polar larut dalam pelarut non-polar (CCl4

dalam C6H6)• Molekul polar larut dalam pelarut polar (C2H5OH dalam

H2O)• Senyawa ionik lebih mudah larut dalam pelarut polar

(NaCl dalam H2O atau NH3)

16


Kelarutan Padatan dan Suhu

solubility increases with increasing

temperature

solubility decreases with increasing

temperature

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Fractional Crystallization • adalah pemisahan

campuran zat menjadi komponen murni atas dasar kelarutannya yang berbeda.

• Soal:100ml Larutan mengandung 90g KNO3 dan 10g NaCl didinginkan dari suhu 60oC menjadi 0oC. Hitung kristal terbentuk.

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Concentration Units• The concentration of a solution is the amount of solute

present in a given quantity of solvent or solution.

• Percent by Mass

% by mass = x 100%mass of solute

mass of solute + mass of solvent

= x 100%mass of solutemass of solution

• Mole Fraction (X)

XA = moles of A

sum of moles of all components

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Concentration Units

Molarity (M)

Molality (m)

M =moles of solute

Volume of solution (liter)

m = moles of solute

mass of solvent (kg)

Soal:What is the molality of a 5.86 M ethanol (C2H5OH) solution whose density is 0.927 g/mL?

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KIMIA DASARstaff.unila.ac.id/sugiharto/files/2015/03/Kimia-Dasar... · 2020. 3. 15. · •Struktur...

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