Solution Manual for Chemistry in Focus A Molecular View
of Our World 5th Edition by Nivaldo J Tro ISBN
1111989060
9781111989064
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CHAPTER 5
CHEMICAL BONDING
ANSWERS TO QUESTIONS:
1. As elements, both sodium and chlorine are very reactive due to their electron configurations. Their reactivity makes them harmful to biological systems; elemental sodium and chlorine are both toxic substances; chlorine was a primitive chemical weapon used in the Great War 1914 - 1918. Sodium has one valence electron, while chlorine has seven valence electrons. Thus, both elements have incomplete octets. Sodium will lose one electron and chlorine will gain one electron to form a chemical bond. In the formation of sodium chloride, both atoms are now stable due to filled octets in their outer Bohr orbits. Therefore sodium and chloride when bonded together are relatively harmless.
2. Sodium is a reactive element because it has only one electron in the outer Bohr orbit. When placed in water, the sodium reacts by donating its valence electrons with water molecules. The fizzing noise and sparks flying are evidence of a chemical reaction occurring.
3. In the Lewis dot structure, the dots surrounding the elements indicate the number of valence electrons. Ionic bonding is represented by moving dots from the Lewis structure of the metal to the Lewis structure of the nonmetal to give both elements a complete octet. This corresponds to a stable configuration, because the outermost occupied Bohr orbit contains an octet for each element. In the process, the metal and nonmetal each acquire a charge, the metal becomes positive (loss of electrons) and the nonmetal becomes negative (gain of electrons). Since opposite charges attract each other, there is a strong attractive force between the ions.
4. Covalent bonding is the sharing of pairs of electrons between two atoms. A consequence of this sharing is that the shared dots (electrons) effectively count towards the octet of each bonded atom. By sharing one or more pairs of electrons, atoms can achieve complete octets in their outer orbits. Two types of electron pairs exist in the Lewis structure. The electrons
between two atoms are called bonding pair electrons, while those on a single atom are called lone pair electrons.
5. The Lewis theory is useful because it explains why elements combine in the observed ratios and allows one to predict the molecules that would form from certain elements. For example, fluorine, chlorine, bromine, and iodine all exist as diatomic molecules in nature, just as predicted by the Lewis theory. In addition, magnesium fluoride is composed of two fluoride ions and one magnesium ion, just as predicted by the Lewis theory.
Bohr electron configuration for sulfur
Lewis dot structure for the S atom:
Na and Cl would be the most chemically reactive. Ar would be the least reactive since it contains an octet of valence electrons.
8. VSEPR is an acronym for valence shell electron pair repulsion. This model or theory is useful in predicting the geometries of molecules formed from nonmetals. The main postulate of this model is that the arrangement of atoms around a given atom in a molecule is determined principally by minimizing electron-pair repulsions. Since electrons are negatively charged, they tend to repel each other so the bonding and nonbonding electron pairs around a given atom will be positioned as far apart as possible. This theory, in combination with the Lewis theory, can be used to predict the approximate shape of a molecule.
9. The shape of a molecule is of primary importance in determining the properties of that substance. Polarity is a property of far-reaching importance that is largely determined by molecular shape. The distribution of charge in chemical bonds determines bond polarity; but the polarity of a molecule will depend on how the bonds are arranged: if the individual bond polarities cancel out, the molecule is non-polar; but if they don’t cancel out, the molecule is polar. Shape will determine whether the polarities will cancel or not
10. A polar covalent bond is a bond in which the bonding electrons are unequally shared between the two atoms. This will occur when the atoms are of different electronegativity. The more electronegative atom draws the electrons away from the less electronegative one. The atom of higher electronegativity has a partial negative charge and the other atom has a partial positive charge.
11. Water is a unique substance for many reasons, and many of its properties are essential for life on earth. Water is a liquid at room temperature, whereas hydrides of the other nonmetals are gases. Ice floats on water, whereas most solids are denser than their liquid phase. Water has a very high heat of vaporization, which makes it a very effective cooling agent. The specific heat of water is unusually high, which means that large amounts of heat are absorbed in heating it, or emitted when it is cooled.
All these properties; each of which alone is unusual, but collectively make for a substance of very special qualities; can be explained by hydrogen bonding. Hydrogen bonding represents a form of intermolecular force resulting from the polar nature of the O – H bond. It is also found to a lesser extent in molecules containing N – H and F – H bonds. In water, it is particularly strong because of the combination of the two lone electron pairs on each O atom being involved in a hydrogen bond with an H atom on each of two neighboring H2O molecules. A network of hydrogen bonds, which makes optimum use of the two lone electron pairs and the two H atoms of every H2O molecule, extends through the water binding the molecules together.
12. A polar molecule has a slightly positive charge at one end and a slightly negative charge at the other end. The separation of charges is due to polar covalent bonds within the molecule. Polar bonds have two poles – a positive pole and a negative pole – due to a difference in electronegativities of the elements bonded. A molecule containing polar bonds is polar unless the symmetry is such that the individual polar bonds cancel each other out Possession of polar bonds is not a sufficient condition for a molecule to be polar. The geometry as well as the polarity of its bonds determines whether a polyatomic molecule is polar or nonpolar. Absence of polarity in a molecule arises when either all the bonds are nonpolar or the polar bonds cancel due to symmetry.
13. In order for a change of state to occur, intermolecular forces must be overcome by the thermal energy of the molecules. As the strength of intermolecular forces rises, so will the melting and boiling points. Polar molecules tend to have higher melting and boiling points than non-polar molecules because the intermolecular forces are greater, owing to the electrostatic interactions between the atoms that have small positive and negative electrical charges.
14. As a general rule, we can say that like molecules attract like molecules and unlike molecules repel unlike molecules. Water is a very polar molecule and owes many of its properties to the hydrogen bonding between molecules. Oil, on the other hand, is a nonpolar molecule. Therefore, the water molecules attract each other strongly, but reject the oil molecules and, since oil is less dense than water, oil forms a layer on the surface of the water.
SOLUTIONS TO PROBLEMS:
a) MgS is ionic: Mg is a metal (group 2A) and S is a nonmetal (group 6A).
b) PI3 is covalent: both P (group 5A) and I (group 7A) are nonmetals.
c) SrCl2 is ionic: Sr is a metal (group 2A) and Cl is a nonmetal (group 7A).
d) CHClO is covalent: All the elements are nonmetals.
a) K2O is ionic: K (group 1A) is a metal and O (group 6A) is a nonmetal.
b) CHClO is covalent: all the elements are nonmetals.
c) SrS is ionic: Sr (group 2A) is a metal and S (group 6A) is a nonmetal.
d) CH3
is covalent: all the elements are nonmetals
a) The Lewis structure is incorrect because Ca and O together form a ionic bond not a covalent bond. The correct Lewis structure is as follows:
b) The Lewis structure is incorrect because there should only be single bonds between the oxygen and both chlorines, there should not be a double bond between the first chlorine and the oxygen. The correct Lewis structure is as follows:
c) The Lewis structure is incorrect because the P does not have a complete octet. The correct Lewis structure is as follows:
d) The Lewis structure is incorrect because there are too many total electrons. Nitrogen only has 5 valence electrons so between the two there should only be 10 total electrons. The correct Lewis structure is as follows:
26.
a) For I2, there are four electron pairs around each I atom, which implies that the electron geometry is tetrahedral around the I atom However, since there is only atom bonded to the I atom, the molecular geometry is inevitably linear.
b) For NF3, the total number of electron pairs around N is four, three bonding pairs and one nonbonding pair. The electron geometry is tetrahedral, but since one of these electron pairs is a lone pair, the resulting molecular geometry is pyramidal.
c) For PCl3, the total number of electron pairs around P is four, three bonding pairs and one nonbonding pair. The electron geometry is tetrahedral, but since one of these electron pairs is a lone pair, the resulting molecular geometry is pyramidal.
d) For SCl2, the total number of electron pairs around S is four, two bonding pairs and two nonbonding pairs. The electron geometry is tetrahedral, but since two of these electron pairs are lone pairs, the resulting molecular geometry is bent.
27.
a) For OF2, the total number of electron pairs around O is four, two bonding pairs and two nonbonding pairs. The electron geometry is tetrahedral, but since two of these electron pairs are lone pairs, the resulting molecular geometry is bent.
b) For NI3, the total number of electron pairs around N is four, three bonding pairs and one nonbonding pair. The electron geometry is tetrahedral, but since one of these electron pairs is a lone pair, the resulting molecular geometry is pyramidal
c) For CS2, the total number of electron “pairs” around C is only two, since each double bond counts as just one charge group. The electron and molecular geometries are both linear.
d) For Cl2CO, the total number of electron “pairs” around C is three, since the C = O double counts as only one group. Therefore, the electron and molecular geometries are both trigonal planar.
28.
a) The total number of electron pairs for the Lewis structure of ClNO is three. Two pairs are bonding and one pair is nonbonding. The electron geometry is trigonal planar and the molecular geometry is bent. ..
b) The total number of electron pairs for the Lewis structure of C2H6 is seven, and they are all bonding. The electron geometry and the molecular geometry are tetrahedral around both carbons.
c) For the N2F2 molecule, both nitrogen atoms have three electron pairs, two bonding pairs (one double bond exists between the nitrogen atoms) and one nonbonding pair. The electron geometry is trigonal planar; however, one of the electron pairs is a lone pair. The correct molecular geometry is a bent structure on both nitrogen atoms. .. : .. .. : .. ..
d) For N2H4, both nitrogen atoms have four electron pairs, three bonding pairs and one nonbonding pair. The electron geometry is tetrahedral but since one of these electron pairs is a lone pair, the resulting molecular geometry is pyramidal on both nitrogen atoms. the indicated three dimensional structure is shown below. ..
a) The total number of electron pairs around each individual carbon is two. Both pairs are bonding therefore both the electron and the molecular geometry is linear.
b) The central atom is carbon in CCl4; it has four electron pairs around it. All four electron pairs are bonding therefore both the electron and the molecular geometry is tetrahedral.
c) The central atom is P; it has four electron pairs around it. Three of the pairs are bonding and one is a lone pair. Therefore, the electron geometry is tetrahedral and the molecular geometry is pyramidal.
d) The oxygens are the two central atoms, each with four electron pairs around it. Each oxygen has two bonding pairs and two lone pairs. Therefore, the electron geometry of each oxygen is tetrahedral and the molecular geometry is bent.
The electronic geometry and molecular geometry are tetrahedral. All the bonds are polar, but since the C-F and C-Cl bonds are different, the polarities do not cancel and the molecular is polar.
31. The Lewis dot structure of CF3CFH2 is
Both carbons are central atoms and the geometry round each is tetrahedral as shown:
Since the C-F bonds are polar, while the C-H bonds are essentially nonpolar, CF3CFH2 will be polar.
32. All molecules are linear diatomics, so polarity of the molecule depends only on the polarity of the bond, which in turn depends on the electronegativity difference between the elements.
a) HBr polar
b) ICl (slightly) polar
c) I2 nonpolar
d) CO polar
33. All the molecules are linear diatomics, so polarity of the molecule depends only on the polarity of the bond, which in turn depends on the electronegativity difference between the elements.
a) HF polar
b) O2 nonpolar
c) NO polar
d) H2 nonpolar
34.
a) NH3 polar (pyramidal geometry – dipoles don’t cancel)
b) CCl4 nonpolar (tetrahedral geometry – dipoles cancel)
c) SO2 polar (bent geometry – dipoles don’t cancel)
d) CH4 nonpolar (tetrahedral geometry – dipoles cancel)
e) CH3OH polar (tetrahedral electron geometries at both the carbon and oxygen make the molecule slightly bent at the oxygen – dipoles don’t cancel)
35.
a) CH3OH polar (tetrahedral electron geometries at both the carbon and oxygen make the molecule slightly bent at the oxygen: dipoles don’t cancel)
b) CH2Cl2 polar (tetrahedral – dipoles don't cancel)
c) N2H2 polar (geometry around each N atom is pyramidal: dipoles don't cancel)
d) CF4 nonpolar (tetrahedral geometry around C – dipoles cancel)
POINTS TO PONDER
36. The point here is that the properties of water depend upon the water molecule being polar. The polarity of a molecule is determined by the arrangements of the polar bonds. If the individual bond polarities cancel out, the molecule is non-polar; if they don’t cancel, the molecule is polar. If water was linear rather than bent, the O – H bond polarities would cancel and water would be non-polar. The world would be a very different place; but we would not be here to experience it.
37. The macroscopic hexagonal snowflake shape reflects the microscopic, molecular level hexagonal arrangement of water molecules in an ice crystal. The covalent O-H bonds and dipolar O∙∙∙H bonds are arranged in hexagons.
38. We can relate the type of bond in the molecule to the energy of the light required to break the bond. A double bond is stronger than a single bond and so a higher energy light would be required to break that bond. O2 molecules are not broken apart by the UV light whereas O3 is; and this is because O2 contains a double bond, whereas the bonds in O3 are between single and double. Therefore, if O3 actually contained double bonds, it too would not be decomposed by the UV light.
39. If in fact the halogens were stable elements, we would be forced to conclude that the atom desired to achieve a septet of electrons rather than an octet. The noble gases would then be expected to be extremely reactive because they would have one electron too many – they would be the equivalent of the alkali metals. Group VI elements, oxygen, sulfur and so on would also be very reactive, because they only have one electron deficient.
40. Answers may vary.
